Robotics with the Boe-Bot
Student Guide
VERSION 3.0
WARRANTY
Parallax warrants its products against defects in materials and workmanship for a period of 90 days from receipt of
product. If you discover a defect, Parallax will, at its option, repair or replace the merchandise, or refund the
purchase price. Before returning the product to Parallax, call for a Return Merchandise Authorization (RMA)
number. Write the RMA number on the outside of the box used to return the merchandise to Parallax. Please enclose
the following along with the returned merchandise: your name, telephone number, shipping address, and a description
of the problem. Parallax will return your product or its replacement using the same shipping method used to ship the
product to Parallax.
14-DAY MONEY BACK GUARANTEE
If, within 14 days of having received your product, you find that it does not suit your needs, you may return it for a
full refund. Parallax will refund the purchase price of the product, excluding shipping/handling costs. This guarantee
is void if the product has been altered or damaged. See the Warranty section above for instructions on returning a
product to Parallax.
COPYRIGHTS AND TRADEMARKS
This documentation is Copyright 2003-2010 by Parallax Inc. By downloading or obtaining a printed copy of this
documentation or software you agree that it is to be used exclusively with Parallax microcontrollers and products.
Any other uses are not permitted and may represent a violation of Parallax copyrights, legally punishable according
to Federal copyright or intellectual property laws. Any duplication of this documentation for commercial uses is
expressly prohibited by Parallax Inc. Duplication for educational use, in whole or in part, is permitted subject to the
following conditions: the material is to be used solely in conjunction with Parallax microcontrollers and products, and
the user may recover from the student only the cost of duplication. Check with Parallax for approval prior to
duplicating any of our documentation in part or whole for any other use.
BASIC Stamp, Board of Education, Boe-Bot, Stamps in Class, and SumoBot are registered trademarks of Parallax
Inc. HomeWork Board, PING))), Parallax, the Parallax logo, Propeller, and Spin are trademarks of Parallax Inc. If
you decide to use any of these words on your electronic or printed material, you must state that “(trademark) is a
(registered) trademark of Parallax Inc.” upon the first use of the trademark name. Other brand and product names
herein are trademarks or registered trademarks of their respective holders.
ISBN 9781928982531
3.0.0-10.11.10-HKTP
DISCLAIMER OF LIABILITY
Parallax Inc. is not responsible for special, incidental, or consequential damages resulting from any breach of
warranty, or under any legal theory, including lost profits, downtime, goodwill, damage to or replacement of
equipment or property, or any costs of recovering, reprogramming, or reproducing any data stored in or used with
Parallax products. Parallax is also not responsible for any personal damage, including that to life and health, resulting
from use of any of our products. You take full responsibility for your BASIC Stamp application, no matter how lifethreatening it may be.
ERRATA
While great effort is made to assure the accuracy of our texts, errors may still exist. Occasionally an errata sheet with
a list of known errors and corrections for a given text will be posted on the related product page at
www.parallax.com. If you find an error, please send an email to editor@parallax.com.
Table of Contents
Preface.........................................................................................................................5
About Version 3.0 ...........................................................................................................6
Audience .........................................................................................................................6
Support Forums ..............................................................................................................7
Resources for Educators ................................................................................................8
Foreign Translations .......................................................................................................9
About the Author .............................................................................................................9
Special Contributors .......................................................................................................9
Chapter 1 : Your Boe-Bot’s Brain ...........................................................................11
Hardware and Software ................................................................................................12
Activity #1 : Getting the Software..................................................................................12
Activity #2 : Using the Help File for Hardware Setup ....................................................17
Summary ......................................................................................................................19
Chapter 2 : Your Boe-Bot’s Servo Motors .............................................................23
Introducing the Continuous Rotation Servo ..................................................................23
Activity #1 : Building and Testing the LED Circuit.........................................................24
Activity #2 : Tracking Time and Repeating Actions with a Circuit .................................27
Activity #3 : Connecting the Servo Motors ....................................................................40
Activity #4 : Centering the Servos.................................................................................49
Activity #5 : How To Store Values and Count ...............................................................53
Activity #6 : Testing the Servos ....................................................................................58
Summary ......................................................................................................................67
Chapter 3 : Assemble and Test Your Boe-Bot.......................................................73
Activity #1 : Assembling the Boe-Bot Robot .................................................................73
Activity #2 : Re-Test the Servos ...................................................................................82
Activity #3 : Start/Reset Indicator Circuit and Program.................................................86
Activity #4 : Testing Speed Control with the Debug Terminal.......................................92
Summary ......................................................................................................................98
Chapter 4 : Boe-Bot Navigation ............................................................................103
Activity #1 : Basic Boe-Bot Maneuvers .......................................................................103
Activity #2 : Tuning the Basic Maneuvers ...................................................................109
Activity #3 : Calculating Distances ..............................................................................112
Activity #4 : Maneuvers—Ramping.............................................................................117
Activity #5 : Simplify Navigation with Subroutines ......................................................120
Activity #6 : Advanced Topic—Building Complex Maneuvers in EEPROM ................126
Summary ....................................................................................................................136
Chapter 5 : Tactile Navigation with Whiskers ..................................................... 143
Tactile Navigation .......................................................................................................143
Activity #1 : Building and Testing the Whiskers ..........................................................144
Activity #2 : Field Testing the Whiskers ......................................................................152
Activity #3 : Navigation with Whiskers ........................................................................155
Activity #4 : Artificial Intelligence and Deciding When You’re Stuck...........................160
Summary ....................................................................................................................165
Chapter 6 : Light-Sensitive Navigation with Phototransistors.......................... 169
Introducing the Phototransistor...................................................................................169
Activity #1 : A Simple Binary Light Sensor .................................................................171
Activity #2 : Measure Light Levels with Phototransistors............................................179
Activity #3 : Light Sensitivity Adjustment ....................................................................189
Activity #4 : Light Measurements for Roaming ...........................................................194
Activity #5 : Routine for Roaming Toward Light .........................................................203
Activity #6 : Test Navigation Routine with the Boe-Bot...............................................212
Summary ....................................................................................................................216
Chapter 7 : Navigating with Infrared Headlights................................................. 221
Infrared Light ..............................................................................................................221
Activity #1 : Building and Testing the IR Object Detectors .........................................223
Activity #2 : Field Testing for Object Detection and Infrared Interference ..................230
Activity #3 : Infrared Detection Range Adjustments ...................................................234
Activity #4 : Object Detection and Avoidance .............................................................237
Activity #5 : High-Performance IR Navigation ............................................................239
Activity #6 : The Drop-Off Detector.............................................................................242
Summary ....................................................................................................................248
Chapter 8 : Robot Control with Distance Detection ........................................... 255
Determining Distance with the Same IR LED/Detector Circuit ...................................255
Activity #1 : Testing the Frequency Sweep ................................................................255
Activity #2 : Boe-Bot Shadow Vehicle ........................................................................262
Activity #3 : Following a Stripe....................................................................................271
Activity #4 : More Boe-Bot Activities and Projects Online...........................................278
Summary ....................................................................................................................280
Appendix A : Parts List and Kit Options.............................................................. 289
Appendix B : Resistor Color Codes and Breadboarding Rules ........................ 293
Appendix C : Boe-Bot Navigation Contests ........................................................ 299
Index ........................................................................................................................ 303
Preface · Page 5
Preface
Robots are used in the auto, medical, and manufacturing industries, in all manner of
exploration vehicles, and, of course, in many science fiction films. The word "robot" first
appeared in a Czechoslovakian satirical play, Rossum's Universal Robots, by Karel
Capek in 1920. Robots in this play tended to be human-like. From this point onward, it
seemed that many science fiction stories involved these robots trying to fit into society
and make sense out of human emotions. This changed when General Motors installed the
first robots in its manufacturing plant in 1961. These automated machines presented an
entirely different image from the “human form” robots of science fiction.
Building and programming a robot is a combination of mechanics, electronics, and
problem solving. What you're about to learn while doing the activities and projects in
this text will be relevant to real-world applications that use robotic control, the only
differences being the size and sophistication. The mechanical principles, example
program listings, and circuits you will use are very similar to, and sometimes the same as,
industrial applications developed by engineers.
The goal of this text is to get students interested in and excited about the fields of
engineering, mechatronics, and software development as they design, construct, and
program an autonomous robot. This series of hands-on activities and projects will
introduce students to basic robotic concepts using the Parallax Boe-Bot® robot, called the
"Boe-Bot." Its name comes from the Board of Education® carrier board that is mounted
on its wheeled chassis. An example of a Boe-Bot with an infrared obstacle detection
circuit built on the Board of Education solderless prototyping area is shown below in
Figure P-1.
Figure P-1
Parallax Inc.’s
Boe-Bot® Robot
The activities and projects in this text begin with an introduction to your Boe-Bot’s brain,
the Parallax BASIC Stamp® 2 microcontroller, and then move on to construction, testing,
Page 6 · Robotics with the Boe-Bot
and calibration of the Boe-Bot. After that, you will program the Boe-Bot for basic
maneuvers, and then proceed to adding sensors and writing programs that make it react to
its surroundings and perform autonomous tasks.
ABOUT VERSION 3.0
This is the first revision of this title since 2004. The major changes include:



Replacement of the cadmium sulfide photoresistor with an RoHS-compliant light
sensor of a type that will be more common in product design going forward. This
required a rewrite of Chapter 6.
Moving the “Setup and Testing” portion of Chapter 1 and the Hardware and
Troubleshooting appendices to the Help file. This was done to support both
serial and USB hardware connections, and other programming connections as
our products and technologies continue to expand. This also allows for the
dynamic maintenance of the Hardware and Troubleshooting material.
Removal of references to the Parallax CD, which has been removed from our
kits, reducing waste and ensuring that customers download the most recent
BASIC Stamp Editor software and USB drivers available for their operating
systems (www.parallax.com/go/Boe-Bot).
In addition, small errata items noted in the previous version (2.2) have been corrected.
The material still aims for the same goals, and all of the same programming concepts and
commands are covered, along with a few new ones. Finally, page numbers have been
changed so the PDF page and the physical page numbers are the same, for ease of use.
AUDIENCE
This text is designed to be an entry point to technology literacy, and an easy learning
curve for embedded programming and introductory robotics. The text is organized so that
it can be used by the widest possible variety of students as well as independent learners.
Middle-school students can try the examples in this text in a guided tour fashion by
simply following the check-marked instructions with instructor supervision. At the other
end of the spectrum, pre-engineering students’ comprehension and problem-solving skills
can be tested with the questions, exercises and projects (with solutions) in each chapter
summary. The independent learner can work at his or her own pace, and obtain
assistance through the Stamps in Class forum cited below.
Preface · Page 7
SUPPORT FORUMS
Parallax maintains free, moderated forums for our customers, covering a variety of
subjects:
 Propeller Chip: for all discussions related to the multicore Propeller
microcontroller and development tools product line.
 BASIC Stamp: Project ideas, support, and related topics for all of the Parallax
BASIC Stamp models.
 Sensors: Discussion relating to Parallax’s wide array of sensors, and interfacing
sensors with Parallax microcontrollers.
 Stamps in Class: Students, teachers, and customers discuss Parallax’s education
materials and school projects here.
 Robotics: For all Parallax robots and custom robots built with Parallax
processors and sensors.
 Wireless: Topics include XBee, GSM/GPRS, telemetry and data communication
over amateur radio.
 PropScope: Discussion and technical assistance for this USB oscilloscope that
contains a Propeller chip.
 The Sandbox: Topics related to the use of Parallax products but not specific to
the other forums.
 Projects: Post your in-process and completed projects here, made from Parallax
products.
Page 8 · Robotics with the Boe-Bot
RESOURCES FOR EDUCATORS
We have a variety of resources for this text designed to support educators.
Stamps in Class “Mini Projects”
To supplement our texts, we provide a bank of projects for the classroom. Designed to
engage students, each “Mini Project” contains full source code, “How it Works”
explanations, schematics, and wiring diagrams or photos for a device a student might like
to use. Many projects feature an introductory video, to promote self-study in those
students most interested in electronics and programming. Just follow the Stamps in Class
“Mini Projects” link at www.parallax.com/Education.
Educators Courses
These hands-on, intensive 1 or 2 day courses for instructors are taught by Parallax
engineers or experienced teachers who are using Parallax educational materials in their
classrooms. Visit www.parallax.com/Education → Educators Courses for details.
Parallax Educator’s Forum
In this free, private forum, educators can ask questions and share their experiences with
using Parallax products in their classrooms. Supplemental education materials are also
posted here. To enroll, email education@parallax.com for instructions; proof of status as
an educator will be required.
Supplemental Educational Materials
Select Parallax educational texts have an unpublished set of questions and solutions
posted in our Parallax Educators Forum; we invite educators to copy and modify this
material at will for the quick preparation of homework, quizzes, and tests. PowerPoint
presentations and test materials prepared by other educators may be posted here as well.
Copyright Permissions for Educational Use
No site license is required for the download, duplication and installation of Parallax
software for educational use with Parallax products on as many school or home
computers as needed. Our Stamps in Class texts and BASIC Stamp Manual are all
available as free PDF downloads, and may be duplicated as long as it is for educational
use exclusively with Parallax microcontroller products and the student is charged no
more than the cost of duplication. The PDF files are not locked, enabling selection of text
and images to prepare handouts, transparencies, or PowerPoint presentations.
Preface · Page 9
FOREIGN TRANSLATIONS
Many of our Stamps in Class texts have been translated into other languages; these texts
are free downloads and subject to the same Copyright Permissions for Educational Use as
our original versions. To see the full list, click on the Tutorials & Translations link at
www.parallax.com/Education. These were prepared in coordination with the Parallax
Volunteer Translator program. If you are interested in participating in our Volunteer
Translator program, email translations@parallax.com.
ABOUT THE AUTHOR
Andy Lindsay joined Parallax Inc. in 1999, and has since authored eleven books and
numerous articles and product documents for the company. The last three versions of
Robotics with the Boe-Bot were designed and updated based on observations and
educator feedback that Andy collected while traveling the nation and abroad teaching
Parallax Educator Courses and events. Andy studied Electrical and Electronic
Engineering at California State University, Sacramento, and is a contributing author to
several papers that address the topic of microcontrollers in pre-engineering curricula.
When he’s not writing educational material, Andy does product and application and
product engineering for Parallax.
SPECIAL CONTRIBUTORS
The Parallax team assembled to prepare this edition includes: excellent department
leadership by Aristides Alvarez, lesson design and technical writing by Andy Lindsay;
cover art by Jen Jacobs; graphic illustrations by Rich Allred and Andy Lindsay;
nitpicking, editing, and layout by Stephanie Lindsay. Special thanks go to Ken Gracey,
founder of the Stamps in Class program, and to Tracy Allen and Phil Pilgrim for
consulting in the selection of the light sensor used in this version to replace the cadmiumsulfide photoresistor. Stephanie is particularly grateful to John Kauffman for his lastminute review of the revised Chapter 6.
Page 10 · Robotics with the Boe-Bot
Your Boe-Bot’s Brain · Page 11
Chapter 1: Your Boe-Bot’s Brain
Parallax, Inc’s Boe-Bot® robot is the focus of the activities, projects, and contests in this
book. The Boe-Bot and a close-up of its BASIC Stamp® 2 programmable microcontroller
brain are shown in Figure 1-1. The BASIC Stamp 2 module is both powerful and easy to
use, especially with a robot.
Figure 1-1
BASIC Stamp
Module on a
Boe-Bot Robot
The activities in this text will guide you through writing simple programs that make the
BASIC Stamp and your Boe-Bot do four essential robotic tasks:
1.
2.
3.
4.
Monitor sensors to detect the world around it
Make decisions based on what it senses
Control its motion (by operating the motors that make its wheels turn)
Exchange information with its Roboticist (that will be you!)
The programming language you will use to accomplish these tasks is called PBASIC, which
stands for:






Parallax - Company that invented and manufactures BASIC Stamp microcontrollers
Beginners - Made for beginners to learn how to program computers
All-purpose - Powerful and useful for solving many different kinds of problems
Symbolic - Using symbols (terms that resemble English word/phrases)
Instruction - To tell a computer what to do
Code - In terms that the computer (and you) can understand
Page 12 · Robotics with the Boe-Bot
What’s a Microcontroller? It’s a programmable device that is designed into your digital
wristwatch, cell phone, calculator, clock radio, etc. In these devices, the microcontroller has
been programmed to sense when you press a button, make electronic beeping noises, and
control the device’s digital display. They are also built into factory machinery, cars,
submarines, and spaceships because they can be programmed to read sensors, make
decisions, and orchestrate devices that control moving parts.
The What’s a Microcontroller? Student Guide is the recommended first text for beginners. It
is full of examples of how to use microcontrollers, and how to make the BASIC Stamp the
brain of your own microcontrolled inventions. It’s available for free download from
www.parallax.com/go/WAM, and it's also included in the BASIC Stamp Editor Help as a PDF
file. It is included in the BASIC Stamp Activity Kit and BASIC Stamp Discovery Kit, which
are carried by many electronic retailers. These kits can also be purchased directly from
Parallax, either online at www.parallax.com/go/WAM or by phone at (888) 512-1024.
HARDWARE AND SOFTWARE
Getting started with BASIC Stamp microcontroller modules is similar to getting started
with a brand-new PC or laptop. The first things that most people have to do is take it out
of the box, plug it in, install and test some software, and maybe even write some software
of their own using a programming language. If this is your first time using a BASIC
Stamp module, you will be doing all these same activities. If you are in a class, your
hardware may already be all set up for you. If this is the case, your teacher may have
other instructions. If not, this chapter will take you through all the steps of getting your
new BASIC Stamp microcontroller up and running.
ACTIVITY #1: GETTING THE SOFTWARE
The BASIC Stamp Editor (version 2.5 or higher) is the software you will use in most of
the activities and projects in this text. You will use this software to write programs that
the BASIC Stamp module will run. You can also use this software to display messages
sent by the BASIC Stamp that help you understand what it senses.
Computer System Requirements
You will need a personal computer to run the BASIC Stamp Editor software. Your
computer will need to have the following features:



Microsoft Windows 2K/XP/Vista/7 or newer operating system
An available serial or USB port
Internet access and an Internet browser program
Your Boe-Bot’s Brain · Page 13
Downloading the Software from the Internet
It is important to always use the latest version of the BASIC Stamp Editor software if
possible. The first step is to go to the Parallax web site and download the software.
 Using a web browser, go to www.parallax.com/basicstampsoftware.
Figure 1-2: BASIC Stamp Editor download page at www.parallax.com/basicstampsoftware
Use the “Click Here to Download” button to get the latest version of the software.
 Click on the Click Here to Download button to download the latest version of the
BASIC Stamp Windows Editor software.
Page 14 · Robotics with the Boe-Bot
 A File Download window will open, asking you if you want to run or to save this
file (Figure 1-3). Click Save.
Figure 1-3
File Download
Window
Click Save, then
save the file to your
computer.
 Follow the prompts that appear. When the download is complete, click Run. You
may see messages from your operating system asking you to verify that you wish
to continue with installation. Always agree that you want to continue.
Figure 1-4
Download Complete
Message
Click Run.
If prompted, always
confirm you want to
continue.
Your Boe-Bot’s Brain · Page 15
 The BASIC Stamp Editor Installer window will open (Figure 1-5). Click Next
and follow the prompts, accepting all defaults.
Figure 1-5
BASIC Stamp Editor
Installer Window
Click Next.
 IMPORTANT: When the “Install USB Driver” message appears (Figure 1-6),
leave the checkmark in place for the Automatically install/update driver
(recommended) box, and then click Next.
Figure 1-6
Install USB Driver
Message
Leave the box
checked, and click
Next.
Page 16 · Robotics with the Boe-Bot
 When the “Ready to Install the Program” message appears, click the Install
button. A progress bar may appear, and this could take a few minutes.
At this point, an additional window may appear behind the current window while the
USB drivers are updating. This window will eventually close on its own when the driver
installation is complete. If you don’t see this window, it does not indicate a problem.
About USB drivers. The USB drivers that install with the BASIC Stamp Windows Editor
installer by default are necessary to use any Parallax hardware connected to your
computer’s USB port. VCP stands for Virtual COM Port, and it will allow your computer’s
USB port to look and be treated as a standard RS232 serial port by Parallax hardware.
USB Drivers for Different Operating Systems The USB VCP drivers included in the
BASIC Stamp Windows Editor software are for certain Windows operating systems only. For
more information, visit www.parallax.com/usbdrivers.
 When the window tells you that installation has been successfully completed,
click Finish (Figure 1-7).
Figure 1-7
BASIC Stamp
Editor Installation
Completed
Click Finish.
Your Boe-Bot’s Brain · Page 17
ACTIVITY #2: USING THE HELP FILE FOR HARDWARE SETUP
In this section you will run the BASIC Stamp Editor’s Help file. Within the Help file, you
will learn about the different BASIC Stamp programming boards available for the Stamps
in Class program, and determine which one you are using. Then, you will follow the steps
in the Help to connect your hardware to your computer and test your BASIC Stamp
programming system.
Running the BASIC Stamp Editor for the first time
 If you see the BASIC Stamp Editor icon on your computer desktop, double-click
it (Figure 1-8).
 Or, click on your computer’s Start menu, then choose All Programs 
Parallax Inc  BASIC Stamp Editor 2.5  BASIC Stamp Editor 2.5.
Figure 1-8
BASIC Stamp Editor
Desktop Icon
Double-click to launch
the program.
 On the BASIC Stamp Editor’s toolbar, click Help on the toolbar (Figure 1-9) and
then select BASIC Stamp Help… from the drop-down menu.
Figure 1-9
Opening the Help Menu
Click Help, then choose
BASIC Stamp Help from
the drop-down menu.
Page 18 · Robotics with the Boe-Bot
Figure 1-10: BASIC Stamp Editor Help
 Click on the Getting Started with Stamps in Class link on the bottom of the
Welcome page, as shown in the lower right corner of Figure 1-10.
Your Boe-Bot’s Brain · Page 19
Following the Directions in the Help File
From here, you will follow the directions in the Help file to complete these tasks:






Identify which BASIC Stamp development board you are using
Connect your development board to your computer
Test your programming connection
Troubleshoot your programming connection, if necessary
Write your first PBASIC program for your BASIC Stamp
Power down your hardware when you are done
When you have completed the activities in the Help file, return to this book and continue
with the Summary below before moving on to Chapter 2.
What do I do if I get stuck?
If you run into problems while following the directions in this book or in the Help file, you
have many options to obtain free Technical Support:




Forums: sign up and post a message in our free, moderated Stamps in Class
forum at forums.parallax.com.
Email: send an email to support@parallax.com.
Telephone: In the Continental United States, call toll-free to 888-99-STAMP
(888-997-8267). All others call (916) 624-8333.
More resources: Visit www.parallax.com/support.
SUMMARY
This chapter guided you through the following:
 An introduction to the BASIC Stamp module
 Where to get the free BASIC Stamp Editor software you will use in just about all
of the experiments in this text
 How to install the BASIC Stamp Editor software
 How to use the BASIC Stamp Editor’s Help and the BASIC Stamp Manual
 An introduction to the BASIC Stamp module, Board of Education, and
HomeWork Board
 How to set up your BASIC Stamp hardware
 How to test your software and hardware
 How to write and run a PBASIC program
 Using the DEBUG and END commands, CR control character, and DEC formatter.
Page 20 · Robotics with the Boe-Bot


A brief introduction to ASCII code
How to disconnect the power to your Board of Education or HomeWork Board
when you’re done
Questions
1. What device will be the brain of your Boe-Bot?
2. When the BASIC Stamp sends a character to your PC/laptop, what type of
numbers are used to send the message through the programming cable?
3. What is the name of the window that displays messages sent from the BASIC
Stamp to your PC/laptop?
4. What PBASIC commands did you learn in this chapter?
Exercises
1. Explain what the asterisk does in this command: DEBUG DEC 7 * 11
2. Guess what the Debug Terminal would display if you ran this command:
DEBUG DEC 7 + 11
3. There is a problem with these two commands. When you run the code, the
numbers they display are stuck together so that it looks like one large number
instead of two small ones. Modify these two commands so that the answers
appear on different lines in the Debug Terminal.
DEBUG DEC 7 * 11
DEBUG DEC 7 + 11
Projects
1. Use DEBUG to display the solution to the math problem: 1 + 2 + 3 + 4.
2. Save FirstProgramYourTurn.bs2 under another name. If you were to place the
DEBUG command shown below on the line just before the END command in the
program, what other lines could you delete and still have it work the same?
Modify the copy of the program to test your hypothesis (your prediction of what
will happen).
DEBUG "What's 7 X 11?", CR, "The answer is: ", DEC 7 * 11
Your Boe-Bot’s Brain · Page 21
Solutions
Q1. A BASIC Stamp 2 microcontroller module.
Q2. Binary numbers, that is, 0’s and 1’s.
Q3. The Debug Terminal.
Q4. DEBUG and END
E1. It multiplies the two operands 7 and 11, resulting in a product of 77. The asterisk
is the multiply operator.
E2. The Debug Terminal would display: 18
E3. To fix the problem, add a carriage return using the CR control character and a
comma.
DEBUG DEC 7 * 11
DEBUG CR, DEC 7 + 11
P1. Here is a program to display a solution to the math problem: 1+2+3+4.
' What's a Microcontroller - Ch01Prj01_Add1234.bs2
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "What's 1+2+3+4?"
DEBUG CR, "The answer is: "
DEBUG DEC 1+2+3+4
END
P2. The last three DEBUG lines can be deleted. An additional CR is needed after the
"Hello" message.
' What's a Microcontroller - Ch01Prj02_ FirstProgramYourTurn.bs2
' BASIC Stamp sends message to Debug Terminal.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Hello, it's me, your BASIC Stamp!", CR
DEBUG "What's 7 X 11?", CR, "The answer is: ", DEC 7 * 11
END
The output from the Debug Terminal is:
Hello, it's me, your BASIC Stamp!
What's 7 X 11?
The answer is: 77
Page 22 · Robotics with the Boe-Bot
This output is the same as it was with the previous code. This is an example of
using commas to output a lot of information, using only one DEBUG command
with multiple elements in it.
Your Boe-Bot’s Servo Motors · Page 23
Chapter 2: Your Boe-Bot’s Servo Motors
This chapter will guide you through connecting, adjusting, and testing the Boe-Bot’s
motors. In order to do that, you will need to understand certain PBASIC commands and
programming techniques that will control the direction, speed, and duration of servo
motions. Therefore, Activities #1, #2, and #5 will introduce you to these programming
tools, and then Activities #3, #4, and #6 will show you how to apply them to the servos.
Since precise servo control is key to the Boe-Bot’s performance, completing these
activities before mounting the servos into the Boe-Bot chassis is both important and
necessary!
INTRODUCING THE CONTINUOUS ROTATION SERVO
The Parallax Continuous Rotation servos shown in Figure 2-1 are the motors that will
make the Boe-Bot’s wheels turn. This figure points out the servos’ external parts. Many
of these parts will be referred to as you go through the instructions in this and the next
chapter.
Figure 2-1 Parallax Continuous Rotation Servo
Control
horn
Mounting
Flange
Access hole
for center
adjusting
feedback
potentiometer
Phillips
screw
Label should
read
“Continuous
Rotation”
Cable
for
power
and
control
signal
Mounting
Flange
Case contains
motor, circuits,
and gears
Plug for RC servo
connection ports on
Board of Education
Note: You might find it useful to bookmark this page so that you can refer back to it later.
Page 24 · Robotics with the Boe-Bot
Standard Servos vs. Continuous Rotation Servos: Standard servos are designed to
receive electronic signals that tell them what position to hold. These servos control the
positions of radio controlled airplane flaps, boat rudders, and car steering. Continuous
rotation servos receive the same electronic signals, but instead of holding certain positions,
they turn at certain speeds and directions. Continuous rotation servos are ideal for
controlling wheels and pulleys.
Servo Control Horn - 4-point Star vs. Round: It doesn’t make a difference. So long as it
is labeled “continuous rotation” it’s the servo for your Boe-Bot. You will be removing the
control horn with a wheel.
ACTIVITY #1: BUILDING AND TESTING THE LED CIRCUIT
Controlling a servo motor’s speed and direction involves a program that makes the
BASIC Stamp send the same message, over and over again. The message has to repeat
itself around 50 times per second for the servo to maintain its speed and direction. This
activity has a few PBASIC example programs that demonstrate how to repeat the same
message over and over again and control the timing of the message.
Displaying Messages at Human Speeds
You can use the PAUSE command to tell the BASIC Stamp to wait for a while before
executing the next command.
PAUSE Duration
The number that you put to the right of the PAUSE command is called the Duration
argument, and it’s the value that tells the BASIC Stamp how long it should wait before
moving on to the next command. The units for the Duration argument are thousandths of
a second (ms). So, if you want to wait for one second, use a value of 1000. Here’s how
the command should look:
PAUSE 1000
If you want to wait for twice as long, try:
PAUSE 2000
Your Boe-Bot’s Servo Motors · Page 25
A second is abbreviated “s.” In this text, when you see 1 s, it means one second.
A millisecond is one thousandth of a second, and it is abbreviated “ms.” The command
PAUSE 1000 delays the program for 1000 ms, which is 1000/1000 of a second, which is
one second, or 1 s. Got it?
Example Program: TimedMessages.bs2
There are lots of different ways to use the PAUSE command. This example program uses
PAUSE to delay between printing messages that tell you how much time has elapsed. The
program should wait one second before it sends the “One second elapsed…” message and
another two seconds before it displays the “Three seconds elapsed . . . ” message.
 If you have a Board of Education, move the 3-postion switch from position-0 to
position-1.
 If you have a HomeWork Board, reconnect the 9 V battery to the battery clip.
 Enter the program below into the BASIC Stamp Editor.
 Save the program under the name TimedMessages.bs2.
 Run the program, and then watch for the delay between messages.
' Robotics with the Boe-Bot - TimedMessages.bs2
' Show how the PAUSE command can be used to display messages at human speeds.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Start timer..."
PAUSE 1000
DEBUG CR, "One second elapsed..."
PAUSE 2000
DEBUG CR, "Three seconds elapsed..."
DEBUG CR, "Done."
END
From here onward, the three instructions that came before this program will be
phrased like this:

Enter, save, and run TimedMessages.bs2.
Page 26 · Robotics with the Boe-Bot
Your Turn – Different Pause Durations
You can change the delay between messages by changing the PAUSE commands’ Duration
arguments.
 Try changing the PAUSE Duration arguments from 1000 and 2000 to 5000 and
10000, for example:
DEBUG "Start timer..."
PAUSE 5000
DEBUG CR, "Five seconds elapsed..."
PAUSE 10000
DEBUG CR, "Fifteen seconds elapsed..."
 Run the modified program.
 Also try it again with numbers like 40 and 100 for the Duration arguments;
they’ll go pretty fast.
 The longest possible Duration argument is 65535. If you've got a minute to
spare, try PAUSE 60000.
Over and Over Again
One of the best things about both computers and microcontrollers is that they never
complain about doing the same boring things over and over again. You can place your
commands between the words DO and LOOP if you want them executed over and over
again. For example, let’s say you want to print a message repeating once every second.
Simply place your DEBUG and PAUSE commands between the words DO and LOOP like
this:
DO
DEBUG "Hello!", CR
PAUSE 1000
LOOP
Example Program: HelloOnceEverySecond.bs2
 Enter, save, and run HelloOnceEverySecond.bs2.
 Verify that the “Hello!” message is printed once every second.
Your Boe-Bot’s Servo Motors · Page 27
' Robotics with the Boe-Bot - HelloOnceEverySecond.bs2
' Display a message once every second.
' {$STAMP BS2}
' {$PBASIC 2.5}
DO
DEBUG "Hello!", CR
PAUSE 1000
LOOP
Your Turn – A Different Message
You can modify your program so that part of it executes once, and another part executes
over an over again.
 Modify the program so that the commands look like this:
DEBUG "Hello!"
DO
DEBUG "!"
PAUSE 1000
LOOP
 Run it and see what happens! Did you anticipate the result?
ACTIVITY #2: TRACKING TIME AND REPEATING ACTIONS WITH A
CIRCUIT
In this activity, you will build circuits that emit light that will allow you to “see” the kind
of signals that are used to control the Boe-Bot’s servo motors.
What’s a Microcontroller? This activity contains selected excerpts from the What’s a
Microcontroller? Student Guide.

Even if you are familiar with this material from What’s a Microcontroller?, don’t
skip this activity.
In the second half of this activity, you will examine the signals that control your servos and
timing diagrams in a different light than they were presented in What’s a Microcontroller?
Bonus! The components in your Boe-Bot kit can be used to complete many of the activities
in What’s a Microcontroller? Go www.paralllax.com/go/WAM for a complete list, and to
download the text.
Page 28 · Robotics with the Boe-Bot
Introducing the LED and Resistor
A resistor is a component that “resists” the flow of electricity. This flow of electricity is
called current. Each resistor has a value that tells how strongly it resists current flow.
This resistance value is called the ohm, and the sign for the ohm is the Greek letter
omega: Ω. The resistor you will be working with in this activity is the 470 Ω resistor
shown in Figure 2-2. The resistor has two wires (called leads and pronounced “leeds”),
one coming out of each end. There is a ceramic case between the two leads, and it’s the
part that resists current flow. Most circuit diagrams that show resistors use the symbol on
the left with the squiggly lines to tell the person building the circuit that he or she must
use a 470 Ω resistor. This is called a schematic symbol. The drawing on the right is a
part drawing used in some beginner level Stamps in Class texts to help you build circuits.
Gold
Silver
or
Blank
470 
Yellow
Violet
Brown
Figure 2-2
470 Ω Resistor Part Drawing
Schematic symbol (left) and
Part Drawing (right)
The colored stripes indicate resistance values. See Appendix B: Resistor Color Codes
and Breadboarding Rules on page 293 for information on how to determine a resistor's
value from the colored stripes on its ceramic case.
A diode is a one-way current valve, and a light-emitting diode (LED) emits light when
current passes through it. Unlike the color codes on a resistor, the color of the LED
usually just tells you what color it will glow when current passes through it. The
important markings on an LED are contained in its shape. Since an LED is a one-way
current valve, you have to make sure to connect it the right way, or it won’t work as
intended.
Figure 2-3 shows an LED’s schematic symbol and part drawing. An LED has two
terminals. One is called the anode, and the other is called the cathode. In this activity,
you will have to build the LED into a circuit, and you will have to pay attention and make
sure the anode and cathode leads are connected to the circuit properly. On the part
drawing, the anode lead is labeled with the plus-sign (+). On the schematic symbol, the
anode is the wide part of the triangle. In this part drawing, the cathode lead is the pin
Your Boe-Bot’s Servo Motors · Page 29
labeled with a minus-sign (-), and on the schematic symbol, the cathode is the line across
the point of the triangle.
Figure 2-3
LED Part Drawing and Schematic
Symbol
Part drawing (above) and schematic
symbol (below)
_
The LED part drawings in later
pictures will have a + next to the
anode leg.
+
LED
When you start building your circuit, make sure to check it against the schematic symbol
and part drawing. If you look closely at the LED’s plastic case in the part drawing, it’s
mostly round, but there is a small flat spot right near one of the leads that that tells you
it’s the cathode. Also note that the LED’s leads are different lengths. In this text, the
anode will be shown with a (+) sign and the cathode will be shown with a (–) sign.
Always check the LED’s plastic case. Usually, the longer lead is connected to the LED’s
anode, and the shorter lead is connected to its cathode. But sometimes the leads have been
clipped to the same length, or a manufacturer does not follow this convention. Therefore, it
is best to always look for the flat spot on the case. If you plug an LED in backwards, it will
not hurt it, but it will not light up.
LED Test Circuit Parts
(2) LEDs – Red
(2) Resistors, 470 Ω (yellow-violet-brown)
Always disconnect power to your board before building or modifying circuits! For the
Board of Education, set the 3-position switch to position-0. For the BASIC Stamp
HomeWork Board, disconnect the 9 V battery from the battery clip. Always double-check
your circuit for errors before reconnecting power.
Page 30 · Robotics with the Boe-Bot
LED Test Circuits
If you completed the What’s a Microcontroller? text, you are no doubt very familiar
with the circuit shown in Figure 2-4. The left side of this figure shows the circuit
schematic, and the right side shows a wiring diagram example of the circuit built on your
board’s prototyping area.
 Build the circuit shown in Figure 2-4.
 Make sure that the shorter pins on each LED (the cathodes) are plugged into
black sockets labeled Vss.
 Make sure the longer pins (the anodes, marked with a  in the wiring diagram)
are connected to the white breadboard sockets exactly as shown.
Vdd
X3
P13
470 
P12
470 
LED
Vss
LED
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vin
Vss
+
+
Figure 2-4
Two LEDs
Connected to
BASIC Stamp
I/O Pins P13
and P12
Schematic
(left) and
wiring
diagram
(right)
What's an I/O pin? I/O stands for input/output. The BASIC Stamp 2 has 24 pins, 16 of
which are I/O pins. In this text, you will program the BASIC Stamp to use I/O pins as
outputs to make LED lights turn on/off, control the speed and direction the Parallax
Continuous Rotation servos turn, make tones with a speaker, and prepare sensors to detect
light and objects. You will also program the BASIC Stamp to use I/O pins as inputs to
monitor sensors that indicate mechanical contact, light level, objects in the Boe-Bot's path,
and even their distance.
New to building circuits? See Appendix B: Resistor Color Codes and Breadboarding
Rules on page 293.
Your Boe-Bot’s Servo Motors · Page 31
Figure 2-5 shows what you will program the BASIC Stamp to do to the LED circuit.
Imagine that you have a 5 volt (5 V) battery. Although a 5 V battery is not common, the
Board of Education has a device called a voltage regulator that supplies the BASIC
Stamp with the equivalent of a 5 V battery. When you connect a circuit to Vss, it’s like
connecting the circuit to the negative terminal of the 5 V battery. When you connect the
other end of the circuit to Vdd, it’s like connecting it to the positive terminal of a 5 V
battery.
Vdd +
-
N
5V
Vss _
+++
+++
+++
--- - -N
-N - N
-
+
+
=
N
Vdd +
-
-
N N
N N N
-
-
-
-
-
-
-
-
5V
Vss _
+++
+++
+++
--- - -N
-N - N
-
Figure 2-5
Current and
Electron Flow
-
Volts is abbreviated V. That means 5 volts is abbreviated 5 V. When you apply voltage to
a circuit, it’s like applying electrical pressure.
Current refers to the rate at which electrons pass through a circuit. You will often see
measurements of current expressed in amps, which is abbreviated A. The amount of
current an electric motor draws is often measured in amps, for example 2 A, 5 A, etc.
However, the currents you will use in the Board of Education are measured in thousandths
of an amp, or milliamps. For example, 10.3 mA passes through the circuit in Figure 2-5.
When these connections are made, 5 V of electrical pressure is applied to the circuit
causing electrons to flow through and the LED to emit light. As soon as you disconnect
the resistor lead from the battery’s positive terminal, the current stops flowing, and the
LED stops emitting light. You can take it one step further by connecting the resistor lead
to Vss, which has the same result. This is the action you will program the BASIC Stamp
to do to make the LED turn on (emit light) and off (not emit light).
Page 32 · Robotics with the Boe-Bot
Programs that Control the LED Test Circuits
The HIGH and LOW commands can be used to make the BASIC Stamp connect an LED
alternately to Vdd and Vss. The Pin argument is a number between 0 and 15 that tells the
BASIC Stamp which I/O pin to connect to Vdd or Vss.
HIGH Pin
LOW Pin
For example, if you use the command:
HIGH 13
...it tells the BASIC Stamp to connect I/O pin P13 to Vdd, which turns the LED on.
Likewise, if you use the command
LOW 13
...it tells the BASIC Stamp to connect I/O pin P13 to Vss, which turns the LED off. Let’s
try this out.
Example Program: HighLowLed.bs2
 Reconnect power to your board.
 Enter, save, and run HighLowLed.bs2.
 Verify that the LED circuit connected to P13 is turning on and off, once every
second.
' Robotics with the Boe-Bot – HighLowLed.bs2
' Turn the LED connected to P13 on/off once every second.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "The LED connected to Pin 13 is blinking!"
DO
HIGH 13
PAUSE 500
LOW 13
PAUSE 500
LOOP
Your Boe-Bot’s Servo Motors · Page 33
How HighLowLed.bs2 Works
Figure 2-6 shows how the BASIC Stamp can connect an LED circuit alternately to Vdd
and Vss. When it’s connected to Vdd, the LED emits light. When it’s connected to Vss,
the LED does not emit light. The command HIGH 13 instructs the BASIC Stamp to
connect P13 to Vdd. The command PAUSE 500 instructs the BASIC Stamp to leave the
circuit in that state for 500 ms. The command LOW 13 instructs the BASIC Stamp to
connect the LED to Vss. Again, the command PAUSE 500 instructs the BASIC Stamp to
leave it in that state for another 500 ms. Since these commands are placed between DO
and LOOP, they execute over and over again.
SOUT
1
SIN
2
ATN
3
VSS
4
P0
5
P1
6
P2
7
P3
8
BS2
Vdd
Vss
24
VIN
23
VSS
SOUT
1
SIN
2
22
21
RES
ATN
3
VDD (+5V)
VSS
4
20
P15
P0
5
19
P14
P1
6
18
P13
P2
7
17
P12
P3
8
BS2
Vdd
Vss
24
VIN
23
VSS
22
RES
21
VDD (+5V)
20
P15
19
P14
18
P13
17
P12
P4
9
16
P11
P4
9
16
P11
P5
10
15
P10
P5
10
15
P10
P6
11
14
P9
P6
11
14
P9
P7
12
13
P8
P7
12
13
P8
BS2-IC
BS2-IC
Figure 2-6
BASIC Stamp
Switching
The BASIC Stamp can
be programmed to
internally connect the
LED circuit’s input to
Vdd or Vss.
A Diagnostic Test for your Computer
A very few computers, such as some laptops, will halt the PBASIC program after the first
time through a DO...LOOP instruction. These computers have a non-standard serial port
design. By placing a DEBUG command the program LedOnOff.bs2, the open Debug
Terminal prevents this from possibly happening. You will next re-run this program
without the DEBUG command to see if your computer has this non-standard serial port
problem. It is not likely, but it would be important for you to know.
 Open HighLowLed.bs2.
 Delete the entire DEBUG instruction.
 Run the modified program while you observe your LED.
If the LED blinks on and off continuously, just as it did when you ran the original
program with the DEBUG command, your computer will not have this problem.
Page 34 · Robotics with the Boe-Bot
If the LED blinked on and off only once and then stopped, you have a computer with a
non-standard serial port design. If you disconnect the programming cable from your
board and press the Reset button, the BASIC Stamp will run the program properly
without freezing. In programs you write yourself, you should add a single command:
DEBUG "Program Running!"
...right after the compiler directives. This will open the Debug Terminal and keep the
COM port open. This will prevent your programs from freezing after one pass through
the DO…LOOP, or any of the other looping commands you will be learning in later
chapters. You will see this command in some of the example programs that would not
otherwise need a DEBUG instruction. So, you should be able to run all of the remaining
programs in this book even if your computer failed the diagnostic test.
Introducing the Timing Diagram
A timing diagram is a graph that relates high (Vdd) and low (Vss) signals to time. In
Figure 2-7, time increases from left to right, and high and low signals align with either
Vdd (5 V) or Vss (0 V). This timing diagram shows you a 1000 ms slice of the high/low
signal you just experimented with. The line of dots (. . .) to the right of the signal is one
way of indicating that the signal repeats itself.
Figure 2-7
Timing Diagram for
HighLowLed.bs2
500 ms
…
Vdd (5 V)
Vss (0 V)
500 ms
1000 ms
The LED on/off states are
shown above the timing
diagram.
Your Boe-Bot’s Servo Motors · Page 35
Your Turn – Blink the Other LED
Blinking the other LED (connected to P12) is a simple matter of changing the Pin
argument in the HIGH and LOW commands and re-running the program.
 Modify the program so that the commands look like this:
DO
HIGH 12
PAUSE 500
LOW 12
PAUSE 500
LOOP
 Run the modified program and verify that it makes the other LED blink on/off.
You can also make both LEDs blink at the same time.
 Modify the program so that the commands look like this:
DO
HIGH 12
HIGH 13
PAUSE 500
LOW 12
LOW 13
PAUSE 500
LOOP
 Run the modified program and verify that it makes both LEDs blink on and off
at roughly the same time.
You can modify the program again to make one LEDs blink alternately on/off, and you
can also change the rates that the LEDs blink by adjusting the PAUSE command’s
Duration argument higher or lower.
 Try it!
Page 36 · Robotics with the Boe-Bot
Viewing a Servo Control Signal with an LED
The high and low signals you will program the BASIC Stamp to send to the servo motors
must last for very precise amounts of time. That’s because the servo motors measure the
amount of time the signal stays high, and use it as an instruction for where to turn. For
accurate servo motor control, the time these signals stay high must be much more precise
than you can get with a HIGH and a PAUSE command. You can only change the PAUSE
command’s Duration argument by 1 ms (remember, that’s 1/1000 of a second) at a time.
There’s a different command called PULSOUT that can deliver high signals for precise
amounts of time. These amounts of time are values you use in the Duration argument,
and they are measured in units that are two millionths of a second!
PULSOUT Pin, Duration
A microsecond is a millionth of a second. It’s abbreviated µs. Be careful when you
write this value, it’s not the letter ‘u’ from our alphabet; it’s the Greek letter mu ‘µ’.
For example, 8 microseconds is abbreviated 8 µs.
You can send a HIGH signal that turns the P13 LED on for 2 µs (that’s two millionths of a
second) by using this command:
PULSOUT 13, 1
This command would turn the LED on for 4 µs:
PULSOUT 13, 2
This command sends a high signal that you can actually view:
PULSOUT 13, 65000
How long does the LED circuit connected to P13 stay on when you send this pulse?
Let’s figure it out. The time it stays on is 65000 times 2 µs. That’s:
Duration  65000  2 s
 65000  0.000002 s
 0 .13 s
...which is still pretty fast, thirteen hundredths of a second.
Your Boe-Bot’s Servo Motors · Page 37
The largest value you can use in a PULSOUT Duration argument is 65535.
Example Program: PulseP13Led.bs2
This timing diagram in Figure 2-8 shows the pulse train you are about to send to the LED
with this new program. This time, the high signal lasts for 0.13 seconds, and the low
signal lasts for 2 seconds. This is 100 times slower than the signal that the servo will
need to control its motion.
0.13 s
0.13 s
Vdd (5 V)
Figure 2-8
Timing Diagram for
PulseP13Led.bs2
Vss (0 V)
2.0 s
 Enter, save, and run PulseP13Led.bs2.
 Verify that the LED circuit connected to P13 pulses for about thirteen
hundredths of a second, once every two seconds.
' Robotics with the Boe-Bot – PulseP13Led.bs2
' Send a 0.13 second pulse to the LED circuit connected to P13 every 2 s.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
DO
PULSOUT 13, 65000
PAUSE 2000
LOOP
Page 38 · Robotics with the Boe-Bot
Example Program: PulseBothLeds.bs2
This example program sends a pulse to the LED connected to P13, and then it sends a
pulse to the LED connected to P12 as shown in Figure 2-9. After that, it pauses for two
seconds.
0.13 s
0.13 s
P13
Figure 2-9
Timing Diagram for
PulseBothLeds.bs2
0.13 s
0.13 s
P12
The LEDs emit light
for 0.13 second
while the signal is
high.
2.26 s
The voltages (Vdd and Vss) in this timing diagram are not labeled. With the BASIC
Stamp, it is understood that the high signal is 5 V (Vdd) and the low signal is 0 V (Vss).
This is a common practice in documents that explain the timing of high and low signals.
Often there are one or more of these documents for each component inside the circuit an
engineer is designing. The engineers who created the BASIC Stamp had to comb through
many of these kinds of documents looking for information needed to help make decisions
while designing the product.
Sometimes the times are also left out, or just shown with a label, like thigh and tlow. Then, the
desired time values for thigh and tlow are listed in a table somewhere after the timing diagram.
This concept is discussed in more detail in Basic Analog and Digital, another Parallax
Stamps in Class Student Guide.
 Enter, save, and run PulseBothLeds.bs2.
 Verify that both LED circuits simultaneously pulse for about thirteen hundredths
of a second, once every two seconds.
Your Boe-Bot’s Servo Motors · Page 39
' Robotics with the Boe-Bot – PulseBothLeds.bs2
' Send a 0.13 second pulse to P13 and P12 every 2 seconds.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
DO
PULSOUT 13, 65000
PULSOUT 12, 65000
PAUSE 2000
LOOP
Your Turn – Viewing the Full Speed Servo Signal
Remember the servo signal is 100 times as fast as the program you just ran. First, let’s
try running the program ten times as fast. That means divide all the Duration arguments
(PULSOUT and PAUSE) by 10.
 Modify the program so that the commands look like this:
DO
PULSOUT 13, 6500
PULSOUT 12, 6500
PAUSE 200
LOOP
 Run it and verify that it makes the LEDs blink ten times as fast.
Now, let’s try 100 times as fast (one hundredth of the duration). Instead of appearing to
flicker, the LED will just appear to be not as bright as it would when you send it a simple
high signal. That’s because the LED is flashing on and off so quickly and for such brief
periods of time that the human eye cannot detect the actual on/off flicker, just a change in
brightness.
 Modify the program so that the commands look like this:
DO
PULSOUT 13, 650
PULSOUT 12, 650
PAUSE 20
LOOP
Page 40 · Robotics with the Boe-Bot
 Run the modified program and verify that it makes both LEDs about the same
brightness.
 Try substituting 850 in the Duration argument for the P13 PULSOUT command.
DO
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
LOOP
 Run the modified program and verify that the P13 LED now appears slightly
brighter than the P12 LED. You may have to cup your hands around the LEDs
and peek inside to see the difference. They differ because the amount of time the
P13 LED stays on is longer than the amount of time the P12 LED stays on.
 Try substituting 750 in the Duration argument for both the PULSOUT commands.
DO
PULSOUT 13, 750
PULSOUT 12, 750
PAUSE 20
LOOP
 Run the modified program and verify that the brightness of both LEDs is the
same again. It may not be obvious, but the brightness level is between those
given by Duration arguments of 650 and 850.
ACTIVITY #3: CONNECTING THE SERVO MOTORS
In this activity, you will build a circuit that connects the servo to a power supply and a
BASIC Stamp I/O pin. The LED circuits you developed in the last activity will be used
later to monitor the signals the BASIC Stamp sends to the servos to control their motion.
Parts for Connecting the Servos
(2) Parallax Continuous Rotation servos
(2) Built and tested LED circuits from the previous activity
Your Boe-Bot’s Servo Motors · Page 41
Finding the Connection Instructions for Your Carrier Board
There are different revisions of the Board of Education and BASIC Stamp HomeWork
Board. Furthermore, there are several variations to the Board of Education, based on
programming interface. In Chapter 1, you used the BASIC Stamp Editor Help file to
determine the type and revision of your board, and special instructions for older boards.
The instructions in this book were written to support the boards that were current at the
time of writing, and previous compatible revisions:



Board of Education Serial - Rev C or higher
Board of Education USB - Rev A or higher
BASIC Stamp HomeWork Board Serial - Rev C or higher
 Examine the labeling on your carrier board and make note of the type and the
revision.
 For older boards, check the BASIC Stamp Editor Help for notes specific to your
board.
(916) 624-8333
www.parallaxinc.com
www.stampsinclass.com
Vdd
Vin
Rev B
P3
P2
P1
P0
X2
Board of Education
Vss
Rev C
X3
© 2000-2003
Rev A
15 14
13 12
STA
in C MPS
LASS
Red
Black
X4
Vdd
X3
X5
Vin
Rev B
Vss
VR1
X3
Vdd
nc
Figure 2-10
BASIC Stamp Switching
The BASIC Stamp can be
programmed to internally
connect the LED circuit’s
input to Vdd or Vss.
Vss
X2
5
 If your board is one of the type and revisions listed above, go to one of the
following pages to continue:


Board of Education: go to page 42.
HomeWork Board: go to page 45.
Page 42 · Robotics with the Boe-Bot
Connecting the Servos to the Board of Education
 Turn off the power by setting the 3-position switch on your Board of Education
to position-0 (see Figure 2-11).
Reset
0
1
Figure 2-11
Turn Off Power
2
Figure 2-12 shows the servo header on the Board of Education. This board features a
jumper that you can use to connect the servo’s power supply to either Vin or Vdd. To
move it, you have to pull it upwards and off the pair of pins it rests on, then push it onto
the pair of pins you want it to rest on.
 If you are using the 6 V battery pack, make sure the jumper between the servo
ports on the Board of Education is set to Vin as shown on the left of Figure 2-12.
About Rechargeable Batteries. The Boe-Bot requires 6 V, easily obtained from 4 AA 1.5 V
batteries. Alkaline AA batteries are 1.5 V. However, many rechargeable AA batteries supply
only 1.2 V, giving a total of 4.8 V, which is not enough to power the BASIC Stamp and BoeBot. If you cannot find 1.5 V rechargeable batteries, you may use the inexpensive Boe-Boost
th
(#30078) to add a 5 1.2 V rechargeable battery, bringing the total back to 6 V.
 If you are using a 7.5 V, 1000 mA center positive DC supply, set the jumper to
Vdd as shown on the right side of Figure 2-12.
CAUTION – Misuse of AC powered DC supplies can damage your servos.
If you are inexperienced with DC supplies, consider sticking with the 6 V battery pack that
comes with the Boe-Bot.
Use only supplies with DC output voltage ratings between 6 and 7.5 V, and current output
ratings of 800 mA or more.
Only use a DC supply that is equipped with the same kind of plug as the Boe-Bot battery
pack (2.1 mm, center-positive).
Your Boe-Bot’s Servo Motors · Page 43
15 14 Vdd 13 12
Red
Black
X4
X5
Vin
Select Vdd if you are using a
DC supply that plugs into an
AC outlet (AC adapter).
Figure 2-12
Selecting Your Servo Ports’
Power Supply on the Board
of Education
15 14 Vdd 13 12
Red
Black
Select Vin if you are using
the battery pack that comes
with the Boe-Bot kits.
X4
X5
Vin
All examples and instructions in this book will use the battery pack. Figure 2-13 shows
the schematic of the circuit you will build on the Board of Education. The jumper is set
to Vin.
 Connect your servos to your Board of Education as shown in Figure 2-13.
Vin
White
Red
Black
P13
White
Red
Black
Vss
15 14 Vdd 13 12
Vin
White
Red
Black
P12
White
Red
Black
Red
Black
X4
Figure 2-13
Servo
Connections for
the Board of
Education
X5
Vss
How do I tell which servo is connected to P13 and which servo is connected to P12?
You just plugged your servos into headers with numbers above them. If the number above
the header where the servo is plugged in is 13, it means the servo is connected to P13. If
the number is 12, it means it’s connected to P12.
 When you are done assembling the system, it should resemble Figure 2-14
(LED circuits not shown).
Page 44 · Robotics with the Boe-Bot
Figure 2-14
Board of Education with Servos and
Battery Pack Connected
 If you removed the LED circuits after Activity #2, make sure to rebuild them as
shown in Figure 2-15. They will be your servo signal monitoring circuits.
Vdd
X3
P13
470 
P12
470 
LED
Vss
LED
Vss
Vin
Vss
+
+
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 2-15
LED Servo Signal
Monitor Circuit
Disconnecting Power for the Board of Education
Never leave the power connected to your system when you are not working on it.

To disconnect power from your Board of Education, move the 3-position switch to
position-0.
 Move on to Activity #4: Centering the Servos on page 49.
Your Boe-Bot’s Servo Motors · Page 45
Connecting the Servos to the BASIC Stamp HomeWork Board
If you are connecting your servos to a BASIC Stamp HomeWork Board, you will need
the parts listed below and shown in Figure 2-16:
Parts List:
(1) Battery pack with tinned leads (not included, see Appendix A)
(2) Parallax Continuous Rotation Servos
(2) 3-pin male-male headers (not included, see Appendix A)
(4) Jumper wires
(4) AA batteries – 1.5 V alkaline
(2) Built and tested LED circuits from the previous activity
Figure 2-16
Servo Centering Parts for the
HomeWork Board
Figure 2-17 shows a schematic of the servo circuits on the HomeWork Board. Before
you start building this circuit, make sure that power is disconnected from the BASIC
Stamp HomeWork Board.
 The 9 V battery should be disconnected from the battery clip, and the battery
pack with tinned leads should not have any batteries loaded.
Page 46 · Robotics with the Boe-Bot
Vbp
White
Red
Black
P13
Figure 2-17
Servo Connection Schematic for the
BASIC Stamp HomeWork Board
Vss
Vbp
White
Red
Black
P12
Note: Vbp stands for Voltage Battery
Pack. See the i-box below.
Vss
 Remove the two LED/resistor circuits, and save the parts.
 Build the servo ports shown on the left side of Figure 2-18.
 Double-check to make sure the black wire with the white stripe is connected to
Vbp, and the solid black wire should be connected to Vss.
 Double-check to make sure that all the connections for P13, Vbp, Vss, Vbp
(another one), and P12 all exactly match the wiring diagram.
 Connect the servo plugs to the male headers as shown in Figure 2-18, on the
right side of the figure.
 Double-check to make sure the servo wire colors match the legend in the figure.
Vbp stands for Voltage battery pack. It refers to the 6 VDC supplied by the four 1.5 V
batteries. This is brought directly to the breadboard to power the servos for Boe-Bots built
with the HomeWork Board. Your BASIC Stamp is still powered by the 9 V battery.
About Rechargeable Batteries. The Boe-Bot requires 6 V, easily obtained from 4 AA 1.5 V
batteries. Alkaline AA batteries are 1.5 V. However, many rechargeable AA batteries supply
only 1.2 V, giving a total of 4.8 V, which is not enough to power the BASIC Stamp and BoeBot. If you cannot find 1.5 V rechargeable batteries, you may use the inexpensive Boe-Boost
th
(#30078) to add a 5 1.2 V rechargeable battery, bringing the total back to 6 V.
Your Boe-Bot’s Servo Motors · Page 47
Figure 2-18: Servo Connection Wiring Diagram for the BASIC Stamp HomeWork Board
Black wire with
white stripe
Solid Black
Wire
(916) 624-8333
www.parallaxinc.com
www.stampsinclass.com
Vdd
Vin
(916) 624-8333
www.parallaxinc.com
www.stampsinclass.com
Rev B
Vdd
Vss
Vss
X3
X3
P15
P14
P13
P12
P11
P10
P9
P8
Vin
Rev B





P13
Vbp
Vss
Vbp
P12
Port connections
P15
P14
P13
P12
P11
P10
P9
P8





White
Red
Black
Red
White
Servo connections by wire color
Your setup will then resemble Figure 2-19.
Figure 2-19
Dual Supplies and Servos Connected
Page 48 · Robotics with the Boe-Bot
 Rebuild the LED circuit as shown in Figure 2-20.
(916) 624-8333
www.parallaxinc.com
www.stampsinclass.com
Vdd
X3
P13
470 
P12
470 
LED
Vss
LED
Vss
Vin
+
Rev B
Vss
Vss
+
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure 2-20
LED Servo
Signal
Monitor
Circuit
© 2002
HomeWork Board
 When all your connections are made and double-checked, load the battery pack
with batteries and reconnect the 9 V battery to the HomeWork Board’s battery
clip.
Disconnecting Power for the HomeWork Board
Never leave the power connected to your system when you are not working with it. From
here onward, disconnecting power takes two steps:


Unplug the 9 V battery from the battery clip to disconnect power from the
HomeWork Board. This disconnects power from the embedded BASIC Stamp,
and the power sockets above the breadboard (Vdd, Vin, and Vss).
Remove one battery from the battery pack. This disconnects power from the
servos.
 Move on to Activity #4: Centering the Servos.
Your Boe-Bot’s Servo Motors · Page 49
ACTIVITY #4: CENTERING THE SERVOS
In this activity, you will run a program that sends the servos a signal, instructing them to
stay still. Because the servos are not pre-adjusted at the factory, they will instead start
turning. You will then use a screwdriver to adjust them so that they stay still. This is
called centering the servos. After the adjustment, you will test the servos to make sure
they are functioning properly. The test programs will send signals that make the servos
turn clockwise and counterclockwise at various speeds.
Servo Tools and Parts
The Parallax screwdriver shown in Figure 2-21 is the only extra tool you will need for
this activity. If needed, any Phillips #1 point screwdriver with a 1/8″ (3.18 mm) shaft
should do the trick.
Figure 2-21
Parallax Screwdriver
Sending the Center Signal
Figure 2-22 shows the signal that has to be sent to the servo connected to P12 to calibrate
it. This is called the center signal, and after the servo has been properly adjusted, this
signal instructs it to stay still. The instruction consists of a series of 1.5 ms pulses with 20
ms pauses between each pulse.
1.5 ms
1.5 ms
P12
20 ms
Figure 2-22
Timing Diagram for
CenterServoP12.bs2
The 1.5 ms pulses instruct
the servo to remain still.
The program for this signal will be a PULSOUT command and a PAUSE command inside a
DO…LOOP. Figuring out the PAUSE command from the timing diagram is easy; it's going
to be PAUSE 20 for the 20 ms between pulses.
Figuring out the PULSOUT command's Pin argument isn't that hard either; it's going to be
12, for I/O pin P12. Next, let's figure out what the PULSOUT command's Duration
Page 50 · Robotics with the Boe-Bot
argument has to be for 1.5 ms pulses. 1.5 ms is 1.5 thousandths of a second, or 0.0015 s.
Remember whatever number is in the PULSOUT command's Duration argument, multiply
that number by 2 μs (2 millionths of a second = 0.000002 s), and you will know how long
the pulse will last. You can also figure out what the PULSOUT command's Duration
argument has to be if you know how long you want the pulse to last. Just divide 2 μs into
the time you want the pulse to last. With this calculation:
Duration argument 
Pulse duration 0 .0015 s
 750
2 s
0.000002 s
...we now know that the command for a 1.5 ms pulse to P12 will be PULSOUT 12, 750.
It’s best to only center one servo at a time, because that way you can hear when the motor
stops as you are adjusting it. This program will only send the center signal to the servo
connected to P12, and these next instructions will guide you through adjusting it. After
you complete the process with the P12 servo, you will repeat it with the servo connected
to P13.
 If you have a Board of Education, make sure to set the 3-position power switch
to position-2 as shown in Figure 2-23.
0 1 2
Figure 2-23
Set the 3-Position Switch
to Position-2
 If you are using the HomeWork Board, check the power connections to both
your BASIC Stamp and your servos. The 9 V battery should be attached to the
battery clip, and the 6 V battery pack should have all four batteries loaded.
If the servos start running (or twitching) as soon as you connect power:
It's probably because the BASIC Stamp is running a program you ran in a previous activity.

Make sure to enter, save, and run CenterServoP12.bs2 before continuing to the
servo centering instructions that follow the example program.
 Enter, save, and run CenterServoP12.bs2, then continue with the instructions that
follow the program.
Your Boe-Bot’s Servo Motors · Page 51
Example Program: CenterServoP12.bs2
' Robotics with the Boe-Bot - CenterServoP12.bs2
' This program sends 1.5 ms pulses to the servo connected to
' P12 for manual centering.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
DO
PULSOUT 12, 750
PAUSE 20
LOOP
If the servo has not yet been centered, its horn will start turning, and you will be able to
hear the motor inside making a whining noise.
 If the servo is not yet centered, use a screwdriver to gently adjust the
potentiometer in the servo as shown in Figure 2-24. Adjust the potentiometer
until you find the setting that makes the servo stop turning.
Caution: do not push too hard with the screwdriver! The potentiometer inside the servo
is pretty delicate, so be careful not to apply any more pressure than necessary.
Figure 2-24
Center Adjusting
a Servo
1) Insert tip of Phillips screwdriver into
potentiometer access hole.
2) Gently turn screwdriver to
adjust potentiometer until the
servo stops moving.
 Verify that the LED signal monitor circuit connected to P12 is showing activity.
It should be emitting light, indicating that the pulses are being transmitted to the
servo connected to P12.
Page 52 · Robotics with the Boe-Bot
If the servo has already been centered, it will not turn. It is unlikely, but a damaged or
defective servo would also not turn. Activity #6 will rule out this possibility before the
servos are installed on your Boe-Bot chassis.
 If the servo does not turn, go to the Your Turn section so that you can test and
center the other servo that’s connected to P13.
What's a Potentiometer? A potentiometer is kind of like an adjustable resistor. The
resistance of a potentiometer is adjusted with a moving part. On some potentiometers, this
moving part is a knob or a sliding bar, others have sockets that can be adjusted with
screwdrivers. The resistance of the potentiometer inside the Parallax Continuous Rotation
servo is adjusted with a #1 point Phillips screwdriver tip. You can learn more about
potentiometers in What's a Microcontroller? and Basic Analog and Digital student guides.
Your Turn – Centering the Servo Connected to P13
 Repeat the process for the servo connected to P13 using this program:
Example Program: CenterServoP13.bs2
' Robotics with the Boe-Bot - CenterServoP13.bs2
' This program sends 1.5 ms pulses to the servo connected to
' P13 for manual centering.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
DO
PULSOUT 13, 750
PAUSE 20
LOOP
Remember to completely disconnect power when you are done.
If you have a Board of Education:

Move the 3-position switch to position-0.
If you have a BASIC Stamp HomeWork Board:


Unplug the 9 V battery from the battery clip to disconnect power to the HomeWork
Board, and:
Remove one battery from the battery pack.
Your Boe-Bot’s Servo Motors · Page 53
ACTIVITY #5: HOW TO STORE VALUES AND COUNT
This activity introduces variables, which are used in PBASIC programs to store values.
Boe-Bot programs later in this book will rely heavily on variables. The most important
thing about being able to store values is that the program can use them to count. As soon
as your program can count, it can both control and keep track of the number of times
something happens.
Your servos do not need to be connected to power for this activity.


If you have a Board of Education, set the 3-position switch to position-1. The
BASIC Stamp, Vdd, Vin, and Vss will all be connected to power, but there will be
no power connected to the servo ports
If you have a BASIC Stamp HomeWork Board, connect the 9 V battery to the
battery clip to power the BASIC Stamp, Vdd, Vin, and Vss. Just leave one battery
out of the battery pack to keep power disconnected from the servos.
Using Variables for Storing Values, Math Operations, and Counting
Variables can be used to store values. Before you can use a variable in PBASIC, you
have to give it a name and specify its size. This is called declaring a variable.
variableName VAR Size
You can declare four different sizes of variables in PBASIC:
Size
Bit
Nib
Byte
Word
–
–
–
–
–
Stores
0 to 1
0 to 15
0 to 255
0 to 65535, or -32768 to + 32767
The next example program just involves a couple of word variables:
value
anotherValue
VAR
VAR
Word
Word
After you have declared a variable, you can also initialize it, which means giving it a
starting, or initial, value.
value = 500
anotherValue = 2000
Page 54 · Robotics with the Boe-Bot
Default Value - If you do not initialize a variable, the program will automatically start by
storing the number zero in that variable. That’s called the variable's default value.
The “=” sign in value = 500 is an example of an operator. You can use other operators
to do math with variables. Here are a couple of multiplication examples:
value = 10 * value
anotherValue = 2 * value
Example Program: VariablesAndSimpleMath.bs2
This program demonstrates how to declare, initialize, and perform operations on
variables.
 Before running the program, predict what each DEBUG command will display.
 Enter, save, and run VariablesAndSimpleMath.bs2.
 Compare the results to your predictions and explain any differences.
'
'
'
'
Robotics with the Boe-Bot - VariablesAndSimpleMath.bs2
Declare variables and use them to solve a few arithmetic problems.
{$STAMP BS2}
{$PBASIC 2.5}
value
anotherValue
VAR
VAR
Word
Word
' Declare variables
value = 500
anotherValue = 2000
' Initialize variables
DEBUG ? value
DEBUG ? anotherValue
' Display values
value = 10 * anotherValue
' Perform operations
DEBUG ? value
DEBUG ? anotherValue
' Display values again
END
How VariablesAndSimpleMath.bs2 Works
This code declares two word variables, value and anotherValue.
value
anotherValue
VAR
VAR
Word
Word
' Declare variables
Your Boe-Bot’s Servo Motors · Page 55
These commands are examples of initializing variables to values that you determine.
After these two commands are executed, value will store 500, and anotherValue will
store 2000.
value = 500
anotherValue = 2000
' Initialize variables
These DEBUG commands help you see what each variable stores after you initialize them.
Since value was assigned 500 and anotherValue was assigned 2000, these DEBUG
commands send the messages “value = 500” and “anotherValue = 2000” to the Debug
Terminal.
DEBUG ? value
DEBUG ? anotherValue
' Display values
The DEBUG command’s “?” formatter can be used before a variable to make the Debug
Terminal display its name, the decimal value it’s storing, and a carriage return. It’s very
handy for looking at the contents of a variable.
The riddle in the next three lines is “What will be displayed?” The answer is that value
will be set equal to ten times anotherValue. Since anotherValue is 2000, value will
be set equal to 20,000. The anotherValue variable is unchanged.
value = 10 * anotherValue
' Perform operations
DEBUG ? value
DEBUG ? anotherValue
' Display values again
Your Turn – Calculations with Negative Numbers
If you want to do calculations that involve negative numbers, you can use the DEBUG
command’s SDEC formatter to display them. Here’s an example that can be made by
modifying VariablesAndSimpleMath.bs2.
 Delete this portion of VariablesAndSimpleMath.bs2:
value = 10 * anotherValue
' Perform operations
DEBUG ? value
' Display values again
Page 56 · Robotics with the Boe-Bot
 Replace it with the following:
value = value - anotherValue
' Answer = -1500
DEBUG "value = ", SDEC value, CR ' Display values again
 Run the modified program and verify that value changes from 500 to -1500.
Counting and Controlling Repetitions
The most convenient way to control the number of times a piece of code is executed is
with a FOR…NEXT loop. Here is the syntax:
FOR Counter = StartValue TO EndValue {STEP StepValue}…NEXT
The three dots “...” indicate that you can put one or more commands between the FOR and
NEXT statements. Make sure to declare a variable for use in the Counter argument. The
StartValue and EndValue arguments can be numbers or variables (or even an expression).
When you see something between curly braces { } in a syntax description, it means it’s
an optional argument. In other words, the FOR…NEXT loop will work without it, but you
can use it for a special purpose.
You don’t have to name the variable “counter.”
“myCounter.”
myCounter
VAR
For example, you can call it
Word
Here’s an example of a FOR…NEXT loop that uses the myCounter variable for counting. It
also displays the value of the myCounter variable each time through the loop.
FOR myCounter = 1 TO 10
DEBUG ? myCounter
PAUSE 500
NEXT
Example Program: CountToTen.bs2
 Enter, save, and run CountToTen.bs2.
Your Boe-Bot’s Servo Motors · Page 57
' Robotics with the Boe-Bot – CountToTen.bs2
' Use a variable in a FOR...NEXT loop.
' {$STAMP BS2}
' {$PBASIC 2.5}
myCounter
VAR
Word
FOR myCounter = 1 TO 10
DEBUG ? myCounter
PAUSE 500
NEXT
DEBUG CR, "All done!"
END
Your Turn – Different Start and End Values and Counting in Steps
You can use different values for the StartValue and EndValue arguments.
 Modify the FOR…NEXT loop so it looks like this:
FOR myCounter = 21 TO 9
DEBUG ? myCounter
PAUSE 500
NEXT
 Run the modified program. Did you notice that the BASIC Stamp counted down
instead of up? It will do this whenever the StartValue argument is larger than the
EndValue argument.
Remember the optional {STEP StepValue} argument? You can use it to make myCounter
count in steps. Instead of 9, 10, 11…, you can make it count by twos (9, 11, 13…) or by
fives (10, 15, 20…), or whatever StepValue you give it, forwards or backwards. Here’s
an example that uses it to count down in steps of 3:
 Add STEP 3 to the FOR…NEXT loop so it looks like this:
FOR myCounter = 21 TO 9 STEP 3
DEBUG ? myCounter
PAUSE 500
NEXT
 Run the modified program and verify that it counts backwards in steps of 3.
Page 58 · Robotics with the Boe-Bot
ACTIVITY #6: TESTING THE SERVOS
There’s one last thing to do before assembling your Boe-Bot, and that’s testing the
servos. In this activity, you will run programs that make the servos turn at different
speeds and directions. By doing this, you will verify that your servos are working
properly before you assemble your Boe-Bot. This is an example of subsystem testing.
Subsystem testing is a worthwhile habit to develop, because it isn’t any fun to take a
robot back apart just to fix a problem that you could have otherwise caught before putting
it together!
Subsystem testing is the practice of testing the individual components before they go into
the larger device. It’s a valuable strategy that can help you win robotics contests. It’s also
an essential skill used by engineers worldwide to develop everything from toys, cars, and
video games to space shuttles and Mars roving robots. Especially in more complex devices,
it can become nearly impossible to figure out a problem if the individual components haven’t
been tested beforehand. In aerospace projects, for example, disassembling a prototype to
fix a problem can cost hundreds of thousands, or even millions of dollars. In those kinds of
projects, subsystem testing is rigorous and thorough.
Pulse Width Controls Speed and Direction
Recall from centering the servos that a signal with a pulse width of 1.5 ms caused the
servos to stay still. This was done using a PULSOUT command with a Duration of 750.
What would happen if the signal’s pulse width is not 1.5 ms?
In the Your Turn section of Activity #2, you programmed the BASIC Stamp to send
series of 1.3 ms pulses to an LED. Let’s take a closer look at that series of pulses and
find out how it can be used to control a servo. Figure 2-25 shows how a Parallax
Continuous Rotation servo turns full speed clockwise when you send it 1.3 ms pulses.
Full speed ranges from 50 to 60 RPM.
1.3 ms
1.3 ms
Vdd (5 V)
standard servo
www.parallax.com
Vss (0 V)
20 ms
Figure 2-25
A 1.3 ms Pulse Train
Turns the Servo Full
Speed Clockwise
Your Boe-Bot’s Servo Motors · Page 59
What’s RPM? Revolutions Per Minute. It’s the number of full circles something turns in a
minute.
What’s a pulse train? Just as a railroad train is a series of cars, a pulse train is a series of
pulses.
ServoP13Clockwise.bs2 sends this pulse train to the servo connected to P13.
 Enter, save, and run ServoP13Clockwise.bs2.
 Verify that the servo’s horn is rotating between 50 and 60 RPM clockwise.
' Robotics with the Boe-Bot – ServoP13Clockwise.bs2
' Run the servo connected to P13 at full speed clockwise.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
DO
PULSOUT 13, 650
PAUSE 20
LOOP
Notice that a 1.3 ms pulse requires a PULSOUT command Duration argument of 650, which
is less than 750. All pulse widths less than 1.5 ms, and therefore PULSOUT Duration
arguments less than 750, will cause the servo to rotate clockwise.
Example Program: ServoP12Clockwise.bs2
By changing the PULSOUT command’s Pin argument from 13 to 12, you can make the
servo connected to P12 turn full speed clockwise.
 Save ServoP13Clockwise.bs2 as ServoP12Clockwise.bs2.
 Modify the program by updating the comments and the PULSOUT command’s Pin
argument to 12.
 Run the program and verify that the servo connected to P12 is now rotating
between 50 and 60 RPM clockwise.
'
'
'
'
Robotics with the Boe-Bot – ServoP12Clockwise.bs2
Run the servo connected to P12 at full speed clockwise.
{$STAMP BS2}
{$PBASIC 2.5}
Page 60 · Robotics with the Boe-Bot
DEBUG "Program Running!"
DO
PULSOUT 12, 650
PAUSE 20
LOOP
Example Program: ServoP12Counterclockwise.bs2
You probably guessed that making the PULSOUT command’s Duration argument greater
than 750 causes the servo to rotate counterclockwise. A Duration of 850 will send 1.7 ms
pulses. This will make the servo turn full speed counterclockwise as shown in Figure
2-26.
1.7 ms
1.7 ms
Vdd (5 V)
standard servo
www.parallax.com
Vss (0 V)
Figure 2-26
A 1.7 ms Pulse
Train Makes the
Servo Turn Full
Speed
Counterclockwise
20 ms
 Save ServoP12Clockwise.bs2 as ServoP12Counterclockwise.bs2.
 Modify the program by changing the PULSOUT command’s Duration argument
from 650 to 850.
 Run the program and verify that the servo connected to P12 is now rotating
between 50 and 60 RPM counterclockwise.
' Robotics with the Boe-Bot – ServoP12Counterclockwise.bs2
' Run the servo connected to P12 at full speed counterclockwise.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
DO
PULSOUT 12, 850
PAUSE 20
LOOP
Your Boe-Bot’s Servo Motors · Page 61
Pulse Width Modulation. A voltage that spends certain amounts of time in two different
states can be considered as a series of resting states and a pulses. Here is a list of different
pulse signals that control your servo speed and direction:

Figure 2-22 on page 49: 1.5 ms high makes the servo hold still.

Figure 2-25 on page 58: 1.3 ms high makes the servo turn clockwise.

Figure 2-26 on page 60: 1.7 ms high makes the servo turn counterclockwise.
These signals spend brief amounts of time at high levels (pulses) that are separated by low
signals (resting states). A program can adjust the pulse duration, which is the amount of
time that the signal is high. This duration is commonly called pulse width because the
amount of time the signal is high looks wider or narrower in a timing diagram or on a device
like an oscilloscope that plots voltage against time.
Modulation is the process of adjusting a property of a signal that is being transmitted to
make it convey certain information. With a servo, the property that is modulated is the pulse
width, the amount of time the signal is high. The information it conveys is servo speed and
direction.
The servo control signals are examples of positive pulses, with low resting states and high
active states. Negative pulses would be the inverted version with high resting states and
low active states.
Your Turn – P13Clockwise.bs2
 Modify the PULSOUT command’s Pin argument so that it makes the servo
connected to P13 turn counterclockwise.
Example Program: ServosP13CcwP12Cw.bs2
You can use two PULSOUT commands to make both servos turn at the same time. You
can also make them turn in opposite directions.
 Enter, save, and run ServosP13CcwP12Cw.bs2.
 Verify that the servo connected to P13 is turning full speed counterclockwise
while the one connected to P12 is turning full speed clockwise.
Page 62 · Robotics with the Boe-Bot
' Robotics with the Boe-Bot - ServosP13CcwP12Cw.bs2
' Run the servo connected to P13 at full speed counterclockwise
' and the servo connected to P12 at full speed clockwise.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
DO
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
LOOP
This will be important soon. Think about it: when the servos are mounted on either side
of the chassis, one will have to rotate clockwise while the other rotates counterclockwise
to make the Boe-Bot roll in a straight line. Does that seem odd? If you can’t picture it,
try this:
 Hold your servos together back-to-back and re-run the program.
Your Turn – Adjusting the Speed and Direction
There are four different combinations of PULSOUT Duration arguments that will be used
repeatedly when programming your Boe-Bot’s motion in the upcoming chapters.
ServosP13CcwP12Cw.bs2 sends one of these combinations, 850 to P13 and 650 to P12.
By testing several possible combinations and filling in the Description column of Table
2-1, you will become familiar with them and build a reference for yourself. You will fill
in the Behavior column after your Boe-Bot is fully assembled, when you can see how
each combination makes it move.
 Try the following PULSOUT Duration combinations, and fill in the Description
column with your results.
Your Boe-Bot’s Servo Motors · Page 63
Table 2-1: PULSOUT Duration Combinations
Durations
P13
P12
850
650
650
850
850
850
650
650
750
850
650
750
750
750
760
740
770
730
850
700
800
650
Description
Full speed, P13 servo counterclockwise, P12 servo clockwise.
Both servos should stay still because
of the centering adjustments made in
Activity #4.
Behavior
Page 64 · Robotics with the Boe-Bot
FOR…NEXT to Control Servo Run Time
Hopefully, by now you fully understand that pulse width controls the speed and direction
of a Parallax Continuous Rotation servo. It’s a pretty simple way to control motor speed
and direction. There is also a simple way to control the amount of time a motor runs, and
that’s with a FOR…NEXT loop.
Here is an example of a FOR…NEXT loop that will make the servo turn for a few seconds:
FOR counter = 1 TO 100
PULSOUT 13, 850
PAUSE 20
NEXT
Let’s figure out the exact length of time this code would cause the servo to turn. Each
time through the loop, the PULSOUT command lasts for 1.7 ms, the PAUSE command lasts
for 20 ms, and it takes around 1.3 ms for the loop to execute.
One time through the loop = 1.7 ms + 20 ms + 1.3 ms = 23.0 ms.
Since the loop executes 100 times, that’s 23.0 ms times 100.
time  100  23 .0 ms
 100  0 .0230 s
 2.30 s
Let’s say you want the servo to run for 4.6 seconds. Your FOR…NEXT loop will have to
execute twice as many times:
FOR counter = 1 TO 200
PULSOUT 13, 850
PAUSE 20
NEXT
Example Program: ControlServoRunTimes.bs2
 Enter, save, and run ControlServoRunTimes.bs2.
 Verify that the P13 servo turns counterclockwise for about 2.3 seconds, followed
by the P12 servo turning for twice as long.
Your Boe-Bot’s Servo Motors · Page 65
'
'
'
'
'
Robotics with the Boe-Bot - ControlServoRunTimes.bs2
Run the P13 servo at full speed counterclockwise for 2.3 s, then
run the P12 servo for twice as long.
{$STAMP BS2}
{$PBASIC 2.5}
DEBUG "Program Running!"
counter VAR Byte
FOR counter = 1 TO 100
PULSOUT 13, 850
PAUSE 20
NEXT
FOR counter = 1 TO 200
PULSOUT 12, 850
PAUSE 20
NEXT
END
Let’s say you want to run both servos, the P13 servo at a pulse width of 850 and the P12
servo at a pulse width of 650. Now, each time through the loop, it will take:
1.7ms
1.3 ms
20 ms
1.6 ms
--------24.6 ms
–
–
–
–
–
Servo connected to P13
Servo connected to P12
Pause duration
Code overhead
-----------------------------Total
If you want to run the servos for a certain amount of time, you can calculate it like this:
Number of pulses = Time s / 0.0246 s = Time / 0.0246
Lets’ say we want to run the servos for 3 seconds. That’s:
Number of pulses = 3 / 0.0246 = 122
Page 66 · Robotics with the Boe-Bot
Now, you can use the value 122 in the EndValue of the FOR…NEXT loop, and it will look
like this:
FOR counter = 1 TO 122
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
Example Program: BothServosThreeSeconds.bs2
Here’s an example of making the servos turn in one direction for three seconds, then
reversing their direction.
 Enter, save, and run BothServosThreeSeconds.bs2.
' Robotics with the Boe-Bot - BothServosThreeSeconds.bs2
' Run both servos in opposite directions for three seconds, then reverse
' the direction of both servos and run another three seconds.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
counter VAR Byte
FOR counter = 1 TO 122
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
FOR counter = 1 TO 122
PULSOUT 13, 650
PULSOUT 12, 850
PAUSE 20
NEXT
END
 Verify that each servo turned one direction for three seconds, and then reversed
direction and turned for three more seconds. Did you notice that while the
servos reversed at the same moment, they were always turning in opposite
directions? Why would this be useful?
Your Boe-Bot’s Servo Motors · Page 67
Your Turn – Predict Servo Run Time




Pick a time (six seconds or less), that you want your servos to turn.
Divide the number of seconds by 0.024.
Your answer is the number of loops you will need.
Modify BothServosThreeSeconds.bs2 so that it makes both servos run for the
amount of time you selected.
 Compare your predicted run time to the actual run time.
 Remember to disconnect power from your system (board and servos) when you
are done.
TIP – To measure the run time, press and hold the Reset button on your Board of Education
(or BASIC Stamp HomeWork Board). When you are ready to start timing, let go of the
Reset button.
SUMMARY
This chapter guided you through connecting, adjusting, and testing the Parallax
Continuous Rotation servos. Along the way, a variety of PBASIC commands were
introduced. The PAUSE command makes the program stop for brief or long periods of
time, depending on the Duration argument you use. DO…LOOP makes repeating a single or
group of PBASIC commands over and over again efficient. HIGH and LOW were
introduced as a way of making the BASIC Stamp connect an I/O pin to Vdd or Vss.
High and low signals were viewed with the help of an LED circuit. These signals were
used to introduce timing diagrams.
The PULSOUT command was introduced as a more precise way to deliver a high or low
signal, and an LED circuit was also used to view signals sent by the PULSOUT command.
DO…LOOP, PULSOUT, and PAUSE were then used to send the Parallax Continuous Rotation
servos the signal to stay still, which is 1.5 ms pulses every 20 ms. The servo was
adjusted with a screwdriver while receiving the 1.5 ms pulses until it stayed still. This
process is called “centering” the servo.
After the servos were centered, variables were introduced as a way to store values.
Variables can be used in math operations and counting. FOR…NEXT loops were introduced
as a way to count. FOR…NEXT loops control the number of times the lines of code between
the FOR and NEXT statements are executed. FOR…NEXT loops were then used to control the
number of pulses delivered to a servo, which in turn controls the amount of time the
servo runs.
Page 68 · Robotics with the Boe-Bot
Questions
1. How do the Parallax Continuous Rotation servos differ from standard servos?
2. How long does a millisecond last? How do you abbreviate it?
3. What PBASIC commands can you use to make other PBASIC commands
execute over and over again?
4. What command causes the BASIC Stamp to internally connect one of its I/O
pins to Vdd? What command makes the same kind of connection, but to Vss?
5. What are the names of the different size variables that can be declared in a
PBASIC program? What size values can each size of variable store?
6. What is the key to controlling a Parallax Continuous Rotation servo’s speed and
direction? How does this relate to timing diagrams? How does it relate to
PBASIC commands? What the command and argument can you adjust to
control a continuous rotation servo’s speed and direction?
Exercises
1. Write a PAUSE command that makes the BASIC Stamp do nothing for 10
seconds.
2. Modify this FOR…NEXT loop so that it counts from 6 to 24 in steps of 3. Also,
write the variable declaration you will need to make this program work.
FOR counter = 9 TO 21
DEBUG ? counter
PAUSE 500
NEXT
Project
1. Write a program that causes an LED connected to P14 to light dimly (on/off with
every pulse) while the P12 servo is turning.
2. Write a program that takes the servos through three seconds of each of the four
different combinations of rotation. Hint: you will need four different FOR…NEXT
loops. First, both servos should rotate counterclockwise, then they should both
rotate clockwise. Then, the P12 servo should rotate clockwise as the P13 servo
rotates counterclockwise, and finally, the P12 servo should rotate
counterclockwise while the P13 servo rotates clockwise.
Your Boe-Bot’s Servo Motors · Page 69
Solutions
Q1. Instead of holding a certain position like a standard servo, the Parallax
Continuous Rotation servos turn a certain direction at a certain speed.
Q2. A millisecond lasts one thousandth of a second, and "ms" is the abbreviation.
Q3. The DO…LOOP command is used to make other PBASIC commands execute over
and over.
Q4. HIGH connects I/O pin to Vdd, LOW connects I/O pin to Vss.
Q5. The variable sizes are bit, nib, byte, and word.
Bit – Stores 0 to 1
Nib – Stores 0 to 15
Byte – Stores 0 to 255
Word – Stores 0 to 65535 or -32768 to +32767
Q6. Pulse width controls servo speed and direction. As seen on a timing diagram, the
pulse width is the high time. In PBASIC, the pulse can be generated with the
PULSOUT command. The PULSOUT command's Duration argument adjusts the
speed and direction.
E1. PAUSE 10000
E2. The key to writing the variable declaration is to choose a variable size large
enough to hold the value 24. A Nib will not work, since the maximum value a
nibble can store is 15. Therefore, choose a Byte variable.
counter VAR Byte
FOR counter = 6 TO 24 STEP 3
DEBUG ? counter
PAUSE 500
NEXT
Page 70 · Robotics with the Boe-Bot
P1. The key to solving this problem is to send a pulse train to the LED as well as the
servo.
' Robotics with the Boe-Bot - Ch02Prj01_DimlyLitLED.bs2
' Run servo and send same signal to dimly light the LED on P14.
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "Program Running!"
DO
PULSOUT 12, 650
PULSOUT 14, 650
PAUSE 20
LOOP
' P12 servo clockwise
' P14 LED lights dimly
P2. First, calculate the number of loops needed to get the servos to run for three
seconds, for each combination of rotation. As given on page 65, the code
overhead is 1.6 ms.
Four combinations (1,2,3,4).
Each combination: Determine PULSOUT Duration arguments:
1. Both counterclockwise:
2. Both clockwise:
3. 12 CW and 13 CCW:
4. 12 CCW and 13 CW:
12, 850 and 13, 850
12, 650 and 13, 650
12, 850 and 13, 650
12, 650 and 13, 850
Each combination: Calculate how long it will take for one loop:
1. one loop = 1.7 + 1.7 + 20 ms + 1.6 = 25.0 ms = 0.025 s
2. one loop = 1.3 + 1.3 + 20 ms + 1.6 = 24.2 ms = 0.0242 s
3. one loop = 1.7 + 1.3 + 20 ms + 1.6 = 24.6 ms = 0.0246 s
4. one loop = 1.3 + 1.7 + 20 ms + 1.6 = 24.6 ms = 0.0246 s
Each combination: Calculate number of pulses needed for 3 s of running:
1. number of pulses = 3 s / 0.025 s = 120
2. number of pulses = 3 s / 0.0242 s = 123.9 = 124
3. number of pulses = 3 s / 0.0246 s = 121.9 = 122
4. number of pulses = 3 s / 0.0246 s = 121.9 = 122
Your Boe-Bot’s Servo Motors · Page 71
Now write four FOR…NEXT loops, using the number of pulses calculated for the
EndValue argument. Include the correct PULSOUT arguments for the combination
of rotation.
' Robotics with the Boe-Bot - Ch02Prj02_4RotationCombinations.bs2
' Move servos through 4 clockwise/counterclockwise rotation
' combinations.
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "Program Running!"
counter
VAR
Word
FOR counter = 1 TO 120
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
' Loop for three seconds
' P13 servo counterclockwise
' P12 servo counterclockwise
FOR counter = 1 TO 124
PULSOUT 13, 650
PULSOUT 12, 650
PAUSE 20
NEXT
' Loop for three seconds
' P13 servo clockwise
' P12 servo clockwise
FOR counter = 1 TO 122
PULSOUT 13, 650
PULSOUT 12, 850
PAUSE 20
NEXT
' Loop for three seconds
' P13 servo clockwise
' P12 servo counterclockwise
FOR counter = 1 TO 122
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
' Loop for three seconds
' P13 servo counterclockwise
' P12 servo clockwise
END
Page 72 · Robotics with the Boe-Bot
Assemble and Test Your Boe-Bot · Page 73
Chapter 3: Assemble and Test Your Boe-Bot
This chapter contains instructions for building and testing your Boe-Bot. It’s especially
important to complete the testing portion before moving on to the next chapter. By doing
so, you can help avoid a number of common mistakes that lead to mystifying Boe-Bot
behavior in later chapters. Here is a summary of what you will do in each of the
activities in this chapter:
Activity
1
2
3
4
Summary
Build the Boe-Bot.
Re-test the servos to make sure they are properly connected .
Connect and test a speaker that can let you know when the Boe-Bot’s
batteries are low.
Use the Debug Terminal to control and test servo speed.
ACTIVITY #1: ASSEMBLING THE BOE-BOT ROBOT
This activity will guide you through assembling the Boe-Bot, step-by-step. In each step,
you will gather a few of the parts, and then assemble them so that they match the
pictures. Each picture has instructions that go with it; make sure to follow them
carefully.
Servo Tools and Parts
All of the tools shown in Figure 3-1 are common and can be found in most households
and school shops. They can also be purchased at local hardware stores.
Tools
(1) Parallax screwdriver
(Phillips #1 point, included)
(1) 1/4″ Combination wrench
(optional but handy)
(1) Needle-nose pliers (optional)
Figure 3-1
Boe-Bot
Assembly
Tools
Page 74 · Robotics with the Boe-Bot
Mounting the Topside Hardware
 Start by gathering this list of parts.
 Then, follow the accompanying instructions.
Parts List:
See Figure 3-2.
(1)
(4)
(4)
(1)
Boe-Bot chassis
1″ Standoffs
Pan head screws, 1/4″ 4-40
Rubber grommet, 13/32″
Instructions:
 Insert the 13/32″ rubber grommet into the
hole in the center of the Boe-Bot chassis.
 Make sure the groove in the outer edge of
the rubber grommet is seated on the edge of
the hole in the chassis.
 Use the four 1/4″ 4-40 screws to attach the
four standoffs to the chassis as shown.
Boe-Bot Parts - The parts for the Boe-Bot are either included in the Boe-Bot full kit or in a
combination of the Board of Education Full Kit and Robotics Parts Kit. If you are using a
HomeWork Board, you need a battery pack with tinned leads instead of a barrel plug, and
two additional 3-pin headers. See Appendix A: Parts List and Kit Options on page 289 for
more information.
Figure 3-2
Chassis and
Topside
Hardware
Parts (left);
assembled
(right)
Assemble and Test Your Boe-Bot · Page 75
Removing the Servo Horns
 Disconnect the power from your BASIC Stamp and servos.
 Remove all of the AA batteries from the battery pack.
 Disconnect the servos from your board.
Parts List:
Instructions:
See Figure 3-3.
(2) Parallax Continuous Rotation
servos, previously centered
 Use a Phillips screwdriver to remove the
screws that hold the servo control horns on
the output shafts.
 Pull each horn upwards and off the servo
output shaft.
 Save the screws; they will be used in a later
step.
Figure 3-3
Chassis and Topside
Hardware
Output
Shaft
Phillips
Screw
Control
Horn
Parts (left);
assembled (right)
Stop!

Before this next step, you must have completed these activities from Chapter 2: Your
Boe-Bot’s Servo Motors


Activity #3: Connecting the Servo Motors; page 40.
Activity #4: Centering the Servos; page 49.
Page 76 · Robotics with the Boe-Bot
Mounting the Servos on the Chassis
Servo Mounting Options - Agile Maneuvers vs. Potentiometer & Maintenance Access
The photographs in this text show the servos mounted from the inside, and oriented so the
potentiometer access port is facing the center of the chassis. This positions the axles close
to the center of the Boe-Bot, allowing for agile maneuvering. If you are diligent about
centering your servos before building your Boe-Bot, this causes no problems.
Many educators prefer the option of mounting the servos from the outside, and oriented so
the potentiometer access port faces the front of the Boe-Bot. This has the advantage of
allowing easy access to adjust these potentiometers on an assembled robot, and also for
quick replacement of damaged servos. However, the Boe-Bot will have a longer, wider
wheel base and be a little less nimble on maneuvers. You may find that you need to adjust
some values in your programs slightly to achieve the same results. The choice is yours.
Parts List:
See Figure 3-4.
(2)
(2)
(8)
(8)
Boe-Bot Chassis (partially assembled.
Parallax Continuous Rotation servos
Pan Head Screws, 3/8″ 4-40
Nuts, 4-40
Instructions:
 Attach the servos to the chassis
using the Phillips screws and nuts.
 Use pieces of masking tape to label
the servos left (L) and right (R).
Figure 3-4
Mounting the
Servos on the
Chassis
Parts (left);
assembled
(right)
Assemble and Test Your Boe-Bot · Page 77
Mounting the Battery Pack
Figure 3-5 shows two different sets of parts. Use the parts on the left if you have a Board
of Education, and the parts on the right if you have a HomeWork Board.
Parts List for Boe-Bot with a
Board of Education:
Parts List for Boe-Bot with a
HomeWork Board:
See Figure 3-5 (left side).
See Figure 3-5 (right side).
(1)
(2)
(2)
(1)
(1)
(2)
(2)
(1)
Boe-Bot chassis (partially assembled)
Flat head Phillips screws, 3/8″ 4-40
Nuts, 4-40
Battery pack with center-positive plug
Boe-Bot chassis (partially assembled)
Flat head Phillips screws, 3/8″ 4-40
Nuts, 4-40
Battery pack with tinned leads
Figure 3-5
Battery Pack
Mounting
Hardware
Instructions:
 Use the flathead screws and nuts to attach the battery pack to underside of the
Boe-Bot chassis as shown on the left side of Figure 3-6.
 Make sure to insert the screws through the battery pack, and then tighten down
the nuts on the topside of the chassis.
Page 78 · Robotics with the Boe-Bot
 As shown on the right side of Figure 3-6, pull the battery pack’s power cord
through the hole with the rubber grommet in the center of the chassis.
 Pull the servo lines through the same hole.
 Arrange the servo lines and supply cable as shown.
Figure 3-6
Battery Pack
Installed
Mounting the Wheels
Parts List:
(1) Partially assembled Boe-Bot
(not shown)
(1) 1/16″ Cotter pin
(1) Tail wheel ball
(2) Rubber band tires
(2) Plastic machined wheels
(2) Screws that were saved in
the Removing the Servo
Horns step
Figure 3-7
Wheel
Hardware
Instructions:
The left side of Figure 3-8 shows the Boe-Bot’s tail wheel mounted on the chassis. The
tail wheel is merely a plastic ball with a hole through the center. A cotter pin holds it to
the chassis and functions as an axle for the wheel.
Assemble and Test Your Boe-Bot · Page 79
 Line the hole in the tail wheel up with the holes in the tail portion of the chassis.
 Run the cotter pin through all three holes (chassis left, tail wheel, chassis right).
 Bend the ends of the cotter pin apart so that it can’t slide back out of the hole.
The right side of Figure 3-8 shows the Boe-Bot’s drive wheels mounted on the servos.
 Stretch each rubber band tire and seat it on the outer edge of each wheel.
 Each plastic wheel has a recess that fits on a servo output shaft. Press each
plastic wheel onto a servo output shaft making sure the shaft lines up with and
sinks into the recess.
 Use the machine screws that you saved when you removed the servo horns to
attach the wheels to the servo output shafts.
Figure 3-8
Mounting the
Wheels
Tail wheel (left);
drive wheels
(right)
Page 80 · Robotics with the Boe-Bot
Attaching the Board to the Chassis
Parts List for Boe-Bot with a
Board of Education:
Parts List for Boe-Bot with a
HomeWork Board:
See left side of Figure 3-9.
See right side of Figure 3-9.
(1) Boe-Bot chassis (partially assembled)
(4) Pan head screws, 1/4″ 4-40
(1) Board of Education with BASIC
Stamp 2
(1) Boe-Bot chassis (partially assembled)
(4) Pan head screws, 1/4″ 4-40
(1) BASIC Stamp HomeWork Board
Figure 3-9
Boe-Bot
Chassis and
Boards
Board of
Education (left);
HomeWork
Board (right)
Figure 3-10 shows the servo ports reconnected for both the Board of Education (left side)
and the HomeWork Board (right side).
 Reconnect the servos to the servo headers.
 Make sure to connect the plug labeled ‘L’ to the P13 port and the plug labeled
‘R’ to the P12 port.
Assemble and Test Your Boe-Bot · Page 81
White
Red
Black
White
Red
Black
White
Stripe
(916) 624-8333
www.parallaxinc.com
www.stampsinclass.com
15 14 Vdd 13 12
Vdd
Red
Black
X4
X5
On Board of Education
Vin
SolidB
Rev
Black
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8





Figure 3-10
Servo Ports
Reconnecte
d
P13 - White
Vbp - Red
Vss - Black
Vbp - Red
P12 - White
On HomeWork Board
Figure 3-11 shows the Boe-Bot chassis with their respective boards attached.
 Set the board on the four standoffs so that they line up with the four holes on the
outer corner of the board.
 Make sure the white breadboard is closer to the drive wheels, not the tail wheel.
 Attach the board to the standoffs with the pan head screws.
Figure 3-11
Boards
Attached to
Boe-Bot
Chassis
With Board of Education
With HomeWork Board
Figure 3-12 shows assembled Boe-Bot robots, the left built with a Board of Education
(Serial Rev C) and the right built with a HomeWork Board.
Page 82 · Robotics with the Boe-Bot
 From the underside of the chassis, pull any excess servo and battery cable
through the hole with the rubber grommet.
 Tuck the excess cable lengths between the servos and the chassis.
Figure 3-12
Assembled
Boe-Bot
Robots
With Board of Education
With HomeWork Board
ACTIVITY #2: RE-TEST THE SERVOS
In this activity, you will test to make sure that the electrical connections between your
board and the servos are correct. Figure 3-13 shows your Boe-Bot’s front, back, left, and
right. We need to make sure that the servo on the right turns when it receives pulses from
P12 and that the servo on the left turns when it receives pulses from P13.
Left
Back
Front
Right
Figure 3-13
Your Boe-Bot robot’s
Front, Back, Left,
and Right
Assemble and Test Your Boe-Bot · Page 83
Testing the Right Wheel
The next example program will test the servo connected to the right wheel, shown in
Figure 3-14. The program will make this wheel turn clockwise for three seconds, then
stop for one second, then turn counterclockwise for three seconds.
Clockwise 3 seconds
Stop 1 second
Figure 3-14
Testing the Right Wheel
Counterclockwise 3 seconds
Example Program: RightServoTest.bs2








Set the Boe-Bot on its nose so that the drive wheels are suspended above ground.
Reload the batteries into the battery pack.
If you have a Board of Education, set the 3-position switch to position-2.
If you have a BASIC Stamp HomeWork Board, connect the 9 V battery to the
battery clip.
Enter, save, and run RightServoTest.bs2.
Verify that the right wheel turns clockwise for three seconds, stops for one
second, then turns counterclockwise for three seconds.
If the right wheel/servo does not behave as predicted, see the Servo
Troubleshooting section. It comes right after RightServoTest.bs2.
If the right wheel/servo does behave properly, then move on to the Your Turn
section, where you will test the left wheel.
' Robotics with the Boe-Bot - RightServoTest.bs2
' Right servo turns clockwise three seconds, stops 1 second, then
' counterclockwise three seconds.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
counter
VAR
FOR counter = 1 TO 122
PULSOUT 12, 650
PAUSE 20
NEXT
Word
' Clockwise just under 3 seconds.
Page 84 · Robotics with the Boe-Bot
FOR counter = 1 TO 40
PULSOUT 12, 750
PAUSE 20
NEXT
' Stop one second.
FOR counter = 1 TO 122
PULSOUT 12, 850
PAUSE 20
NEXT
' Counterclockwise three seconds.
END
Your Turn – Testing the Left Wheel
Now, it’s time to run the same test on the left wheel as shown in Figure 3-15. This
involves modifying RightServoTest.bs2 so that the PULSOUT commands are sent to the
servo connected to P13 instead of the servo connected to P12.
All you have to do is change the three PULSOUT commands so that they read PULSOUT 13
instead of PULSOUT 12.
Clockwise 3 seconds
Stop 1 second
Figure 3-15
Testing the Left Wheel
Counterclockwise 3 seconds
 Save RightServoTest.bs2 as LeftServoTest.bs2.
 Change the three PULSOUT commands so that they read PULSOUT 13 instead of
PULSOUT 12.
 Save and then run the program.
 Verify that it makes the left servo turn clockwise for 3 seconds, stops for 1
second, then makes the servo turn counterclockwise for 3 seconds.
 If the left wheel/servo does not behave as predicted, see the Servo
Troubleshooting box below.
 If the left wheel/servo does behave properly, then your Boe-Bot is functioning
properly, and you are ready to move on to the next activity.
Assemble and Test Your Boe-Bot · Page 85
Servo Troubleshooting: Here is a list of some common symptoms and how to fix them.
The servo doesn’t turn at all.




If you are using a Board of Education, make sure the 3-position switch is set to
position-2. You can then re-run the program by pressing and releasing the Reset
button.
If you are using a BASIC Stamp HomeWork Board, make sure the battery pack
has fresh batteries, all oriented properly in the case.
Double-check your servo connections Figure 3-10 on page 81 as a guide. If you
are using a HomeWork Board, you may also want to take a second look at Figure
2-18 on page 47.
Check and make sure you entered the program correctly.
The right servo doesn’t turn, but the left one does.
This means that the servos are swapped. The servo that’s connected to P12 should be
connected to P13, and the servo that’s connected to P13 should be connected to P12.






Disconnect power.
Unplug both servos.
Connect the servo that was connected to P12 to P13.
Connect the other servo (that was connected to P13) to P12.
Reconnect power.
Re-run RightServoTest.bs2.
The wheel does not fully stop; it turns slowly.
This means that the servo may not be exactly centered. There are two ways to fix this:


Adjusting in hardware: Go back and re-do the Chapter 2, Activity #4: Centering the
Servos on page 49. If the servos are not mounted to give easy access to the
potentiometer ports, consider re-orienting them for re-assembly.
Adjusting in software: If the wheel turns slowly counterclockwise, use a value
that’s a little smaller than 750. If it’s turning clockwise, use a value that’s a little
larger than 750. This new value will be used in place of 750 for all PULSOUT
commands for that wheel as you do the experiments in this book.
The wheel doesn’t stop for one second between the clockwise and counterclockwise
rotations.
The wheel might turn rapidly for three seconds in one direction and four in the other. It
might also turn rapidly for three seconds, then just a little slower for one second, then turn
rapidly again for three seconds. Or, it might turn rapidly in the same direction for seven
seconds. Regardless, it means the potentiometer is out of adjustment.

Remove the wheels, un-mount the servos and repeat the exercise in Activity #4:
Centering the Servos on page 49.
Page 86 · Robotics with the Boe-Bot
ACTIVITY #3: START/RESET INDICATOR CIRCUIT AND PROGRAM
When the voltage supply drops below the level a device needs to function properly, it’s
called brownout. The BASIC Stamp protects itself from brownout by making its
processor and program memory chips go dormant until the power supply voltage returns
to normal levels. A drop below 5.2 V at Vin results in a drop below 4.3 V at the BASIC
Stamp’s internal voltage regulator output. A circuit called a brownout detector on the
BASIC Stamp is always on the lookout for this condition. When brownout occurs, the
brownout detector disables the BASIC Stamp’s processor and program memory.
When the supply voltage comes back above 5.2 V, the BASIC Stamp starts running
again, but not at the same place in the program. Instead, it starts from the beginning of
the program. This is actually the same thing that happens when you unplug power and
plug it back in, and it’s also the same thing that happens if you press and release the
Reset button on your board.
When the Boe-Bot’s batteries are running low, brownouts can cause the program to
restart when you’re not expecting it to. This can lead to some really mystifying Boe-Bot
behavior. In some cases, the Boe-Bot will be running whatever course it’s programmed
to navigate, and all of a sudden, it might seem to get lost and go in an unexpected
direction. If low batteries are the cause, it could be the fact that the Boe-Bot’s program
went back to the beginning and started over again. In other cases, the Boe-Bot can end
up doing a confused dance because every time the servos start turning, it overtaxes the
already low batteries. The program attempts to make the servos turn for a split second,
then restarts, over and over again.
These situations make a program start/reset indicator an extremely useful diagnostic
device as well as a useful robot tool. One way to indicate resets is to include an
unmistakable signal at the beginning of all the Boe-Bot’s programs. The signal occurs
every time the power gets plugged in, but it also occurs every time a reset due to
brownout conditions occurs. One effective signal for resets is a speaker that emits a tone
each time the BASIC Stamp program runs from the beginning or resets.
BASIC Stamp HomeWork Board Special Instructions
Although the reset indicator will tell you when the 9 V battery supplying the BASIC Stamp is
running low, it will not tell you when the servo supply (the battery pack) is running low. You
can always tell when your battery pack is running low because the servos will gradually
move slower and slower during normal operation. When you observe this symptom, replace
the dead batteries with new or freshly recharged batteries.
Assemble and Test Your Boe-Bot · Page 87
This exercise will introduce a device called a piezoelectric speaker (piezospeaker) that
you can use to generate tones. This speaker can make different tones depending on the
frequency of high/low signals it receives from the BASIC Stamp. The schematic symbol
and part drawing for the piezoelectric speaker are shown in Figure 3-16. This speaker
will be used for emitting the tones when the BASIC Stamp is reset in this activity as well
as in the rest of the activities in this text.
Figure 3-16
Piezospeaker
What’s frequency? It’s the measurement of how often something occurs in a given amount
of time.
What’s a piezoelectric element and how can it make sound? It’s a crystal that changes
shape slightly when voltage is applied to it. By applying high and low voltages to a
piezoelectric crystal at a rapid rate, it causes the piezoelectric crystal to rapidly change
shape. The result is vibration. Vibrating objects cause the air around them to vibrate also.
This is what our ear detects as sounds and tones. Every rate of vibration has a different
tone. For example, if you pluck a single guitar string, it will vibrate at one frequency, and
you will hear a particular tone. If you pluck a different guitar string, it will vibrate at a
different frequency and make a different tone.
Piezoelectric elements have many uses. For example, when force is applied to a
piezoelectric element, it can create voltage. Some piezoelectric elements have a frequency
at which they naturally vibrate. These can be used to create voltages at frequencies that
function as the clock oscillator for many computers and microcontrollers.
Parts Required
(1) Assembled and tested Boe-Bot
(1) Piezospeaker
(misc.) Jumper wires
If your piezospeaker has a label that says “Remove seal after washing” just peel it off
and proceed. Your piezospeaker does not need to be washed!
Page 88 · Robotics with the Boe-Bot
Building the Start/Reset Indicator Circuit
Figure 3-17 shows piezospeaker alarm circuit schematics for both the Board of Education
and BASIC Stamp HomeWork Board. Figure 3-18 shows a wiring diagram for each
board.
Always disconnect power before building or modifying circuits!


If you have a Board of Education, set the 3-position switch to position-0.
If you have a BASIC Stamp HomeWork Board, disconnect the 9 V battery from the
battery clip and remove a battery from the Battery Pack.
 Build the circuit shown in Figure 3-17 and Figure 3-18.
The piezospeaker and servo circuits will remain connected to your board for the rest
of the activities in this text.
All circuit schematics from this point onward will show circuits that should be added
to the existing servo and piezospeaker circuits.
All wiring diagrams will show the circuit from the schematic that comes just before it
along with the servo and piezospeaker circuit connections.
P4
Figure 3-17
Program Start/Reset Indicator
Circuit Schematic
Vss
Assemble and Test Your Boe-Bot · Page 89
To Servos
To Servos
15 14 Vdd 13 12
(916) 624-8333
Rev B
www.parallax.com
www.stampsinclass.com
Red
Black
X4
Vdd
X5
Vin
Vdd
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
+
Board of Education
Rev C
© 2000-2003
Vin
Vss
X3
Figure 3-18
Wiring Diagrams
for the Program
Start/Reset
Indicator Circuit
Board of
Education (left)
and HomeWork
Board (right)
+
HomeWork Board
Programming the Start/Reset Indicator
The next example program tests the piezospeaker. It uses the FREQOUT command to send
precisely timed high/low signals to a speaker. Here is the FREQOUT command’s syntax:
FREQOUT Pin, Duration, Freq1 {,Freq2}
Here’s an example of a FREQOUT command that’s used in the next example program.
FREQOUT 4, 2000, 3000
The Pin argument is 4, meaning that the high/low signals will be sent to I/O pin P4. The
Duration argument, which is how long the high/low signals will last, is 2000, which is
2000 ms or 2 seconds. The Freq1 argument is the frequency of the high/low signals. In
this example, the high/low signals will make a 3000 hertz, or 3 kHz, tone.
Page 90 · Robotics with the Boe-Bot
Frequency can be measured in hertz (Hz). The hertz is a frequency measurement of how
many times per second something happens. One hertz is simply one time-per-second, and
it’s abbreviated 1 Hz. One kilohertz is one-thousand-times-per-second, and it’s abbreviated
1 kHz.
FREQOUT digitally synthesizes tones. The FREQOUT command applies high/low pulses of
varying durations that make a piezospeaker’s vibration more closely resemble natural
vibrations of music strings.
Example Program: StartResetIndicator.bs2
This example program makes a beep at the beginning of the program, then it goes on to
run a program that sends DEBUG messages every half second. These messages will
continue indefinitely because they are nested between DO and LOOP. If the power to the
BASIC Stamp is interrupted while it is in the middle of its DO…LOOP, the program will
start at the beginning again. When it starts over, it will beep again. You can simulate a
brownout condition by either pressing and releasing the Reset button on your board or
disconnecting and reconnecting your board’s battery supply.
Learn how to create sound effects and music with your BASIC Stamp! Download
What’s a Microcontroller? from www.parallax.com/go/WAM, and try the example circuit and
programs in Chapter 8.
To learn even more about the signals FREQOUT generates, download Understanding Signals
with the PropScope from www.parallax.com/go/PropScope, and read Chapter 7.
 Reconnect power to your board.
 Enter, save, and run StartResetIndicator.bs2.
 Verify that the piezospeaker made a clearly audible tone for two seconds before
the “Waiting for reset…” messages started to display in the Debug Terminal.
 If you did not hear a tone, check your wiring and code for errors. Repeat until
you get an audible tone from your speaker.
 If you did hear an audible tone, try simulating the brownout condition by
pressing and releasing the Reset button on your board. Verify that the
piezospeaker makes a clearly audible tone after each reset.
 Also try disconnecting and reconnecting your battery supply, and verify that this
results in the reset warning tone as well.
'
'
'
'
Robotics with the Boe-Bot - StartResetIndicator.bs2
Test the piezospeaker circuit.
{$STAMP BS2}
' Stamp directive.
{$PBASIC 2.5}
' PBASIC directive.
Assemble and Test Your Boe-Bot · Page 91
DEBUG CLS, "Beep!!!"
FREQOUT 4, 2000, 3000
' Display while speaker beeps.
' Signal program start/reset.
DO
'
'
'
'
DEBUG CR, "Waiting for reset…"
PAUSE 500
LOOP
DO...LOOP
Display message
every 0.5 seconds
until hardware reset.
How StartResetIndicator.bs2 Works
StartResetIndicator.bs2 starts by displaying the message “Beep!!!” Then, immediately
after printing the message, the FREQOUT command plays a 3 kHz tone on the piezoelectric
speaker for 2 s. Because the instructions are executed so rapidly by the BASIC Stamp, it
should seem as though the message is displayed at the same instant the piezospeaker
starts to play the tone.
When the tone is done, the program enters a DO…LOOP, displaying the same “Waiting for
reset…” message over and over again. Each time the reset button on the Board of
Education is pressed or the power is disconnected and reconnected, the program starts
over again, with the "Beep!!!" message and the 3 kHz tone.
Your Turn – Adding StartResetIndicator.bs2 to a Different Program
The FREQOUT command in the battery indicator program will be used at the beginning of
every example program from here onward. You could consider it part of the
“initialization routine” or “boot routine” for every Boe-Bot program.
An initialization routine is comprised of all the commands necessary to get a device or
program up and running. It often includes setting certain variable values, beeping noises,
and for more complex devices, self testing and calibration.
 Open HelloOnceEverySecond.bs2.
 Copy the FREQOUT command from the StartResetIndicator.bs2 program into
HelloOnceEverySecond.bs2 program, above the DO…LOOP section.
 Run the modified program and verify that it responds with a warning tone every
time the BASIC Stamp is reset (either by pressing and releasing the Reset button
on the board or disconnecting and reconnecting the battery supply).
Page 92 · Robotics with the Boe-Bot
ACTIVITY #4: TESTING SPEED CONTROL WITH THE DEBUG TERMINAL
In this activity, you will graph servo speed vs. pulse width. One thing that can make this
process go much more quickly is the Debug Terminal’s Transmit windowpane, which is
shown in Figure 3-19. You can use the Transmit windowpane to send the BASIC Stamp
messages. By sending messages that tell the BASIC Stamp what pulse width to deliver to
the servo, you can test the servo speed at various pulse widths.
Transmit
Windowpane
Receive
Windowpane
Figure 3-19
Debug
Terminal’s
Transmit and
Receive
Windowpanes
Pulse width is a common way to describe how long a pulse lasts. The reason it is
called pulse "width" is because the amount of time a pulse lasts is related to how
wide it is on a timing diagram. Pulses which last longer are wider on timing diagrams
and pulses which last for short periods of time are narrow.
Using the DEBUGIN Command
By now, you are probably familiar with the DEBUG command and how it can be used to
send messages from the BASIC Stamp to the Debug Terminal. The place the messages
are viewed is called the Receive windowpane because it's the place where messages
received from the BASIC Stamp are displayed. The Debug Terminal also has a Transmit
windowpane, which allows you to send information to your BASIC Stamp while a
program is running. You can use the DEBUGIN command to make the BASIC Stamp
receive what you type into the Transmit windowpane and store it in one or more
variables.
Assemble and Test Your Boe-Bot · Page 93
The DEBUGIN command places the value you type in the Transmit windowpane into a
variable. In the next example program, a word variable named pulseWidth will be used
to store the values the DEBUGIN command receives.
pulseWidth
VAR
Word
Now, the DEBUGIN command can be used to capture a decimal value that you enter into
the Debug Terminal’s Transmit windowpane and store it in pulseWidth:
DEBUGIN DEC pulseWidth
You can then program the BASIC Stamp to use this value. Here it is used in the
PULSOUT command’s Duration argument:
PULSOUT 12, pulseWidth
Example Program: TestServoSpeed.bs2
This program allows you to set the PULSOUT command’s Duration argument by entering it
into the Debug Terminal's Transmit windowpane.
 Continue this activity with the Boe-Bot sitting on its nose so that the wheels do
not touch the ground.
 Enter, save, and run TestServoSpeed.bs2.
 Point at the Debug Terminal’s Transmit windowpane with your mouse, and click
it to activate the cursor in that window for typing.
 Type 650 and then press the Enter key.
 Verify that the servo turns full speed clockwise for six seconds.
When the servo is done turning, you will be prompted to enter another value.
 Type 850 and then press the Enter key.
 Verify that the servo turns full speed counterclockwise.
Try measuring the wheel's rotational speed in RPM (revolutions per minute) for a range
of pulse widths between 650 and 850. Here's how:
 Place a mark on the wheel so that you can see how far it turns in 6 seconds.
Page 94 · Robotics with the Boe-Bot
 Use the Debug Terminal to test how far the wheel turns for each of these pulse
widths: 650, 660, 670, 680, 690, 700, 700, 710, 720, 730, 740, 750, 760, 770,
780, 790, 800, 810, 820, 830, 840, 850
 For each pulse width, multiply the number of turns by 10 to get the RPM. For
example, if the wheel makes 3.65 full turns, it was rotating at 36.5 RPM.
 Explain in your own words how you can use pulse width to control Continuous
Rotation servo speed.
'
'
'
'
Robotics with the Boe-Bot - TestServoSpeed.bs2
Enter pulse width, then count revolutions of the wheel.
The wheel will run for 6 seconds
Multiply by 10 to get revolutions per minute (RPM).
'{$STAMP BS2}
'{$PBASIC 2.5}
counter
VAR
pulseWidth
VAR
pulseWidthComp VAR
Word
Word
Word
FREQOUT 4, 2000, 3000
' Signal program start/reset.
DO
DEBUG "Enter pulse width: "
DEBUGIN DEC pulseWidth
pulseWidthComp = 1500 - pulseWidth
FOR counter = 1 TO 244
PULSOUT 12, pulseWidth
PULSOUT 13, pulseWidthComp
PAUSE 20
NEXT
LOOP
How TestServoSpeed.bs2 Works
Three variables are declared, counter for the FOR…NEXT loop, pulseWidth for the
DEBUGIN and PULSOUT commands, and pulseWidthComp which stores a value that is
used in a second PULSOUT command.
counter
VAR
pulseWidth
VAR
pulseWidthComp VAR
Word
Word
Word
Assemble and Test Your Boe-Bot · Page 95
The FREQOUT command is used to signal that the program has started.
FREQOUT 4,2000,3000
The remainder of the program is nested within a DO…LOOP, so it will execute over and
over again. The Debug Terminal’s operator (that's you) is asked to enter a pulse width.
The DEBUGIN command stores this value in the pulseWidth variable.
DEBUG "Enter pulse width: "
DEBUGIN DEC pulseWidth
To make the measurement more accurate, two PULSOUT commands have to be sent. By
making one PULSOUT command the same amount below 750 as the other is above 750,
the sum of the two PULSOUT Duration arguments is always 1500. That ensures that the
two PULSOUT commands combined take the same amount of time. The result is that no
matter the Duration of your PULSOUT command, the FOR…NEXT loop will still take the
same amount of time to execute. This will make the RPM measurements you will take in
the Your Turn section more accurate.
This next command takes the pulse width you entered, and calculates a pulse width that
will make 1500 when the two are added together. If you enter a pulse width of 650,
pulseWidthComp will be 850. If you enter a pulse width of 850, pulseWidthComp will
be 650. If you enter a pulse width of 700, pulseWidthComp will be 800. Try a few other
examples. They will all add up to 1500.
pulseWidthComp = 1500 - pulseWidth
A FOR…NEXT loop that runs for 6 seconds sends pulses to the right (P12) servo. The
pulseWidthComp value is sent to the left (P13) servo, making it turn in the opposite
direction.
FOR counter = 1 TO 244
PULSOUT 12, pulseWidth
PULSOUT 13, pulseWidthComp
PAUSE 20
NEXT
Your Turn – Advanced Topic: Graphing Pulse Width vs. Rotational Velocity
Figure 3-20 shows an example of a transfer curve for a continuous rotation servo. The
horizontal axis shows the pulse width in ms, and the vertical axis shows the rotational
Page 96 · Robotics with the Boe-Bot
velocity in RPM. In this graph, clockwise is negative and counterclockwise is positive.
This particular servo’s transfer curve ranges from about -48 RPM to 48 RPM over the
range of test pulse widths that range from 1.3 ms to 1.7 ms.
Rotational Velocity vs. Pulse Width for Servo
60
Rotational Velocity, RPM
40
20
Figure 3-20
Transfer Curve Example
for Parallax Continuous
Rotation Servo
0
-20
-40
-60
1.300
1.350
1.400
1.450
1.500
1.550
1.600
1.650
1.700
Pulse Width, m s
Right Servo
Remember that the PULSOUT command’s Duration argument is in 2 µs units. PULSOUT
12, 650 sends pulses that last 1.3 ms to P12.
PULSOUT 12, 655 sends pulses of
1.31 ms, PULSOUT 12, 660 sends pulses of 1.32 ms, and so on.
Duration  650  2 s
 650  0.000002 s
 0 .0013 s
 1 .3 m s
Duration  655  2 s
 655  0.000002 s
 0 .00131 s
 1 .31 m s
Duration  660  2 s
 660  0.000002 s
 0 .00132 s
 1 .32 m s
You can use Table 3-1 to record the data for your own transfer curve. Keep in mind that
the example program is controlling the right wheel with the values you enter. The left
wheel turns in the opposite direction.
 Mark your right wheel so that you have a reference point to count the
revolutions.
 Run TestServoSpeed.bs2.
Assemble and Test Your Boe-Bot · Page 97
Table 3-1: Pulse Width and RPM for Parallax Continuous Rotation Servo
Pulse
Width
(ms)
Rotational
Velocity
(RPM)
Pulse
Width
(ms)
Rotational
Velocity
(RPM)
Pulse
Width
(ms)
Rotational
Velocity
(RPM)
Pulse
Width
(ms)
1.300
1.400
1.500
1.600
1.310
1.410
1.510
1.610
1.320
1.420
1.520
1.620
1.330
1.430
1.530
1.630
1.340
1.440
1.540
1.640
1.350
1.450
1.550
1.650
1.360
1.460
1.560
1.660
1.370
1.470
1.570
1.670
1.380
1.480
1.580
1.680
1.390
1.490
1.590
1.690
Rotational
Velocity
(RPM)
1.700
 Click the Debug Terminal’s Transmit windowpane.
 Enter the value 650.
 Count how many turns the wheel made.
Since the servo turns for 6 seconds, you can multiply this value by 10 to get revolutions
per minute (RPM).






Multiply this value by 10 and enter the result next to the 1.3 ms table entry.
Enter the value 655, and count how many turns the wheel made.
Multiply this value by 10 and enter the result next to the 1.31 ms table entry.
Keep increasing your durations by 5 (0.01 ms) until you are up to 850 (1.7 ms).
Use a spreadsheet, calculator, or graph paper to graph the data.
Repeat this process for your other servo.
To repeat these measurements for the left wheel, modify the PULSOUT commands so that
pulses with a Duration of pulseWidth are sent to P13 and pulses with a Duration of
pulseWidthComp are sent to P12.
Page 98 · Robotics with the Boe-Bot
SUMMARY
This chapter covered Boe-Bot assembly and testing. This involved mechanical assembly,
such as connecting the various moving parts to the Boe-Bot chassis. It also involved
circuit assembly, connecting the servos and piezospeaker. The testing involved retesting
the servos after they were disconnected to build the Boe-Bot.
The concept of brownout was introduced along with what this condition does to a
program running on the BASIC Stamp. Brownout causes the BASIC Stamp to shut
down, and then start running the program from the beginning. A piezospeaker was added
to signal the start of a program. If the piezospeaker sounds in the middle of a running
program when it’s not supposed to, this can indicate a brownout condition. Brownout
conditions can in turn indicate low batteries. To make the piezospeaker play a tone to
indicate a reset, the FREQOUT command was introduced. This command is part of an
initialization routine that will be used at the beginning of all Boe-Bot programs.
Until this chapter, the Debug Terminal has been used to display messages sent to the
computer by the BASIC Stamp. These messages were displayed in the Receive
windowpane. The Debug Terminal also has a Transmit windowpane that you can use to
send values to the BASIC Stamp. The BASIC Stamp can capture these values by
executing the DEBUGIN command, which receives a value sent by the Debug Terminal's
Transmit windowpane and stores it in a variable. The value can then be used by the
PBASIC program. This technique was used to set the pulse widths to control and test
servo speed and direction. It was also used as a data collection aid for plotting the
transfer curve of a Parallax Continuous Rotation servo.
Assemble and Test Your Boe-Bot · Page 99
Questions
1.
2.
3.
4.
5.
What are some of the symptoms of brownout on the Boe-Bot?
How can a piezospeaker be used to detect brownout?
What is a reset?
What is an initialization routine?
What are three (or more) possible mistakes that can occur when disconnecting
and reconnecting the servos?
6. What command do you have to change in RightServoTest.bs2 to test the left
wheel instead of the right wheel?
Exercises
1. Write a FREQOUT command that makes a tone that sounds different from the reset
detect tone to signify the end of a program.
2. Write a FREQOUT command that makes a tone (different from beginning or
ending tones) that signifies an intermediate step in a program has been
completed. Try a value with a 100 ms duration at a 4 kHz frequency.
Projects
1. Modify RightServoTest.bs2 so that it makes a tone signifying the test is
complete.
2. Modify TestServoSpeed.bs2 so that you can use DEBUGIN to enter the pulse
width for the left and the right servo as well as the number of pulses to deliver in
the FOR…NEXT loop. Use this program to control your Boe-Bot’s motion via the
Debug Terminal’s Transmit windowpane.
Page 100 · Robotics with the Boe-Bot
Solutions
Q1. Symptoms include erratic behavior such as going in unexpected directions or
doing a confused dance.
Q2. A FREQOUT command at the beginning of all Boe-Bot programs causes the
piezospeaker to play a tone. This tone will therefore occur every time an
accidental reset happens due to brownout conditions.
Q3. A reset is when the power is interrupted and the BASIC Stamp program starts
running again from the beginning of the program.
Q4. An initialization routine consists of the lines of code that are used at the
beginning of the program. These lines of code run each time the program starts
from the beginning.
Q5. 1) The servo lines P12 and P13 are swapped. 2) One or both servos is plugged in
backwards, so that the white-red-black color coding is incorrect. 3) The power
switch is not on position-2. 4) The 9V or AA batteries are not installed. 5) The
servo centering potentiometer is out of adjustment.
Q6. The PULSOUT commands must be changed to read PULSOUT 13 instead of
PULSOUT 12.
E1. The key is to modify the FREQOUT command used for the StartResetIndicator.bs2
program, that is, FREQOUT, 4, 2000, 3000. For example: FREQOUT, 4, 500,
3500 would work.
E2. FREQOUT 4, 100, 4000.
P1. The key to solving this program is to add the line from Exercise 1 above the END
command in the RightServoTest.bs2 program.
'
'
'
'
Robotics with the Boe-Bot - Ch03Prj01_TestCompleteTone.bs2
Right servo turns clockwise three seconds, stops 1 second, then
counterclockwise three seconds.
A tone signifies that the
test is complete.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
counter
VAR
Word
FREQOUT 4, 2000, 3000
' Signal start of program.
FOR counter = 1 TO 122
PULSOUT 12, 650
PAUSE 20
NEXT
' Clockwise just under 3 seconds.
FOR counter = 1 TO 40
' Stop one second.
Assemble and Test Your Boe-Bot · Page 101
PULSOUT 12, 750
PAUSE 20
NEXT
FOR counter = 1 TO 122
PULSOUT 12, 850
PAUSE 20
NEXT
' Counterclockwise three seconds.
FREQOUT 4, 500, 3500
' Signal end of program
END
P2. To solve this problem, TestServoSpeed.bs2 must be expanded to receive three
pieces of data: left servo pulsewidth, right servo pulsewidth, and number of
pulses. Then, a FOR…NEXT loop with two servo PULSOUT commands must be
added to actually move the servo motors. Furthermore, all variables must be
declared in the beginning of the program. An example solution is shown below.
Note: This project is best tested with the Boe-Bot's wheels propped up.
' Robotics with the Boe-Bot - Ch03Prj02_DebuginMotion.bs2
' Enter servo pulsewidth & duration for both wheels via Debug Terminal.
'{$STAMP BS2}
'{$PBASIC 2.5}
ltPulseWidth
rtPulseWidth
pulseCount
counter
VAR
VAR
VAR
VAR
Word
Word
Byte
Word
'
'
'
'
Left servo pulse width
Right servo pulse width
Number of pulses to servo
Loop counter
DO
DEBUG "Enter left servo pulse width: "
DEBUGIN DEC ltPulseWidth
' Enter values in Debug
' Terminal
DEBUG "Enter right servo pulse width: "
DEBUGIN DEC rtPulseWidth
DEBUG "Enter number of pulses:
DEBUGIN DEC pulseCount
FOR counter = 1 TO pulseCount
PULSOUT 13, ltPulseWidth
PULSOUT 12, rtPulseWidth
PAUSE 20
NEXT
LOOP
"
' Send specific number of pulses
' Left servo motion
' Right servo motion
Page 102 · Robotics with the Boe-Bot
Boe-Bot Navigation · Page 103
Chapter 4: Boe-Bot Navigation
The Boe-Bot can be programmed to perform a variety of maneuvers. The maneuvers and
programming techniques introduced in this chapter will be reused in later chapters. The
only difference is that in this chapter, the Boe-Bot will blindly perform the maneuvers.
In later chapters, the Boe-Bot will perform similar maneuvers in response to conditions it
detects with its sensors.
This chapter also introduces ways to tune and calibrate the Boe-Bot’s navigation.
Included are techniques to straighten a Boe-Bot’s forward drive, more precise turns, and
calculating distances.
Activity
1
2
3
4
5
6
Summary
Program the Boe-Bot to perform the basic maneuvers: forward, backward,
rotate left, rotate right, and pivoting turns.
Tune the maneuvers from Activity #1 so that they are more precise.
Use math to calculate the number of pulses to deliver to make the Boe-Bot
travel a predetermined distance.
Instead of programming the Boe-Bot to make abrupt starts and stops, write
programs that make the Boe-Bot gradually accelerate into and decelerate out
of maneuvers.
Write subroutines to perform the basic maneuvers so that each subroutine can
be used over and over again in a program.
Record complex maneuvers in the BASIC Stamp module's unused program
memory and write programs that play back these maneuvers.
ACTIVITY #1: BASIC BOE-BOT MANEUVERS
Figure 4-1 shows your Boe-Bot’s front, back, left, and right. When the Boe-Bot goes
forward, in the picture, it would have to roll to the right edge of the page. Backward
would be toward the left edge of the page. A left turn would be make the Boe-Bot ready
to drive off the top of the page, and a right turn would have it facing the bottom of the
page.
Page 104 · Robotics with the Boe-Bot
Left Turn
Backward
Forward
Figure 4-1
Your Boe-Bot and
Driving Directions
Right Turn
Moving Forward
Here’s a funny thing: to make the Boe-Bot go forward, the Boe-Bot’s left wheel has to
turn counterclockwise, but its right wheel has to turn clockwise. If you haven’t already
grasped this, take a look at Figure 4-2 and see if you can convince yourself that it’s true.
Viewed from the left, the wheel has to turn counterclockwise for the Boe-Bot to move
forward. Viewed from the right, the other wheel has to turn clockwise for the Boe-Bot to
move forward.
Counterclockwise
Forward
Left Side
Clockwise
Forward
Figure
4-2
Wheel
Rotation
for
Forward
Motion
Right Side
Remember from Chapter 2 that the PULSOUT command’s Duration argument controls the
speed and direction the servo turns. The StartValue and EndValue arguments of a
FOR…NEXT loop control the number of pulses that are delivered. Since each pulse takes
Boe-Bot Navigation · Page 105
the same amount of time, the EndValue argument also controls the time the servo runs.
Here’s an example program that will make the Boe-Bot roll forward for about three
seconds.
Example Program: BoeBotForwardThreeSeconds.bs2
 Make sure power is connected to the BASIC Stamp and servos.
 Enter, save, and run BoeBotForwardThreeSeconds.bs2.
' Robotics with the Boe-Bot - BoeBotForwardThreeSeconds.bs2
' Make the Boe-Bot roll forward for three seconds.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
counter
VAR
Word
FREQOUT 4, 2000, 3000
' Signal program start/reset.
FOR counter = 1 TO 122
' Run servos for 3 seconds.
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
END
How BoeBotForwardThreeSeconds.bs2 Works
From chapter 2, you already have lots of experience with the elements of this program: a
variable declaration, a FOR…NEXT loop, PULSOUT commands with Pin and Duration
arguments, and PAUSE commands. Here’s a review of what each does and how it relates
to the servos’ motions.
First a variable is declared that will be used in the FOR...NEXT loop.
counter VAR Word
You should recognize this next command; it generates a tone to signal the start of the
program. It will be used in all programs that run the servos.
FREQOUT 4, 2000, 3000
' Signal program start/reset.
Page 106 · Robotics with the Boe-Bot
This FOR…NEXT loop sends 122 sets of pulses to the servos, one each to P13 and P12,
pausing for 20 ms after each set and then returning to the top of the loop.
FOR counter = 1 TO 122
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
PULSOUT 13, 850 causes the left servo to rotate counterclockwise while PULSOUT 12,
650 causes the right servo to rotate clockwise. Therefore, both wheels will be turning
toward the front end of the Boe-Bot, causing it to drive forward. It takes about 3 seconds
for the FOR…NEXT loop to execute 122 times, so the Boe-Bot drives forward for about 3
seconds.
Your Turn – Adjusting Distance and Speed
 By changing the FOR…NEXT loop’s EndValue argument from 122 to 61, you can
make the Boe-Bot move forward for half the time. This in turn will make the
Boe-Bot move forward half the distance.
 Save BoeBotForwardThreeSeconds.bs2 under a new name.
 Change the FOR...NEXT loop's EndValue from 122 to 61.
 Run the program and verify that it ran at half the time and covered half the
distance.
 Try these steps over again, but this time, change the FOR…NEXT loop’s EndValue
to 244.
The PULSOUT Duration arguments of 650 and 850 caused the servos to rotate near their
maximum speed. By bringing each of the PULSOUT Duration arguments closer to the staystill value of 750, you can slow down your Boe-Bot.
 Modify your program with these PULSOUT commands:
PULSOUT 13, 780
PULSOUT 12, 720
 Run the program, and verify that your Boe-Bot moves slower.
Boe-Bot Navigation · Page 107
Moving Backward, Rotating, and Pivoting
All it takes to get other motions out of your Boe-Bot are different combinations of the
PULSOUT Duration arguments. For example, these two PULSOUT commands can be used
to make your Boe-Bot go backwards:
PULSOUT 13, 650
PULSOUT 12, 850
These two commands will make your Boe-Bot rotate in a left turn (counterclockwise as
you are looking at it from above):
PULSOUT 13, 650
PULSOUT 12, 650
These two commands will make your Boe-Bot rotate in a right turn (clockwise as you are
looking at it from above):
PULSOUT 13, 850
PULSOUT 12, 850
You can combine all these commands into a single program that makes the Boe-Bot
move forward, turn left, turn right, then move backward.
Example Program: ForwardLeftRightBackward.bs2
 Enter, save, and run ForwardLeftRightBackward.bs2.
TIP – To enter this program quickly, use the BASIC Stamp Editor's Edit menu tools (Copy
and Paste) to make four copies of a FOR…NEXT loop. Then, adjust only the PULSOUT
Duration values and FOR…NEXT loop EndValues.
' Robotics with the Boe-Bot - ForwardLeftRightBackward.bs2
' Move forward, left, right, then backward for testing and tuning.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
counter
VAR
FREQOUT 4, 2000, 3000
Word
' Signal program start/reset.
Page 108 · Robotics with the Boe-Bot
FOR counter = 1 TO 64
' Forward
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
PAUSE 200
FOR counter = 1 TO 24
' Rotate left - about 1/4 turn
PULSOUT 13, 650
PULSOUT 12, 650
PAUSE 20
NEXT
PAUSE 200
FOR counter = 1 TO 24
' Rotate right - about 1/4 turn
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
PAUSE 200
FOR counter = 1 TO 64
' Backward
PULSOUT 13, 650
PULSOUT 12, 850
PAUSE 20
NEXT
END
Your Turn – Pivoting
You can make the Boe-Bot turn by pivoting around one wheel. The trick is to keep one
wheel still while the other rotates. For example, if you keep the left wheel still and make
the right wheel turn clockwise (forward), the Boe-Bot will pivot to the left.
PULSOUT 13, 750
PULSOUT 12, 650
Boe-Bot Navigation · Page 109
If you want to pivot forward and to the right, simply stop the right wheel, and make the
left wheel turn counterclockwise (forward).
PULSOUT 13, 850
PULSOUT 12, 750
These are the PULSOUT commands for pivoting backwards and to the right.
PULSOUT 13, 650
PULSOUT 12, 750
Finally, these are the PULSOUT commands for pivoting backwards and to the left.
PULSOUT 13, 750
PULSOUT 12, 850
 Save ForwardLeftRightBackward.bs2 as PivotTests.bs2.
 Substitute the PULSOUT commands just discussed in place of the forward, left,
right, and backward routines.
 Adjust the run time of each maneuver by changing each FOR…NEXT loop’s
EndValue to 30.
 Be sure to change the comment next to each FOR…NEXT loop to reflect each new
pivot action.
 Run the modified program and verify that the different pivot actions work.
ACTIVITY #2: TUNING THE BASIC MANEUVERS
Imagine writing a program that instructs the Boe-Bot to travel full-speed forward for
fifteen seconds. What if the Boe-Bot curves slightly to the left or right during its travel,
when it’s supposed to be traveling straight ahead? There’s no need to take the Boe-Bot
back apart and re-adjust the servos with a screwdriver to fix this. You can simply adjust
the program slightly to get both Boe-Bot wheels traveling the same speed. While the
screwdriver approach would be called a “hardware adjustment,” the programming
approach is called a “software adjustment.”
Straightening the Boe-Bot’s Path
The first step is to examine your Boe-Bot’s travel for long enough to find out if it’s
curving either to the left or to the right when it’s supposed to be going straight ahead.
Ten seconds of forward travel should be enough. This can be accomplished with a
simple modification to BoeBotForwardThreeSeconds.bs2 from the previous activity.
Page 110 · Robotics with the Boe-Bot
Example Program: BoeBotForwardTenSeconds.bs2
 Open BoeBotForwardThreeSeconds.bs2.
 Rename and save it as BoeBotForwardTenSeconds.bs2.
 Change the EndValue of the FOR counter from 122 to 407, so it reads like this:
' Robotics with the Boe-Bot - BoeBotForwardTenSeconds.bs2
' Make the Boe-Bot roll forward for ten seconds.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
counter
VAR
Word
FREQOUT 4, 2000, 3000
' Signal program start/reset.
FOR counter = 1 TO 407
' Number of pulses – run time.
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
' Left servo full speed ccw.
' Right servo full speed cw.
NEXT
END
 Run the program, and watch closely to see if your Boe-Bot veers to the right or
left as it travels forwards for ten seconds.
Your Turn – Adjusting Servo Speed to Straighten the Boe-Bot’s Path
If your Boe-Bot goes perfectly straight, try this example anyway. If you follow the
instructions, it should adjust your Boe-Bot so that it curves slightly to the right.
Let’s say that the Boe-Bot turns slightly to the left. There are two ways to think about
this problem: either the left wheel is turning too slowly, or the right wheel is turning too
quickly. Since the Boe-Bot is already at full speed, speeding up the left wheel isn’t going
to be practical, but slowing down the right wheel should help remedy the situation.
Remember that servo speed is determined by the PULSOUT command’s Duration
argument. The closer the Duration is to 750, the slower the servo turns. This means you
should change the 650 in the command PULSOUT 12,650 to something a little closer
Boe-Bot Navigation · Page 111
to 750. If the Boe-Bot is only just a little off course, maybe PULSOUT 12,663 will do the
trick. If the servos are severely mismatched, maybe it needs to be PULSOUT 12,690.
It will probably take several tries to get the right value. Let’s say that your first guess is
that PULSOUT 12,663 will do the trick, but it turns out not to be enough because the BoeBot is still turning slightly to the left. So try PULSOUT 12,670. Maybe that overcorrects,
and it turns out that PULSOUT 12,665 gets it exactly right. This is called an iterative
process, meaning a process that takes repeated tries and refinements to get to the right
value.
If your Boe-Bot curved to the right instead of the left, it means you need to slow down
the left wheel by reducing the Duration of 850 in the PULSOUT 13,850 command. Again,
the closer this value gets to 750, the slower the servo will turn.
 Modify BoeBotForwardTenSeconds.bs2 so that it makes your Boe-Bot go
straight forward.
 Use masking tape or a sticker to label each servo with the best PULSOUT values.
 If your Boe-Bot already travels straight forward, try the modifications just
discussed to see the effect. It should cause the Boe-Bot to travel in a curve
instead of a straight line.
You might find that there’s an entirely different situation when you program your BoeBot to roll backward.
 Modify BoeBotForwardTenSeconds.bs2 so that it makes the Boe-Bot roll
backward for ten seconds.
 Repeat the test for straight line.
 Repeat the steps for correcting the PULSOUT command’s Duration argument to
straighten the Boe-Bot’s backward travel.
Tuning the Turns
Software adjustments can also be made to get the Boe-Bot to turn to a desired angle, such
as 90°. The amount of time the Boe-Bot spends rotating in place determines how far it
turns. Because the FOR…NEXT loop controls run time, you can adjust the FOR…NEXT loop’s
EndValue argument to get very close to the turning angle you want.
Page 112 · Robotics with the Boe-Bot
Here’s the left turn routine from ForwardLeftRightBackward.bs2:
FOR counter = 1 TO 24
' Rotate left - about 1/4 turn
PULSOUT 13, 650
PULSOUT 12, 650
PAUSE 20
NEXT
Let’s say that the Boe-Bot turns just a bit more than 90° (1/4 of a full circle). Try FOR
counter = 1 TO 23, or maybe even FOR counter = 1 TO 22. If it doesn’t turn far
enough, increase the run time of the rotation by increasing the FOR…NEXT loop’s EndValue
argument to whatever value it takes to complete the quarter turn.
If you find yourself with one value slightly overshooting 90° and the other slightly
undershooting, try choosing the value that makes it turn a little too far, then slow down
the servos slightly. In the case of the rotate left, both PULSOUT Duration arguments
should be changed from 650 to something a little closer to 750. As with the straight line
exercise, this will also be an iterative process.
Your Turn – 90° Turns
 Modify ForwardLeftRightBackward.bs2 so that it makes precise 90° turns.
 Update ForwardLeftRightBackward.bs2 with the PULSOUT values that you
determined for straight forward and backward travel.
 Update the label on each servo with a notation about the appropriate EndValue
for a 90° turn.
Carpeting can cause navigation errors. If you are running your Boe-Bot on carpeting,
don’t expect perfect results! A carpet is a bit like a golf green—the way the carpet pile is
inclined can affect the way your Boe-Bot travels, especially over long distances. For more
precise maneuvers, use a smooth surface.
ACTIVITY #3: CALCULATING DISTANCES
In many robotics contests, more precise robot navigation lends itself to better scores.
One popular entry level robotics contest is called dead reckoning. The entire goal of this
contest is to make your robot go to one or more locations and then return to exactly
where it started.
Boe-Bot Navigation · Page 113
You might remember asking your parents this question, over and over again, while on
your way to a vacation destination or relatives’ house:
“Are we there yet?”
Perhaps when you got a little older, and learned division in school, you started watching
the road signs to see how far it was to the destination city. Next, you checked the
speedometer in your car. By dividing the speed into the distance, you got a pretty good
estimate of the time it would take to get there. You may not have been thinking in these
exact terms, but here is the equation you were using:
time 
distance
speed
Example – Time for English Distance
Example – Time for Metric Distance
If you’re 140 miles away from your
destination, and you’re traveling 70 miles
per hour, it’s going to take 2 hours to get
there.
If you’re 200 kilometers away from your
destination, and you’re traveling 100
kilometers per hour, it’s going to take 2
hours to get there.
140 miles
70 miles/hour
1 hour
 140 miles 
70 miles
 2 hours
time 
200 kilometers
100 kilometers/hour
1 hour
 200 km 
100 km
time 
 2 hours
You can do the same exercise with the Boe-Bot, except you have control over how far
away the destination is. Here’s the equation you will use:
servo run time 
Boe - Bot distance
Boe - Bot speed
You will have to test the Boe-Bot speed. The easiest way to do this is to set the Boe-Bot
next to a ruler and make it travel forward for one second. By measuring how far your
Boe-Bot traveled, you will know your Boe-Bot’s speed. If your ruler has inches, your
answer will be in inches per second (in/s), if it has centimeters your answer will be in
centimeters per second (cm/s).
Page 114 · Robotics with the Boe-Bot
 Enter, save, and run ForwardOneSecond.bs2.
 Place your Boe-Bot next to a ruler as shown in Figure 4-3.
 Make sure to line up the point where the wheel touches the ground with the 0
in/cm mark on the ruler.
Figure 4-3: Measuring Boe-Bot Distance
6-9VDC
15 14 Vdd 1 3 12
9 Vdc
Battery
Red
Black
X4
Pwr
STAM
inCLA PS
SS
TM
1
Sout
Sin
ATN
Vss
P0
P1
P2
P3
P4
P5
P6
P7
U1
Vin
Vss
Rst
Vdd
P15
P14
P13
P12
P11
P10
P9
P8
Vss
P1
P3
P5
P7
P9
P11
P1 3
P1 5
Vin
Vss
P0
P2
P4
P6
P8
P1 0
P1 2
P1 4
Vd d
X1
Reset
Vdd
X5
Vin
Vss
X3
P1 5
P1 4
P1 3
P1 2
P11
P1 0
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
0
1 2
www.stampsinclass.com
Board of Education
Rev C
© 2000-2003
Measured Distance
inch
cm
1
1
2
2
3
4
5
4
3
6
7
8
5
6
7
8
9
10
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
 Press the Reset button on your board to re-run the program.
 Measure how far your Boe-Bot traveled by recording the measurement where the
wheel is now touching the ground here:__________________ in / cm.
'
'
'
'
'
Example Program: ForwardOneSecond.bs2
Robotics with the Boe-Bot - ForwardOneSecond.bs2
Make the Boe-Bot roll forward for one second.
{$STAMP BS2}
{$PBASIC 2.5}
DEBUG "Program Running!"
counter
VAR
Word
FREQOUT 4, 2000, 3000
FOR counter = 1 TO 41
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
END
' Signal program start/reset.
Boe-Bot Navigation · Page 115
You can also think about the distance you just recorded as your Boe-Bot’s speed, in units
per second. Let’s say that your Boe-Bot traveled 9 in (23 cm). Since it took one second
for your Boe-Bot to travel that far, it means your Boe-Bot travels at around 9 in/s
(23 cm/s). Now, you can figure out how many seconds your Boe-Bot has to travel to go a
particular distance.
Inches and centimeters per second
The abbreviation for inches is in, and the abbreviation for centimeters is cm. Likewise,
inches per second is abbreviated in/s, and centimeters per second is abbreviated cm/s.
Both are convenient speed measurements for the Boe-Bot. There are 2.54 cm in 1 in. You
can convert inches to centimeters by multiplying the number of inches by 2.54. You can
convert centimeters to inches by dividing the number of centimeters by 2.54.
Example – Time for 20 Inches
Example – Time for 51 Centimeters
At 9 in/s, your Boe-Bot has to travel for
2.22 s to travel 20 in.
At 23 cm/s, your Boe-Bot has to travel for
2.22 s to travel 51 cm.
time 
20 in
9 in/s
 20 in 
 2.22 s
time 
1s
9 in
51 cm
23 cm/s
 51 cm 
1s
23 cm
 2.22 s
In Chapter 2, Activity #6, we learned that it takes 24.6 ms (0.024 s) each time the two
servo PULSOUT and one PAUSE commands are executed in a FOR…NEXT loop. The
reciprocal of this value is the number of pulses per second that the loop transmits to each
servo. A reciprocal is when you swap a fraction's numerator and denominator. Another
way to take a reciprocal is to divide a number or fraction into the number one. In other
words, 1  0.024 s/pulse = 40.65 pulses/s.
Since you know the amount of time you want your Boe-Bot to move forward (2.22 s) and
the number of pulses the BASIC Stamp sends to the servos each second (40.65 pulses/s),
you can use these values to calculate how many pulses to send to the servos. This is the
number you will have to use for your FOR…NEXT loop's EndValue argument.
Page 116 · Robotics with the Boe-Bot
40 .65 pulses
s
 90.24 ... pulses
 90 pulses
pulses  2 .22 s 
The calculations in this example took two steps. First, figure out how long the servos
have to run to make the Boe-Bot travel a certain distance, then figure out how many
pulses it takes to make the servos run for that long. Since you know you have to multiply
by 40.65 to get from run time to pulses, you can reduce this to one step.
pulses 
Boe  Bot dis tan ce 40.65 pulses

Boe  Bot speed
s
Example – Time for 20 Inches
Example – Time for 51 Centimeters
At 9 in/s, your Boe-Bot has to travel for
2.22 s to travel 20 in.
At 23 cm/s, your Boe-Bot has to travel for
2.22 s to travel 51 cm.
20 in 40 .65 pulses

s
9 in/s
1 s 40 .65 pulses
 20 in 

9 in
1s
 20  9  40.65 pulses
pulses 
51 cm 40.65 pulses

s
23 cm/s
1s
40 .65 pulses
 51 cm 

23 cm
1s
 51  23  40.65 pulses
pulses 
 90.333...pulses
 90 pulses
 90.136...pulses
 90 pulses
Your Turn – Your Boe-Bot’s Distance
Now, it’s time to try this out with distances that you choose.
 If you have not already done so, use a ruler and the ForwardOneSecond.bs2
program to determine your Boe-Bot’s speed in in/s or cm/s.
 Decide how far you want your Boe-Bot to travel.
 Use the pulses equation to figure out how many pulses to deliver to the BoeBot’s servos:
pulses 
Boe  Bot dis tan ce 40.65 pulses

Boe  Bot speed
s
Boe-Bot Navigation · Page 117
 Modify BoeBotForwardOneSecond.bs2 so that it delivers the number of pulses
you determined for your distance.
 Run the program and test to see how close you got.
This technique has sources of error. The activity you just completed does not take into
account the fact that it took a certain number of pulses for the Boe-Bot to get up to full
speed. Nor did it take into account any distance the Boe-Bot might coast before it comes to
a full stop. The servo speeds will also go slower as the batteries lose their charge.
You can increase the accuracy of your Boe-Bot distances with devices called encoders,
which count the holes in the Boe-Bot's wheels as they pass. Encoder kits and other BoeBot specific accessories are available from www.parallax.com/go/Boe-Bot.
ACTIVITY #4: MANEUVERS—RAMPING
Ramping is a way to gradually increase or decrease the speed of the servos instead of
abruptly starting or stopping. This technique can increase the life expectancy of both
your Boe-Bot’s batteries and your servos.
Programming for Ramping
The key to ramping is to use variables along with constants for the PULSOUT command’s
Duration argument. Figure 4-4 shows a FOR…NEXT loop that can ramp the Boe-Bot’s
speed from full stop to full speed ahead. Each time the FOR…NEXT loop repeats itself, the
pulseCount variable increases by 1. The first time through, pulseCount is 1, so it’s
like using the commands PULSOUT 13, 751 and PULSOUT 12, 749. The second time
through the loop, the value of pulseCount is 2, so it’s like using the commands PULSOUT
13, 752 and PULSOUT 12, 748. As the value of the pulseCount variable increases, so
does the speed of the servos. By the hundredth time through the loop, the pulseCount
variable is 100, so it’s like using the commands PULSOUT 13, 850 and PULSOUT 12,
650, which is full-speed ahead for the Boe-Bot.
pulseCount
VAR
Word
FOR pulseCount = 1 TO 100
PULSOUT 13, 750 + pulseCount
PULSOUT 12, 750 - pulseCount
PAUSE 20
NEXT
1, 2, 3,
…100
Figure 4-4
Ramping Example
Page 118 · Robotics with the Boe-Bot
Recall from Chapter 2, Activity #5 that FOR…NEXT loops can also count downward from a
higher number to a lower number. You can use this to ramp the speed back down again
by using FOR pulseCount = 100 TO 1. Here is an example program that uses
FOR…NEXT loops to ramp up to full speed, then ramp back down.
Example Program: StartAndStopWithRamping.bs2
 Enter, save, and run StartAndStopWithRamping.bs2.
 Verify that the Boe-Bot gradually accelerates to full speed, maintains full speed
for a while, and then gradually decelerates to a full stop.
' -----[ Title ]-------------------------------------------------------------' Robotics with the Boe-Bot - StartAndStopWithRamping.bs2
' Ramp up, go forward, ramp down.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
pulseCount
VAR
Word
' FOR...NEXT loop counter.
' -----[ Initialization ]---------------------------------------------------FREQOUT 4, 2000, 3000
' Signal program start/reset.
' -----[ Main Routine ]------------------------------------------------------' Ramp up forward.
FOR pulseCount = 1 TO 100
PULSOUT 13, 750 + pulseCount
PULSOUT 12, 750 - pulseCount
PAUSE 20
'
'
'
'
Loop ramps up for 100 pulses.
Pulse = 1.5 ms + pulseCount.
Pulse = 1.5 ms – pulseCount.
Pause for 20 ms.
'
'
'
'
Loop sends 75 forward pulses.
1.7 ms pulse to left servo.
1.3 ms pulse to right servo.
Pause for 20 ms.
NEXT
' Continue forward for 75 pulses.
FOR pulseCount = 1 TO 75
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
Boe-Bot Navigation · Page 119
' Ramp down from going forward to a full stop.
FOR pulseCount = 100 TO 1
PULSOUT 13, 750 + pulseCount
PULSOUT 12, 750 - pulseCount
PAUSE 20
NEXT
'
'
'
'
Loop ramps down for 100 pulses.
Pulse = 1.5 ms + pulseCount.
Pulse = 1.5 ms - pulseCount.
Pause for 20 ms.
END
' Stop until reset.
Your Turn
You can also create routines to combine ramping up or down with the other maneuvers.
Here’s an example of how to ramp up to full speed going backward instead of forward.
The only difference between this routine and the forward ramping routine is that the
value of pulseCount is subtracted from 750 in the PULSOUT 13 command, where before
it was added. Likewise, pulseCount is added to the value of 750 in the PULSOUT 12
command, where before it was subtracted.
' Ramp up to full speed going backwards
FOR pulseCount = 1 TO 100
PULSOUT 13, 750 - pulseCount
PULSOUT 12, 750 + pulseCount
PAUSE 20
NEXT
You can also make a routine for ramping into a turn by adding the value of pulseCount
to 750 in both PULSOUT commands. By subtracting pulseCount from 750 in both
PULSOUT commands, you can ramp into a turn the other direction. Here’s an example of
a quarter turn with ramping. The servos don’t get an opportunity to get up to full speed
before they have to slow back down again.
' Ramp up right rotate.
FOR pulseCount = 0 TO 30
PULSOUT 13, 750 + pulseCount
PULSOUT 12, 750 + pulseCount
PAUSE 20
NEXT
Page 120 · Robotics with the Boe-Bot
' Ramp down right rotate
FOR pulseCount = 30 TO 0
PULSOUT 13, 750 + pulseCount
PULSOUT 12, 750 + pulseCount
PAUSE 20
NEXT
 Open ForwardLeftRightBackward.bs2 from Activity #1, and save it as
ForwardLeftRightBackwardRamping.bs2.
 Modify the new program so your Boe-Bot will ramp into and out of each
maneuver. Hint: you might use the code snippets above, and similar snippets
from StartAndStopWithRamping.bs2.
ACTIVITY #5: SIMPLIFY NAVIGATION WITH SUBROUTINES
In the next chapter, your Boe-Bot will have to perform maneuvers to avoid obstacles.
One of the key ingredients to avoiding obstacles is executing pre-programmed
maneuvers. One way of executing pre-programmed maneuvers is with subroutines. This
activity introduces subroutines, and also two different approaches to creating reusable
maneuvers with subroutines.
Inside the Subroutine
There are two parts of a PBASIC subroutine. One part is the subroutine call. It’s the
command in the program that tells it to jump to the reusable part of code, then come back
when it’s done. The other part is the actual subroutine. It starts with a label that serves
as its name and ends with a RETURN command. The commands between the label and the
RETURN command make up the code block that does the job you want the subroutine to
do.
Figure 4-5 shows part of a PBASIC program that contains a subroutine call and a
subroutine. The subroutine call is the GOSUB My_Subroutine command. The actual
subroutine is everything from the My_Subroutine: label through the RETURN command.
Here’s how it works. When the program gets to the GOSUB My_Subroutine command,
it looks for the My_Subroutine: label. As shown by arrow (1), the program jumps to
the My_Subroutine: label and starts executing commands. The program keeps going
down line by line from the label, so you’ll see the message “Command in subroutine” in
Boe-Bot Navigation · Page 121
your Debug Terminal. PAUSE 1000 causes a one second pause. Then, when the program
gets to the RETURN command, arrow (2) shows how it jumps back to the command
immediately after the GOSUB command. In this case, it’s a DEBUG command that displays
the message “After subroutine.”
DO
DEBUG
PAUSE
GOSUB
DEBUG
PAUSE
LOOP
2
"Before subroutine",CR
1000
My_Subroutine
"After subroutine", CR
1000
1
Figure 4-5
Subroutine Basics
My_Subroutine:
DEBUG "Command in subroutine", CR
PAUSE 1000
RETURN
Example Program – OneSubroutine.bs2
 Enter, save, and run OneSubroutine.bs2.
' Robotics with the Boe-Bot - OneSubroutine.bs2
' This program demonstrates a simple subroutine call.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG
PAUSE
GOSUB
DEBUG
END
"Before subroutine",CR
1000
My_Subroutine
"After subroutine", CR
My_Subroutine:
DEBUG "Command in subroutine", CR
PAUSE 1000
RETURN
 Watch your Debug Terminal, and press the Reset button a few times. You
should get the same set of three messages in the right order each time.
Page 122 · Robotics with the Boe-Bot
Here’s an example program that has two subroutines. One subroutine makes a highpitched tone while the other makes a low-pitched tone. The commands between DO and
LOOP call each of the subroutines in turn. Try this program and note the effect.
Example Program – TwoSubroutines.bs2
 Enter, save, and run TwoSubroutines.bs2.
' Robotics with the Boe-Bot - TwoSubroutines.bs2
' This program demonstrates that a subroutine is a reusable block of commands.
' {$STAMP BS2}
' {$PBASIC 2.5}
DO
GOSUB
DEBUG
PAUSE
GOSUB
DEBUG
PAUSE
DEBUG
LOOP
High_Pitch
"Back in main", CR
1000
Low_Pitch
"Back in main again", CR
1000
"Repeat...",CR,CR
High_Pitch:
DEBUG "High pitch", CR
FREQOUT 4, 2000, 3500
RETURN
Low_Pitch:
DEBUG "Low pitch", CR
FREQOUT 4, 2000, 2000
RETURN
Let’s try putting the forward, left, right, and backward navigation routines inside
subroutines. Here’s an example:
Example Program – MovementsWithSubroutines.bs2
 Enter, save, and run MovementsWithSubroutines.bs2. Hint: you can use the Edit
menu in the BASIC Stamp Editor to copy and paste code blocks from one
program to another.
Boe-Bot Navigation · Page 123
'
'
'
'
Robotics with the Boe-Bot - MovementsWithSubroutines.bs2
Make forward, left, right, and backward movements in reusable subroutines.
{$STAMP BS2}
{$PBASIC 2.5}
DEBUG "Program Running!"
counter
VAR
FREQOUT 4, 2000, 3000
GOSUB
GOSUB
GOSUB
GOSUB
Forward
Left
Right
Backward
END
Forward:
FOR counter = 1 TO 64
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
PAUSE 200
RETURN
Left:
FOR counter = 1 TO 24
PULSOUT 13, 650
PULSOUT 12, 650
PAUSE 20
NEXT
PAUSE 200
RETURN
Right:
FOR counter = 1 TO 24
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
PAUSE 200
RETURN
Backward:
FOR counter = 1 TO 64
PULSOUT 13, 650
PULSOUT 12, 850
PAUSE 20
NEXT
RETURN
Word
' Signal program start/reset.
Page 124 · Robotics with the Boe-Bot
You should recognize the pattern of movement your Boe-Bot makes; it is the same one
made by ForwardLeftRightBackward.bs2. Clearly there are many different ways to
structure a program that will result in the same movements. A third approach is given in
the example below.
Example Program – MovementsWithVariablesAndOneSubroutine.bs2
Here’s another example program that causes your Boe-Bot to perform the same
maneuvers, but it only uses one subroutine and some variables to do it.
You have surely noticed that up to this point each Boe-Bot maneuver has been
accomplished with similar code blocks. Compare these two snippets:
' Forward full speed
' Ramp down from full speed backwards
FOR counter = 1 TO 64
FOR pulseCount = 100 TO 1
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
PULSOUT 13, 750 - pulseCount
PULSOUT 12, 750 + pulseCount
PAUSE 20
NEXT
What causes these two code blocks to perform different maneuvers are changes to the
FOR StartValue and EndValue arguments, and the PULSOUT Duration arguments. These
arguments can be variables, and these variables can be changed repeatedly during
program run time to generate different maneuvers. Instead of using separate subroutines
with specific PULSOUT Duration arguments for each maneuver, the program below uses
the same subroutine over and over. The key to making different maneuvers is to set the
variables to the correct values for the maneuver you want before calling the subroutine.
 Enter, save, and run MovementWithVariablesAndOneSubroutine.bs2.
' Robotics with the Boe-Bot - MovementWithVariablesAndOneSubroutine.bs2
' Make a navigation routine that accepts parameters.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
Boe-Bot Navigation · Page 125
counter
pulseLeft
pulseRight
pulseCount
VAR
VAR
VAR
VAR
Word
Word
Word
Byte
FREQOUT 4, 2000, 3000
' Signal program start/reset.
' Forward
pulseLeft = 850: pulseRight = 650: pulseCount = 64: GOSUB Navigate
' Left turn
pulseLeft = 650: pulseRight = 650: pulseCount = 24: GOSUB Navigate
' Right turn
pulseLeft = 850: pulseRight = 850: pulseCount = 24: GOSUB Navigate
' Backward
pulseLeft = 650: pulseRight = 850: pulseCount = 64: GOSUB Navigate
END
Navigate:
FOR counter = 1 TO pulseCount
PULSOUT 13, pulseLeft
PULSOUT 12, pulseRight
PAUSE 20
NEXT
PAUSE 200
RETURN
Did your Boe-Bot perform the familiar forward-left-right-backward sequence? This
program may be difficult to read at first, because the instructions are arranged in a new
way. Instead of having each variable statement and each GOSUB command on a different
line, they are grouped together on the same line and separated by colons. Here, the
colons function the same as a carriage return to separate each PBASIC instruction. Using
colons this way allows all of the new variable values for a given maneuver to be stored
together, and on the same line as the subroutine call.
Your Turn
Here is your "dead reckoning" contest mentioned earlier.
 Modify MovementWithVariablesAndOneSubroutine.bs2 to make your Boe-Bot
drive in a square, facing forwards on the first two sides and backwards on the
second two sides. Hint: you will need to use your own PULSOUT EndValue
argument that you determined in Activity #2, page 109.
Page 126 · Robotics with the Boe-Bot
ACTIVITY #6: ADVANCED TOPIC—BUILDING COMPLEX MANEUVERS
IN EEPROM
When you download PBASIC program to your BASIC Stamp, the BASIC Stamp Editor
converts your program to numeric values called tokens. These tokens are what the
BASIC Stamp uses as instructions for executing the program. They are stored in one of
the two smaller black chips on top of your BASIC Stamp. This chip is a special type of
computer memory called EEPROM, which stands for electrically erasable programmable
read only memory (EEPROM). The BASIC Stamp’s EEPROM can hold 2048 bytes
(2 KB) of information. What’s not used for program storage (which builds from address
2047 toward address 0) can be used for data storage (which builds from address 0 toward
address 2047).
If the data you store in EEPROM collides with your program, the PBASIC program
won't execute properly.
EEPROM memory is different from RAM (random access memory) variable storage in
several respects:



EEPROM takes more time to store a value, sometimes up to several
milliseconds.
EEPROM can accept a finite number of write cycles, around 10 million writes.
RAM has unlimited read/write capabilities.
The primary function of the EEPROM is to store programs; data can be stored in
leftover space.
You can view the contents of the BASIC Stamp’s EEPROM in the BASIC Stamp Editor
by clicking Run and selecting Memory Map. Figure 4-6 shows the Memory Map for
MovementsWithSubroutines.bs2. Note the condensed EEPROM Map on the left side of
the figure. This shaded area in the small box at the bottom shows the amount of
EEPROM that MovementsWithSubroutines.bs2 occupies.
The Memory Map images shown in this activity were taken from the BASIC Stamp Editor
v2.1. If you are using a different version of the BASIC Stamp Editor, your memory map will
contain the same information, but it may be formatted differently.
Boe-Bot Navigation · Page 127
Figure 4-6
BASIC Stamp
Editor Memory Map
While we are here, note also that the counter variable we declared as a word is visible in
Register 0 of the RAM Map.
This program might have seemed large while you were typing it in, but it only takes up
136 of the available 2048 bytes of program memory. There currently is enough room for
quite a long list of instructions. Since a character occupies a byte in memory, there is
room for 1912 one-character direction instructions.
EEPROM Navigation
Up to this point we have tried three different programming approaches to make your BoeBot drive forward, turn left, turn right, and drive back again. Each technique has its
merits, but all would be cumbersome if you wanted your Boe-Bot to execute a longer,
more complex set of maneuvers. The upcoming program examples will use the nowfamiliar code blocks in subroutines for each basic maneuver. Each maneuver is given a
one-letter code as a reference. Long lists of these code letters can be stored in EEPROM
and then read and decoded during program execution. This avoids the tedium of
repeating long lists of subroutines, or having to change the variables before each GOSUB
command.
This programming approach requires some new PBASIC instructions: the DATA directive,
and READ and SELECT...CASE...ENDSELECT commands. Let’s take a look at each
before trying out an example program.
Page 128 · Robotics with the Boe-Bot
Each of the basic maneuvers is given a single letter code that will correspond to its
subroutine: F for Forward, B for Backward, L for Left_Turn, and R for Right_Turn.
Complex Boe-Bot movements can be quickly choreographed by making a string of these
code letters. The last letter in the string is a Q, which will mean “quit” when the
movements are over. The list is saved in EEPROM during program download with the
DATA directive, which looks like this:
DATA
"FLFFRBLBBQ"
Each letter is stored in a byte of EEPROM, beginning at address 0 (unless we tell it to
start somewhere else). The READ command can then be used to get this list back out of
EEPROM while the program is running. These values can be read from within a
DO…LOOP like this:
DO UNTIL (instruction = "Q")
READ address, instruction
address = address + 1
' PBASIC code block omitted here.
LOOP
The address variable is the location of each byte in EEPROM that is holding a code
letter. The instruction variable will hold the actual value of that byte, our code letter.
Notice that each time through the loop, the value of the address variable is increased by
one. This will allow each letter to be read from consecutive bytes in the EEPROM,
starting at address 0.
The DO…LOOP command has optional conditions that are handy for different
circumstances. The DO UNTIL (condition)...LOOP allows the loop to repeat until a
certain condition occurs. DO WHILE (condition)...LOOP allows the loop to repeat only
while a certain condition exists. Our example program will use DO…LOOP UNTIL
(condition). In this case, it causes the DO…LOOP to keep repeating until the character “Q”
is read from EEPROM.
A SELECT...CASE...ENDSELECT statement can be used to select a variable and evaluate
it on a case-by-case basis and execute code blocks accordingly. Here is the code block
that will look at each letter value held in the instruction variable and then call the
appropriate subroutine for each instance, or case, of a given letter.
SELECT instruction
CASE "F": GOSUB Forward
CASE "B": GOSUB Backward
Boe-Bot Navigation · Page 129
CASE "R": GOSUB Right_Turn
CASE "L": GOSUB Left_Turn
ENDSELECT
Here are these concepts, all together in a single program.
Example Program: EepromNavigation.bs2
 Carefully read the code instructions and comments in EepromNavigation.bs2 to
understand what each part of the program does.
 Enter, save, and run EepromNavigation.bs2.
'
'
'
'
Robotics with the Boe-Bot - EepromNavigation.bs2
Navigate using characters stored in EEPROM.
{$STAMP BS2}
' Stamp directive.
{$PBASIC 2.5}
' PBASIC directive.
DEBUG "Program Running!"
' -----[ Variables ]---------------------------------------------------------pulseCount
address
instruction
VAR
VAR
VAR
Word
Byte
Byte
' Stores number of pulses.
' Stores EEPROM address.
' Stores EEPROM instruction.
' -----[ EEPROM Data ]-------------------------------------------------------'
'
DATA
Address: 0123456789
||||||||||
"FLFFRBLBBQ"
' These two commented lines show
' EEPROM address of each datum.
' Navigation instructions.
' -----[ Initialization ]----------------------------------------------------FREQOUT 4, 2000, 3000
' Signal program start/reset.
' -----[ Main Routine ]------------------------------------------------------DO UNTIL (instruction = "Q")
READ address, instruction
address = address + 1
' Data at address in instruction.
' Add 1 to address for next read.
SELECT instruction
CASE "F": GOSUB Forward
CASE "B": GOSUB Backward
CASE "L": GOSUB Left_Turn
CASE "R": GOSUB Right_Turn
ENDSELECT
'
'
'
'
LOOP
Call a different subroutine
for each possible character
that can be fetched from
EEPROM.
Page 130 · Robotics with the Boe-Bot
END
' Stop executing until reset.
' -----[ Subroutine - Forward ]----------------------------------------------Forward:
FOR pulseCount = 1 TO 64
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
RETURN
'
'
'
'
'
Forward subroutine.
Send 64 forward pulses.
1.7 ms pulse to left servo.
1.3 ms pulse to right servo.
Pause for 20 ms.
' Return to Main Routine loop.
' -----[ Subroutine - Backward ]---------------------------------------------Backward:
FOR pulseCount = 1 TO 64
PULSOUT 13, 650
PULSOUT 12, 850
PAUSE 20
NEXT
RETURN
'
'
'
'
'
Backward subroutine.
Send 64 backward pulses.
1.3 ms pulse to left servo.
1.7 ms pulse to right servo.
Pause for 20 ms.
' Return to Main Routine loop.
' -----[ Subroutine - Left_Turn ]--------------------------------------------Left_Turn:
FOR pulseCount = 1 TO 24
PULSOUT 13, 650
PULSOUT 12, 650
PAUSE 20
NEXT
RETURN
'
'
'
'
'
Left turn subroutine.
Send 24 left rotate pulses.
1.3 ms pulse to left servo.
1.3 ms pulse to right servo.
Pause for 20 ms.
' Return to Main Routine loop.
' -----[ Subroutine – Right_Turn ]-------------------------------------------Right_Turn:
FOR pulseCount = 1 TO 24
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
RETURN
'
'
'
'
'
right turn subroutine.
Send 24 right rotate pulses.
1.7 ms pulse to left servo.
1.7 ms pulse to right servo.
Pause for 20 ms.
' Return to Main Routine section.
Did your Boe-Bot drive in a rectangle, going forward on the first two sides and
backwards on the second two? If it looked more like a trapezoid, you may want to adjust
the FOR...NEXT loop's EndValue arguments in the turning subroutines to make precise
90-degree turns.
Boe-Bot Navigation · Page 131
Your Turn
 With EepromNavigation.bs2 active in the BASIC Stamp Editor, click Run and
select Memory Map.
Your stored instructions will appear highlighted in blue at the beginning of the Detailed
EEPROM Map as shown in Figure 4-7. The numbers shown are the hexadecimal ASCII
codes that correspond to the characters you entered in your DATA statement.
Figure 4-7
Memory Map with
Stored Instructions
Visible in EEPROM
Map
 Click the Display ASCII checkbox near the lower left corner of the Memory Map
window.
Now the direction instructions will appear in a more familiar format shown in Figure 4-8.
Instead of ASCII codes, they appear as the actual characters you recorded using the DATA
directive.
Figure 4-8
Close-up of the
Detailed EEPROM
Map after Display
ASCII Box is
Checked
Page 132 · Robotics with the Boe-Bot
This program stored a total of 10 characters in EEPROM. These ten characters were
accessed by the READ command’s address variable. The address variable was declared
as a byte, so it can access up to 256 locations, well over the 10 we needed. If the
address variable is re-declared to be a word variable, you could theoretically access up
to 65535, far more locations than are available. Keep in mind that if your program gets
larger, the number of available EEPROM addresses for holding data gets smaller.
You can modify the existing data string to a new set of directions. You can also add
additional DATA statements. The data is stored sequentially, so the first character in the
second data string will get stored immediately after the last character in the first data
string.
 Try changing, adding, and deleting characters in the DATA directive, and rerunning the program. Remember that the last character in the DATA directive
should always be a “Q.”
 Modify the DATA directive to make your Boe-Bot perform the familiar forwardleft-right-backward sequence of movements.
 Try adding a second DATA directive. Remember to remove the “Q” from the end
of the first DATA directive and add it to the end of the second. Otherwise, the
program will execute only the commands in the first DATA directive.
Example Program – EepromNavigationWithWordValues.bs2
This next example program looks complicated at first, but it is a very efficient way to
design programs for custom Boe-Bot choreography. This example program uses
EEPROM data storage, but does not use subroutines. Instead, a single code block is
used, with variables in place of the FOR...NEXT loop's EndValue and PULSOUT Duration
arguments.
By default, the DATA directive stores bytes of information in EEPROM. To store wordsized data items, you can add the Word modifier to the DATA directive, before each data
item in your string. Each word-sized data item will use two bytes of EEPROM storage,
so the data will be accessed via every other address location. When using more than one
DATA directive, it is most convenient to assign a label to each one. This way, your READ
commands can refer to the label to retrieve data items without you having to figure out at
which EEPROM address each string of data items begins.
Take a look at this code
snippet:
Boe-Bot Navigation · Page 133
' addressOffset
Pulses_Count DATA
Pulses_Left DATA
Pulses_Right DATA
0
Word 64,
Word 850,
Word 650,
2
Word 24,
Word 650,
Word 650,
4
Word 24,
Word 850,
Word 850,
6
8
Word 64, Word 0
Word 650
Word 850
Each of the three DATA statements begins with its own label. The Word modifier goes
before each data item, and the items are separated by commas. These three strings of
data will be stored in EEPROM one after another. We don’t have to do the math to
figure out the address number of a given data item, because the labels and the
addressOffset variable will do that automatically. The READ command uses each label
to determine the EEPROM address where that string begins, and then adds the value of
the addressOffset variable to know how many address numbers to shift over to find the
correct DataItem. The DataItem found at the resulting Address will be stored in the READ
command's Variable argument. Notice that the Word modifier also comes before the
variable that stores the value fetched from EEPROM.
DO
READ Pulses_Count + addressOffset, Word pulseCount
READ Pulses_Left + addressOffset, Word pulseLeft
READ Pulses_Right + addressOffset, Word pulseRight
addressOffset = addressOffset + 2
' PBASIC code block omitted here.
LOOP UNTIL (pulseCount = 0)
The first time through the loop, addressOffset = 0. The first READ command will
retrieve a value of 64 from the first address at the Pulses_Count label, and place it in the
pulseCount variable. The second READ command retrieves a value of 850 from the first
address specified by the Pulses_Left label, and places it in the pulseLeft variable.
The third READ command retrieves a value of 650 from the first address specified by the
Pulses_Right label and places it in the pulseRight variable. Notice that these are the
three values in the “0” column of the code snippet above. When the value of those
variables are placed in the code block that follows, this:
FOR counter = 1 TO pulseCount
PULSOUT 13, pulseLeft
PULSOUT 12, pulseRight
PAUSE 20
NEXT
....becomes....
FOR counter = 1 TO 64
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
Page 134 · Robotics with the Boe-Bot
Do you recognize the basic maneuver generated by this code block?
 Look at the other columns of the code snippet on page 133 and anticipate what
the FOR…NEXT code block will look like on the second, third, and fourth times
through the loop.
 Look at the LOOP UNTIL (pulseCount = 0) statement in the program below.
The <> operator stands for “not equal to.” What will happen on the fifth time
through the loop?
 Enter, save, and run EepromNavigationWithWordValues.bs2.
' Robotics with the Boe-Bot - EepromNavigationWithWordValues.bs2
' Store lists of word values that dictate.
' {$STAMP BS2}
' {$PBASIC 2.5}
' Stamp directive.
' PBASIC directive.
DEBUG "Program Running!"
' -----[ Variables ]---------------------------------------------------------counter
pulseCount
addressOffset
instruction
pulseRight
pulseLeft
VAR
VAR
VAR
VAR
VAR
VAR
Word
Word
Byte
Byte
Word
Word
'
'
'
'
Stores
Stores
Stores
Stores
number of pulses.
offset from label.
EEPROM instruction.
servo pulse widths.
' -----[ EEPROM Data ]-------------------------------------------------------' addressOffset
Pulses_Count DATA
Pulses_Left DATA
Pulses_Right DATA
0
Word 64,
Word 850,
Word 650,
2
Word 24,
Word 650,
Word 650,
4
Word 24,
Word 850,
Word 850,
6
8
Word 64, Word 0
Word 650
Word 850
' -----[ Initialization ]----------------------------------------------------FREQOUT 4, 2000, 3000
' Signal program start/reset.
' -----[ Main Routine ]------------------------------------------------------DO
READ Pulses_Count + addressOffset, Word pulseCount
READ Pulses_Left + addressOffset, Word pulseLeft
READ Pulses_Right + addressOffset, Word pulseRight
addressOffset = addressOffset + 2
Boe-Bot Navigation · Page 135
FOR counter = 1 TO pulseCount
PULSOUT 13, pulseLeft
PULSOUT 12, pulseRight
PAUSE 20
NEXT
LOOP UNTIL (pulseCount = 0)
END
' Stop executing until reset.
Did your Boe-Bot perform the familiar forward-left-right-backwards movements? Are
you thoroughly bored with it by now? Do you want to see your Boe-Bot do something
else, or to choreograph your own routine?
Your Turn – Making Your Own Custom Navigation Routines
 Save EepromNavigationWithWordValues.bs2. under a new name.
 Replace the DATA directives with the ones below.
 Run the modified program and see what your Boe-Bot does.
Pulses_Count DATA Word
Word
Pulses_Left DATA Word
Word
Pulses_Right DATA Word
Word
60,
110,
850,
740,
650,
760,
Word
Word
Word
Word
Word
Word
80,
100,
800,
715,
700,
785,
Word
Word
Word
Word
Word
Word
100,
80,
785,
700,
715,
800,
Word
Word
Word
Word
Word
Word
110,
60,
760,
650,
740,
850,
Word
Word
Word
Word
Word
0
750,
750
750,
750
 Make a table with three rows, one for each DATA directive, and a column for each
Boe-Bot maneuver you want to make, plus one for the Word 0 item in the
Pulses_Count row.
 Use the table to plan out your Boe-Bot choreography, filling in the FOR...NEXT
loop's EndValue and PULSOUT Duration arguments you will need for each
maneuver’s code block.
 Modify your program with your newly charted DATA directives.
 Enter, save, and run your custom program. Did your Boe-Bot do what you
wanted it to do? Keep working on it until it does.
Page 136 · Robotics with the Boe-Bot
SUMMARY
This chapter introduced the basic Boe-Bot maneuvers: forward, backward, rotating in
place to turn to the right or left, and pivoting. The type of maneuver is determined by the
PULSOUT commands’ Duration arguments. How far the maneuver goes is determined by
the FOR…NEXT loop’s StartValue and EndValue arguments.
Chapter 2 included a hardware adjustment, physically centering the Boe-Bot’s servos
with a screwdriver. This chapter focused on fine tuning adjustments made by
manipulating the software. Specifically, a difference in rotation speed between the two
servos was compensated for by changing the PULSOUT command’s Duration argument for
the faster of the two servos. This changes the Boe-Bot’s path from a curve to a straight
line if the servos are not perfectly matched. To refine turning so that the Boe-Bot turns to
the desired angle, the StartValue and EndValue arguments of a FOR…NEXT loop can be
adjusted.
Programming the Boe-Bot to travel a pre-defined distance can be accomplished by
measuring the distance it travels in one second, with the help of a ruler. Using this
distance, and the number of pulses in one second of run time, you can calculate the
number of pulses required to cover a desired distance.
Ramping was introduced as a way to gradually accelerate and decelerate. It’s kinder to
the servos, and we recommended that you use your own ramping routines in place of the
abrupt start and stop routines shown in the example programs. Ramping is accomplished
by taking the same variable that’s used as the Counter argument in a FOR…NEXT loop and
adding it to or subtracting it from 750 in the PULSOUT command’s Duration argument.
Subroutines were introduced as a way to make pre-programmed maneuvers reusable by a
PBASIC program. Instead of writing an entire FOR…NEXT loop for each new maneuver, a
single subroutine that contains a FOR…NEXT loop can be executed as needed with the
GOSUB command. A subroutine begins with a label, and ends with the RETURN command.
A subroutine is called from the main program with a GOSUB command. When the
subroutine is finished and it encounters the RETURN command, the next command to be
executed is the one immediately following the GOSUB command.
The BASIC Stamp’s EEPROM stores the program it runs, but you can take advantage of
any unused portion of the program to store values. This is a great way to store custom
navigation routines. The DATA directive can store values in EEPROM. Bytes are stored
Boe-Bot Navigation · Page 137
by default, but adding the Word modifier to each data item allows you to store values up
to 65535 in two bytes’ worth of EEPROM memory space. You can read values back out
of EEPROM using the READ command. If you are retrieving a word-sized variable, make
sure to place a Word modifier before the variable that will receive the value that READ
fetches. SELECT…CASE was introduced as a way of evaluating a variable on a case by
case basis, and executing a different code block depending on the case. Optional
DO…LOOP conditions are helpful in certain circumstances; DO UNTIL (Condition)...LOOP
and DO...LOOP UNTIL (Condition) were demonstrated as ways to keep executing a
DO…LOOP until a particular condition is detected.
Questions
1. What direction does the left wheel have to turn to make the Boe-Bot go forward?
What direction does the right wheel have to turn?
2. When the Boe-Bot pivots to the left, what are the right and left wheels doing?
What PBASIC commands do you need to make the Boe-Bot pivot left?
3. If your Boe-Bot veers slightly to the left when you are running a program to
make it go straight ahead, how do you correct this? What command needs to be
adjusted and what kind of adjustment should you make?
4. If your Boe-Bot travels 11 in/s, how many pulses will it take to make it travel 36
inches?
5. What’s the relationship between a FOR…NEXT loop’s Counter argument and the
PULSOUT command’s Duration argument that makes ramping possible?
6. What directive can you use to pre-store values in the BASIC Stamp’s EEPROM
before running a program?
7. What command can you use to retrieve a value stored in EEPROM and copy it to
a variable?
8. What code block can you use to select a particular variable and evaluate it on a
case by case basis and execute a different code block for each case?
9. What are the different conditions that can be used with DO…LOOP?
Exercises
1. Write a routine that makes the Boe-Bot back up for 350 pulses.
2. Let’s say that you tested your servos and discovered that it takes 48 pulses to
make a 180° turn with right-rotate. With this information, write routines to make
the Boe-Bot perform 30, 45, and 60 degree turns.
3. Write a routine that makes the Boe-Bot go straight forward, then ramp in and out
of a pivoting turn, and then continue straight forward.
Page 138 · Robotics with the Boe-Bot
Projects
1. It is time to fill in column 3 of Table 2-1 on page 63. To do this, modify the
PULSOUT Duration arguments in the program BoeBotForwardThreeSeconds.bs2
using each pair of values from column 1. Record your Boe-Bot’s resultant
behavior for each pair in column 3. Once completed, this table will serve as a
reference guide when you design your own custom Boe-Bot maneuvers.
2. Figure 4-9 shows two simple courses. Write a program that will make your
Boe-Bot navigate along each figure. Assume straight line distances (including
the diameter of the circle) to be either 1 yd or 1 m.
Figure 4-9
Simple Courses
Solutions
Q1. Left wheel counterclockwise, right wheel clockwise.
Q2. The right wheel is turning clockwise (forward), and the left wheel is not moving.
PULSOUT 13, 750
PULSOUT 12, 650
Q3. You can slow down the right wheel to correct a veer to the left. The PULSOUT
command for the right wheel needs to be adjusted.
PULSOUT 12, 650
Adjust the 650 to something closer to 750 to slow the wheel down.
PULSOUT 12, 663
Q4. Given the data below, it should take 133 pulses to travel 36 inches:
Boe-Bot speed = 11 in/s
Boe-Bot distance = 36 in/s
pulses = (Boe-Bot distance / Boe-Bot speed) * (40.65 pulses / s)
= (36 / 11 ) * (40.65)
= 133.04
= 133
Boe-Bot Navigation · Page 139
Q5. The FOR…NEXT loop's pulseCount variable can be used as an offset (plus or
minus) to 750 (the center position) in the Duration argument.
FOR pulseCount = 1 to 100
PULSOUT 13, 750 + pulseCount
PULSOUT 12, 750 – pulseCount
PAUSE 20
NEXT
Q6. The DATA directive.
Q7. The READ command
Q8. SELECT...CASE...ENDSELECT.
Q9. UNTIL and WHILE.
E1. FOR counter = 1 to 350
' Backward
PULSOUT 13, 650
PULSOUT 12, 850
PAUSE 20
NEXT
E2. FOR counter = 1 to 8
' Rotate right 30 degrees
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
FOR counter = 1 to 12
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
' Rotate right 45 degrees
FOR counter = 1 to 16
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
' Rotate right 60 degrees
E3. FOR counter = 1 to 100
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
' Forward
Page 140 · Robotics with the Boe-Bot
FOR counter = 0 TO 30
' Ramping pivot turn
PULSOUT 13, 750 + counter
PULSOUT 12, 750
PAUSE 20
NEXT
FOR counter = 30 TO 0
PULSOUT 13, 750 + counter
PULSOUT 12, 750
PAUSE 20
NEXT
FOR counter = 1 to 100
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
' Forward
P1.
P13
P12
Description
Behavior
850
650
Full Speed: P13 CCW, P12 CW
Forward
650
850
Full Speed: P13 CW, P12 CCW
Backward
850
850
Full Speed: P13 CCW, P12 CCW
Right rotate
650
650
Full Speed: P13 CW, P12 CW
Left rotate
750
850
P13 Stopped, P12 CCW Full speed
Pivot back left
650
750
P13 CW Full Speed, P12 Stopped
Pivot back right
750
750
P13 Stopped, P12 Stopped
Stopped
Forward slow
760
740
P13 CCW Slow, P12 CW Slow
770
730
P13 CCW Med, P12 CW Med
Forward medium
850
700
P13 CCW Full Speed, P12 CW Medium
Veer right
800
650
P13 CCW Medium, P12 CW Full Speed
Veer left
P2. The circle can be implemented by veering right continuously. Trial and error, a
yard or meter stick, will help you arrive at the right PULSOUT value. Circle with a
one-yard diameter:
' Robotics with the Boe-Bot - Chapter 4 - Circle.bs2
' Boe-Bot navigates a circle of 1 yard diameter.
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "Program running!"
Boe-Bot Navigation · Page 141
pulseCount
VAR
Word
FREQOUT 4, 2000, 3000
' Pulse count to servos
' Signal program start/reset.
' -----[ Main Routine ]-----------------------------------------------Main:
DO
PULSOUT 13, 850
' Veer right
PULSOUT 12, 716
PAUSE 20
LOOP
To make the triangle, first calculate the number of pulses required for a one
meter or yard straight line, as in Question 4. Then fine-tune your distances to
match your Boe-Bot and particular surface. For a triangle pattern, the Boe-Bot
must travel 1 meter/yard forward, and then make a 120 degree turn. This should
be repeated three times for the three sides of the triangle. You may have to adjust
the pulseCount EndValue in the Right_Rotate120 subroutine to get a precise
120 degree turn.
' Robotics with the Boe-Bot - Chapter 4 - Triangle.bs2
' Boe-Bot navigates triangle shape with 1 yard sides.
' Go forward, then turn 120 degrees. Repeat three times.
'{$STAMP BS2}
'{$PBASIC 2.5}
DEBUG "Program running!"
counter
pulseCount
VAR
VAR
Nib
Word
FREQOUT 4, 2000, 3000
Main:
FOR counter = 1 TO 3
GOSUB Forward
GOSUB Right_Rotate120
NEXT
END
Forward:
FOR pulseCount = 1 TO 163
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
NEXT
RETURN
' Triangle has 3 sides
' Pulse count to servos
' Signal program start/reset.
' Repeat 3 times for triangle
' Forward 1 yard
Page 142 · Robotics with the Boe-Bot
Right_Rotate120:
FOR pulseCount = 1 TO 21
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
RETURN
' Rotate right 120 degrees
Tactile Navigation with Whiskers · Page 143
Chapter 5: Tactile Navigation with Whiskers
Many types of robotic machinery rely on a variety of tactile switches. For example, a
tactile switch may detect when a robotic arm has encountered an object. The robot can
be programmed to pick up the object and place it elsewhere. Factories use tactile
switches to count objects on a production line, and also for aligning objects during
industrial processes. In all these instances, the switches provide inputs that dictate some
other form of programmed output. The inputs are electronically monitored by the
product, be it a robot, or a calculator, or a production line. Based on the state of the
switches, the robot arm grabs an object, or the calculator display updates, or the factory
production line reacts with motors or servos to guide products.
In this chapter, you will build tactile switches, called whiskers, onto your Boe-Bot and
test them. You will then program the Boe-Bot to monitor the state of these switches, and
to decide what to do when it encounters an obstacle. The end result will be autonomous
navigation by touch.
TACTILE NAVIGATION
The whiskers are so named because that is what these bumper switches look like, though
some argue they look more like antennae. At any rate, these whiskers are shown
mounted on a Boe-Bot in Figure 5-1. Whiskers give the Boe-Bot the ability to sense the
world around it through touch, much like the antennae on an ant or the whiskers on a cat.
The activities in this chapter use the whiskers by themselves, but they can also be
combined with other sensors you will learn about in later chapters to increase your BoeBot’s functionality.
Page 144 · Robotics with the Boe-Bot
Figure 5-1
Boe-Bot with Whiskers
ACTIVITY #1: BUILDING AND TESTING THE WHISKERS
Before moving on to programs that make the Boe-Bot navigate based on what it can
touch, it’s essential to build and test the whiskers first. This activity will guide you
through building and testing the whiskers.
Whisker Circuit and Assembly
 Gather the whiskers hardware shown in Figure 5-2.
 Disconnect power from your board and servos.
Tactile Navigation with Whiskers · Page 145
Parts List
(2) Whisker wires
(2) 7/8″ pan head 4-40
Phillips screws
(2) ½″ round spacer
(2) Nylon washers, size #4
(2) 3-pin m/m headers
(2) Resistors, 220 Ω
(red-red-brown)
(2) Resistors, 10 kΩ
(brown-black-orange)
Figure 5-2
Whiskers
Hardware
Building the Whiskers




Remove the two front screws that hold your board to the front standoffs.
Refer to Figure 5-3 while following the remaining instructions.
Thread a nylon washer and then a ½″ round spacer on each of the 7/8″ screws.
Attach the screws through the holes in your board and into the standoffs below,
but do not tighten them all the way yet.
 Slip the hooked ends of the whisker wires around the screws, one above a
washer and the other below a washer, positioning them so they cross over each
other without touching.
 Tighten the screws into the standoffs.
Whisker
below
washer
Whisker
above
washer
Board of Education / HomeWork Board
Figure 5-3
Mounting the Whiskers
Page 146 · Robotics with the Boe-Bot
The next step is add the whiskers circuit shown in Figure 5-4 to the piezospeaker and
servo circuits you built and tested in Chapter 2 and Chapter 3.
 If you have a Board of Education, build the whiskers circuit shown in Figure 5-4
using the wiring diagram in Figure 5-5 on page 147 as a reference.
 If you have a HomeWork Board, build the whiskers circuit shown in Figure 5-4
using the wiring diagram in Figure 5-6 on page 148 as a reference.
 Make sure to adjust each whisker so that it is close to, but not touching, the 3-pin
header on the breadboard. A distance of about 1/8″ (3 mm) is a recommended
starting point.
Vdd
Vdd
10 k
10 k
P7
220 
Figure 5-4
Whiskers Schematic
P5
220 
Right
Whisker
Vss
Left
Whisker
Vss
Tactile Navigation with Whiskers · Page 147
Figure 5-5: Whisker Wiring Diagram for the Board of Education
Left
Whisker
To Servos
15 14 Vdd 13 12
Red
Black
X4
Vdd
X5
Vin
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
+
Board of Education
Rev C
© 2000-2003
Right
Whisker
Use the 220 Ω resistors (red-red-brown color codes) to connect P5 and P7 to their
corresponding 3-pin headers. Use the 10 kΩ resistors (brown-black-orange color
codes) to connect Vdd to each 3-pin header.
Page 148 · Robotics with the Boe-Bot
Figure 5-6: Whisker Wiring Diagram for the HomeWork Board
Left
Whisker
To Servos
(916) 624-8333
Rev B
www.parallax.com
www.stampsinclass.com
Vdd
Vin
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
+
HomeWork Board
Right
Whisker
Use the 220 Ω resistors (red-red-brown color codes) to connect P5 and P7 to their
corresponding 3-pin headers. Use the 10 kΩ resistors (brown-black-orange color
codes) to connect Vdd to each 3-pin header.
Tactile Navigation with Whiskers · Page 149
Testing the Whiskers
Take a second look at the whiskers schematic (Figure 5-7). Each whisker is both the
mechanical extension and the ground electrical connection of a normally open, singlepole, single-throw switch. The reason the whiskers are connected to ground (Vss) is
because the plated holes at the outer edge of the board are all connected to Vss. This is
true for both the Board of Education and the BASIC Stamp HomeWork Board. The
metal standoffs and screw provide the electrical connection to each whisker.
Vdd
Vdd
10 k
10 k
P7
220 
P5
220 
Right
Whisker
Vss
Left
Whisker
Figure 5-7
Whiskers Schematic
A Second Look
Vss
The BASIC Stamp can be programmed to detect when a whisker is pressed. I/O pins
connected to each switch circuit monitor the voltage at the 10 kΩ pull-up resistor.
Figure 5-8 illustrates how this works. When a given whisker is not pressed, the voltage at
the I/O pin connected to that whisker is 5 V. When a whisker is pressed, the I/O line is
shorted to ground (Vss), so the I/O line sees 0 V.
All I/O pins default to input every time a PBASIC program starts. This means that the
I/O pins connected to the whiskers will function as inputs automatically. As an input, an
I/O pin connected to a whisker circuit will cause its input register to store a 1 if the
voltage is 5 V (whisker not pressed) or a 0 if the voltage is 0 V (whisker pressed). The
Debug Terminal can be used to display these values.
Page 150 · Robotics with the Boe-Bot
How do you get the BASIC Stamp to tell you whether it’s reading a 1 or 0?
Because the circuit is connected to P7, this 1 or 0 value will appear in a variable named
IN7. IN7 is called an input register. Input register variables are built-in and do not have to
be declared in the beginning of your program. You can see the value this variable is storing
by using the command DEBUG BIN1 IN7. The BIN1 is a formatter that tells the Debug
Terminal to display one binary digit (either 1 or 0).
Figure 5-8
Detecting
Electrical
Contacts
Example Program: TestWhiskers.bs2
This next example program is designed to test the whiskers to make sure they are
functioning properly. By displaying the binary digits stored in the P7 and P5 input
registers (IN7 and IN5), the program will show you whether the BASIC Stamp detects
contact with a whisker. When the value stored in a given input register is 1, the whisker
is not pressed. When it is 0, the whisker is pressed.
 Reconnect power to your board and servos.
 Enter, save, and run TestWhiskers.bs2.
 This program makes use of the Debug Terminal, so leave the programming cable
connected to the BASIC Stamp while the program is running.
'
'
'
'
Robotics with the Boe-Bot - TestWhiskers.bs2
Display what the I/O pins connected to the whiskers sense.
{$STAMP BS2}
' Stamp directive.
{$PBASIC 2.5}
' PBASIC directive.
Tactile Navigation with Whiskers · Page 151
DEBUG "WHISKER STATES", CR,
"Left
Right", CR,
"-----------"
DO
DEBUG CRSRXY, 0, 3,
"P5 = ", BIN1 IN5,
"
P7 = ", BIN1 IN7
PAUSE 50
LOOP
 Note the values displayed in the Debug Terminal; it should display that both P7
and P5 are equal to 1.
 Check Figure 5-5 on page 147 (or Figure 5-6 on page 148) so you know which r
is the “left whisker” and which is the “right whisker.”
 Press the right whisker into its three-pin header, and note the values displayed in
the Debug Terminal. It should now read:
P5 = 1 P7 = 0
 Press the left whisker into its three-pin header, and note the value displayed in
the Debug Terminal again. This time it should read:
P5 = 0 P7 = 1
 Press both whiskers against both three-pin headers. Now it should read
P5 = 0 P7 = 0
 If the whiskers passed all these tests, you’re ready to move on; otherwise, check
your program and circuits for errors.
What is a Cursor? What is CRSRXY?
According to Merriam-Webster online dictionary, a cursor is: “A moveable item used to mark
a position as…a visual cue on a video display that indicates position.” As you move your
mouse, the pointer that moves on your screen is a cursor. The Debug Terminal’s cursor is
somewhat different because it doesn’t flash or do anything to indicate its position. But,
wherever the Debug Terminal’s cursor is, that’s where the next character gets printed.
It is a formatter that allows you to conveniently arrange information your program sends to
the Debug Terminal. The formatter CRSRXY 0, 3, in the command:
DEBUG CRSRXY, 0, 3,
"P5 = ", BIN1 IN5,
"
P7 = ", BIN1 IN7
...places the cursor at column 0, row 3 in the Debug Terminal. This makes it display nicely
below the “Whisker States” table heading. Each time through the loop, the new values
overwrite the old values because the cursor keeps going back to the same place.
Page 152 · Robotics with the Boe-Bot
ACTIVITY #2: FIELD TESTING THE WHISKERS
Assume that you may have to test the whiskers at some later time away from a computer.
Since the Debug Terminal won’t be available, what can you do? One solution would be
to program the BASIC Stamp so that it sends an output signal that corresponds to the
input signal it’s receiving. This can be done with a pair of LED circuits and a program
that turns the LEDs on and off based on the whisker inputs.
Parts List:
(2) Resistors, 220 Ω (red-red-brown)
(2) LEDs, red
Building the LED Whisker Testing Circuits
 Disconnect power from your board and servos.
 If you have a Board of Education, add the circuit shown in Figure 5-9 with the
help of the wiring diagram in Figure 5-10 (page 153).
 If you have a HomeWork Board, add the circuit shown in Figure 5-9 with the
help of the wiring diagram in Figure 5-11 (page 154).
P10
220 
Figure 5-9
LED Whisker Testing
Schematic
P1
220 
LED
Vss
LED
Add these LED circuits.
Vss
Remember that an LED is a one way current valve.
If it is plugged in backwards, it will not let current pass through, and so will not emit light.
For the LED to emit light when the BASIC Stamp sends a high signal, the LED's anode must
be connected to the 220 Ω resistor, and its cathode must be connected to Vss. See Figure
5-10 or Figure 5-11.
Tactile Navigation with Whiskers · Page 153
Figure 5-10 Whisker Plus LED Wiring Diagram for the Board of Education
Left
Whisker
To Servos
15 14 Vdd 13 12
This
lead
is the
anode
Red
Black
X4
Vdd
X5
Vin
Vss
Flat spot on
plastic case
indicates
cathode.
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
+
Board of Education
Rev C
© 2000-2003
This lead is
the anode.
Right
Whisker
Page 154 · Robotics with the Boe-Bot
Figure 5-11 Whisker Plus LED Wiring Diagram for the HomeWork Board
Left
Whisker
To Servos
(916) 624-8333
Rev B
www.parallax.com
www.stampsinclass.com
Vdd
Vin
Vss
X3
Connect P15
P14
the
P13
anode P12
to the P11
P10
220 Ω P9
resistor. P8
P7
P6
P5
P4
P3
P2
P1
P0
+
Flat spot on
plastic case
indicates
cathode
X2
HomeWork Board
The anode
connects to
the 220 Ω
resistor.
Right
Whisker
Programming the LED Whisker Testing Circuits
 Reconnect power to your board.
 Save TestWhiskers.bs2 as TestWhiskersWithLeds.bs2.
 Insert these two IF...THEN statements between the PAUSE 50 and LOOP
commands.
IF (IN7 = 0) THEN
HIGH 1
ELSE
LOW 1
ENDIF
Tactile Navigation with Whiskers · Page 155
IF (IN5 = 0) THEN
HIGH 10
ELSE
LOW 10
ENDIF
These are called IF…THEN statements, and they will be more fully introduced in the next
activity. These statements are used to make decisions in PBASIC. The first of the two
IF…THEN statements sets P1 high, which turns the LED on when the whisker connected to
P7 is pressed (IN7 = 0). The ELSE portion of the statement makes P1 go low, which turns
the LED off when the whisker is not pressed. The second IF…THEN statement does the
same thing for the whisker connected to P5 and the LED connected to P10.
 RunTestWhiskersWithLeds.bs2.
 Test the program by gently pressing the whiskers. The red LEDs should light up
when each whisker has made contact with its 3-pin header.
ACTIVITY #3: NAVIGATION WITH WHISKERS
In the previous activity, the BASIC Stamp was programmed to detect whether a given
whisker was pressed. In this activity, the BASIC Stamp will be programmed to take
advantage of this information to guide the Boe-Bot. When the Boe-Bot is rolling along
and a whisker is pressed, it means the Boe-Bot bumped into something. A navigation
program needs to take this input, decide what it means, and call a set of maneuvers that
will make the Boe-Bot back up from the obstacle, turn, and go in a different direction.
Programming the Boe-Bot to Navigate Based on Whisker Inputs
This next program makes the Boe-Bot go forward until it encounters an obstacle. In this
case, the Boe-Bot knows when it encounters an obstacle by bumping into it with one or
both of its whiskers. As soon as the obstacle is detected by the whiskers, the navigation
routines and subroutines developed in Chapter 4 will make the Boe-Bot back up and turn.
Then, the Boe-Bot resumes forward motion until it bumps into another obstacle.
In order to do that, the Boe-Bot needs to be programmed to make decisions. PBASIC has
a command called an IF…THEN statement that makes decisions. The syntax for IF…THEN
statements is:
IF (condition) THEN…{ELSEIF (condition)}…{ELSE}…ENDIF
Page 156 · Robotics with the Boe-Bot
The “…” means you can place a code block (one or more commands) between the
keywords. The next example program makes decisions based on the whisker inputs, and
then calls subroutines to make the Boe-Bot take action. The subroutines are similar to the
ones you developed in Chapter 4. Here is how IF…THEN is used.
IF (IN5 = 0) AND (IN7 = 0) THEN
GOSUB Back_Up
' Both whiskers detect obstacle,
GOSUB Turn_Left
' back up & U-turn (left twice)
GOSUB Turn_Left
ELSEIF (IN5 = 0) THEN
' Left whisker contacts
GOSUB Back_Up
' Back up & turn right
GOSUB Turn_Right
ELSEIF (IN7 = 0) THEN
' Right whisker contacts
GOSUB Back_Up
' Back up & turn left
GOSUB Turn_Left
ELSE
' Both whiskers 1, no contacts
GOSUB Forward_Pulse
' Apply a forward pulse &
ENDIF
' check again
Example Program: RoamingWithWhiskers.bs2
This program demonstrates one way of evaluating the whisker inputs and deciding which
navigation subroutine to call using IF…THEN.
 Reconnect power to your board and servos.
 Enter, save, and run RoamingWithWhiskers.bs2.
 Try letting the Boe-Bot roam. When it contacts obstacles in its path, it should
back up, turn, and then roam in a new direction.
' -----[ Title ]-------------------------------------------------------------' Robotics with the Boe-Bot - RoamingWithWhiskers.bs2
' Boe-Bot uses whiskers to detect objects, and navigates around them.
' {$STAMP BS2}
' {$PBASIC 2.5}
' Stamp directive.
' PBASIC directive.
DEBUG "Program Running!"
' -----[ Variables ]---------------------------------------------------------pulseCount
VAR
Byte
' FOR...NEXT loop counter.
' -----[ Initialization ]----------------------------------------------------FREQOUT 4, 2000, 3000
' Signal program start/reset.
Tactile Navigation with Whiskers · Page 157
' -----[ Main Routine ]------------------------------------------------------DO
IF (IN5 = 0) AND (IN7 = 0) THEN
GOSUB Back_Up
GOSUB Turn_Left
GOSUB Turn_Left
ELSEIF (IN5 = 0) THEN
GOSUB Back_Up
GOSUB Turn_Right
ELSEIF (IN7 = 0) THEN
GOSUB Back_Up
GOSUB Turn_Left
ELSE
GOSUB Forward_Pulse
ENDIF
LOOP
' Both whiskers detect obstacle
' Back up & U-turn (left twice)
' Left whisker contacts
' Back up & turn right
' Right whisker contacts
' Back up & turn left
' Both whiskers 1, no contacts
' Apply a forward pulse
' and check again
' -----[ Subroutines ]-------------------------------------------------------Forward_Pulse:
PULSOUT 13,850
PULSOUT 12,650
PAUSE 20
RETURN
' Send a single forward pulse.
Turn_Left:
FOR pulseCount = 0 TO 20
PULSOUT 13, 650
PULSOUT 12, 650
PAUSE 20
NEXT
RETURN
' Left turn, about 90-degrees.
Turn_Right:
FOR pulseCount = 0 TO 20
PULSOUT 13, 850
PULSOUT 12, 850
' Right turn, about 90-degrees.
PAUSE 20
NEXT
RETURN
Back_Up:
FOR pulseCount = 0 TO 40
PULSOUT 13, 650
PULSOUT 12, 850
PAUSE 20
NEXT
RETURN
' Back up.
Page 158 · Robotics with the Boe-Bot
How Roaming with Whiskers Works
The IF...THEN statements in the Main Routine section first check the whiskers for any
states that require attention. If both whiskers are pressed (IN5 = 0 and IN7 = 0), a U-turn
is executed by calling the Back_Up subroutine followed by calling the Turn_Left
subroutine twice in a row. If just the left whisker is pressed (IN5 = 0), then the program
calls the Back_Up subroutine followed by the Turn_Right subroutine. If the right
whisker is pressed (IN7 = 0), the Back_Up subroutine is called, followed by the
Turn_Left subroutine. The only possible combination that has not been covered is if
neither whisker is pressed (IN5 = 1 and IN7 = 1). The ELSE command calls the
Forward_Pulse subroutine in this case.
IF (IN5 = 0) AND (IN7 = 0) THEN
GOSUB Back_Up
GOSUB Turn_Left
GOSUB Turn_Left
ELSEIF (IN5 = 0) THEN
GOSUB Back_Up
GOSUB Turn_Right
ELSEIF (IN7 = 0) THEN
GOSUB Back_Up
GOSUB Turn_Left
ELSE
GOSUB Forward_Pulse
ENDIF
The Turn_Left, Turn_Right, and Back_Up subroutines should look fairly familiar, but
the Forward_Pulse subroutine has a twist. It just sends one pulse, then returns. This is
really important, because it means the Boe-Bot can check its whiskers between each
forward pulse. That means the Boe-Bot checks for obstacles roughly 40 times per second
as it travels forward.
Forward_Pulse:
PULSOUT 12,650
PULSOUT 13,850
PAUSE 20
RETURN
Since each full speed forward pulse makes the Boe-Bot roll around half a centimeter, it’s
a really good idea to only send one pulse, then go back and check the whiskers again.
Since the IF…THEN statement is inside a DO…LOOP, each time the program returns from a
Tactile Navigation with Whiskers · Page 159
Forward_Pulse, it gets to LOOP, which sends the program back up to DO. What happens
then? The IF…THEN statement checks the whiskers all over again.
Your Turn
The FOR...NEXT loop EndValue arguments in the Back_Right and Back_Left routines
can be adjusted for more or less turn, and the Back_Up routine can have its EndValue
adjusted to back up less for navigation in tighter spaces.
 Experiment with the FOR...NEXT loop EndValue arguments in the navigation
routines in RoamingWithWhiskers.bs2.
You can also modify your IF…THEN statements to make the LED indicators from the
previous activity broadcast what maneuver the Boe-Bot is in by adding HIGH and LOW
commands to control the LED circuits. Here is an example.
IF (IN5 = 0) AND (IN7 = 0) THEN
HIGH 10
HIGH 1
GOSUB Back_Up
GOSUB Turn_Left
GOSUB Turn_Left
ELSEIF (IN5 = 0) THEN
HIGH 10
GOSUB Back_Up
GOSUB Turn_Right
ELSEIF (IN7 = 0) THEN
HIGH 1
GOSUB Back_Up
GOSUB Turn_Left
ELSE
LOW 10
LOW 1
GOSUB Forward_Pulse
ENDIF
 Modify the IF…THEN statement in RoamingWithWhiskers.bs2 to make the BoeBot broadcast its maneuver using the LED indicators.
Page 160 · Robotics with the Boe-Bot
ACTIVITY #4: ARTIFICIAL INTELLIGENCE AND DECIDING WHEN
YOU’RE STUCK
You may have noticed that the Boe-Bot gets stuck in corners. As the Boe-Bot enters the
corner, its whisker touches the wall on the left, so it turns right. When the Boe-Bot
moves forward again, its right whisker bumps the wall on the right, so it turns left. Then
it turns and bumps the left wall again, and the right wall again, and so on, until somebody
rescues it from its predicament.
Programming to Escape Corners
RoamingWithWhiskers.bs2 can be modified to detect this problem and act upon it. The
trick is to count the number of times that alternate whiskers are contacted. One important
thing about this trick is that the program has to remember what state each whisker was in
during the previous contact. It has to compare that to the whisker states of the current
contact. If they are opposite, then add one to the counter. If the counter goes over a
threshold that you (the programmer) have determined, then, it’s time to do a U-turn and
reset that alternate whisker counter.
This next program also relies on the fact that you can “nest” IF…THEN statements. In
other words, the program checks for one condition, and if that condition is true, it checks
for another condition within the first condition. Here is a pseudo code example of how
it can be used.
IF condition1 THEN
Commands for condition1
IF condition2 THEN
Commands for both condition2 and condition1
ELSE
Commands for condition1 but not condition2
ENDIF
ELSE
Commands for not condition1
ENDIF
There is an example of nested IF…THEN statements in the routine that detects consecutive
alternate whisker contacts in the next program.
Example Program: EscapingCorners.bs2
This program will cause your Boe-Bot to execute a U-turn at either the fourth or fifth
alternate corner, depending on which whisker was pressed first.
Tactile Navigation with Whiskers · Page 161
 Enter, save, and run EscapingCorners.bs2.
 Test this program by pressing alternate whiskers as the Boe-Bot roams.
Depending on which whisker you started with, the Boe-Bot should execute its UTurn maneuver after either the fourth or fifth consecutive whisker press.
'
'
'
'
'
-----[ Title ]-------------------------------------------------------------Robotics with the Boe-Bot - EscapingCorners.bs2
Boe-Bot navigates out of corners by detecting alternating whisker presses.
{$STAMP BS2}
' Stamp directive.
{$PBASIC 2.5}
' PBASIC directive.
DEBUG "Program Running!"
' -----[ Variables ]---------------------------------------------------------pulseCount
counter
old7
old5
VAR
VAR
VAR
VAR
Byte
Nib
Bit
Bit
'
'
'
'
FOR...NEXT loop counter.
Counts alternate contacts.
Stores previous IN7.
Stores previous IN5.
' -----[ Initialization ]----------------------------------------------------FREQOUT 4, 2000, 3000
counter = 1
old7 = 0
old5 = 1
' Signal program start/reset.
' Start alternate corner count.
' Make up old values.
' -----[ Main Routine ]------------------------------------------------------DO
' --- Detect Consecutive Alternate Corners -----------------------' See the "How EscapingCorners.bs2 Works" section that follows this program.
IF (IN7 <> IN5) THEN
IF (old7 <> IN7) AND (old5 <> IN5) THEN
counter = counter + 1
old7 = IN7
old5 = IN5
IF (counter > 4) THEN
counter = 1
GOSUB Back_Up
GOSUB Turn_Left
GOSUB Turn_Left
ENDIF
ELSE
counter = 1
ENDIF
ENDIF
'
'
'
'
'
'
'
'
One or other is pressed.
Different from previous.
Alternate whisker count + 1.
Record this whisker press
for next comparison.
If alternate whisker count = 4,
reset whisker counter
and execute a U-turn.
'
'
'
'
'
'
ENDIF counter > 4.
ELSE (old7=IN7) or (old5=IN5),
not alternate, reset counter.
ENDIF (old7<>IN7) and
(old5<>IN5).
ENDIF (IN7<>IN5).
Page 162 · Robotics with the Boe-Bot
' ---
Same navigation routine from RoamingWithWhiskers.bs2 ------------------
IF (IN5 = 0) AND (IN7 = 0) THEN
GOSUB Back_Up
GOSUB Turn_Left
GOSUB Turn_Left
ELSEIF (IN5 = 0) THEN
GOSUB Back_Up
GOSUB Turn_Right
ELSEIF (IN7 = 0) THEN
GOSUB Back_Up
GOSUB Turn_Left
ELSE
GOSUB Forward_Pulse
ENDIF
' Both whiskers detect obstacle
' Back up & U-turn (left twice)
' Left whisker contacts
' Back up & turn right
' Right whisker contacts
' Back up & turn left
' Both whiskers 1, no contacts
' Apply a forward pulse
' and check again
LOOP
' -----[ Subroutines ]-------------------------------------------------------Forward_Pulse:
PULSOUT 13,850
PULSOUT 12,650
PAUSE 20
RETURN
' Send a single forward pulse.
Turn_Left:
FOR pulseCount = 0 TO 20
PULSOUT 13, 650
PULSOUT 12, 650
PAUSE 20
NEXT
RETURN
' Left turn, about 90-degrees.
Turn_Right:
FOR pulseCount = 0 TO 20
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
RETURN
Back_Up:
FOR pulseCount = 0 TO 40
PULSOUT 13, 650
PULSOUT 12, 850
PAUSE 20
NEXT
RETURN
' Right turn, about 90-degrees.
' Back up.
Tactile Navigation with Whiskers · Page 163
How EscapingCorners.bs2 Works
Since this program is a modified version of RoamingWithWhiskers.bs2, only new
features related to detecting and escaping corners are discussed here.
Three extra variables are created for detecting a corner. The nibble variable counter can
store a value between 0 and 15. Since our target value for detecting a corner is 4, the size
of the variable is reasonable. Remember that a bit variable can store a single bit, either a
1 or a 0. The next two variables (old7 and old5) are both bit variables. These are also
the right size for the job since they are used to store old values of IN7 and IN5, which are
also bit variables.
counter
old7
old5
VAR
VAR
VAR
Nib
Bit
Bit
These variables have to be initialized (given initial values). For the sake of making the
program easier to read, counter is set to 1, and when it gets to 4 due to the fact that the
Boe-Bot is stuck in a corner, it is reset to 1. The old7 and old5 variables have to be set
so that it looks like one of the two whiskers was pressed some time before the program
started. This has to be done because the routine for detecting alternate corners always
compares an alternating pattern, either (IN5 = 1 and IN7 = 0) or (IN5 = 0 and IN7 = 1).
Likewise, old5 and old7 have to be different from each other.
counter = 1
old7 = 0
old5 = 1
Now we get to the Detect Consecutive Alternate Corners section. The first thing we want
to check for is if one or the other whisker is pressed. A simple way to do this is to ask “is
IN7 different from IN5?” In PBASIC, we can use the not-equal operator <> in an IF
statement:
IF (IN7 <> IN5) THEN
If it is indeed one whisker that is pressed, the next thing to check for is whether or not it’s
the exact opposite pattern as the previous time. In other words, is (old7 <> IN7) and is
(old5 <> IN5)? If that’s true, then, it’s time to add one to the counter that tracks
alternate whisker contacts. It’s also time to remember the current whisker pattern by
setting old7 equal to the current IN7 and old5 equal to the current IN5.
Page 164 · Robotics with the Boe-Bot
IF (old7 <> IN7) AND (old5 <> IN5) THEN
counter = counter + 1
old7 = IN7
old5 = IN5
If it turns out that this is the fourth consecutive whisker contact, then it’s time to reset
counter to 1 and execute a U-turn.
IF (counter > 4) THEN
counter = 1
GOSUB Back_Up
GOSUB Turn_Left
GOSUB Turn_Left
This ENDIF ends the code block that is executed if counter > 4.
ENDIF
This ELSE statement is connected to the IF (old7 <> IN7) AND (old5 <> IN5)
THEN statement. The ELSE statement covers what happens if the IF statement is not
true. In other words, it must not be an alternate whisker that was pressed, so reset
counter because the Boe-Bot is not stuck in a corner.
ELSE
counter = 1
This ENDIF statement ends the decision making process for the IF (old7 <> IN7)
AND (old5 <> IN5) THEN statement.
ENDIF
ENDIF
The remainder of the program is the same as before.
Your Turn
One of the IF...THEN statements in EscapingCorners.bs2 checks to see if counter has
reached 4.
 Try increasing the value to 5 and 6 and note the effect.
 Try also reducing the value and see if it has any effect on normal roaming.
Tactile Navigation with Whiskers · Page 165
SUMMARY
In this chapter, instead of navigating from a pre-programmed list, the Boe-Bot was
programmed to navigate based on sensory inputs. The sensory inputs used in this chapter
were whiskers, which served as normally open contact switches. When properly wired,
these switches can show one voltage (5 V) at the switch’s contact point when it’s open
and a different voltage (0 V) when it’s closed. The BASIC Stamp I/O pin’s input
registers store “1” if they detect Vdd (5 V) and “0” if they detect Vss (0 V).
The BASIC Stamp was programmed to test the whisker sensors and display the test
results using two different media, the Debug Terminal and LEDs. PBASIC programs
were developed to make the BASIC Stamp check the whiskers between each servo pulse.
Based on the state of the whiskers, IF…THEN statements in the program’s Main Routine
section called navigation subroutines similar to the ones developed in the previous
chapter to guide the Boe-Bot away from obstacles. As a simple example of artificial
intelligence, an additional routine was developed that enabled the Boe-Bot to detect when
it got stuck in a corner. This routine involved storing old whisker states, comparing them
against the current whisker states, and counting the number of alternate object detections.
This chapter introduced sensor-based Boe-Bot navigation. The next three chapters will
focus on using different types of sensors to give the Boe-Bot vision. Both vision and
touch open up lots of opportunities for the Boe-Bot to navigate in increasingly complex
environments.
Questions
1. What kind of electrical connection is a whisker?
2. When a whisker is pressed, what voltage occurs at the I/O pin monitoring it?
What binary value will occur in the input register? If I/O pin P8 is used to
monitor the input pin, what value does IN8 have when a whisker is pressed, and
what value does it have when a whisker is not pressed?
3. If IN7 = 1, what does that mean? What does it mean if IN7 = 0? How about
IN5 = 1 and IN5 = 0?
4. What command is used to jump to different subroutines depending on the value
of a variable? What command is used to decide which subroutine to jump to?
What are these decisions based on?
5. What is the purpose of having nested IF…THEN statements?
Page 166 · Robotics with the Boe-Bot
Exercises
1. Write a DEBUG command for TestWhiskers.bs2 that updates each whisker state
on a new line. Adjust the PAUSE command so that it is 250 instead of 50.
2. Using RoamingWithWhiskers.bs2 as a reference, write a Turn_Away subroutine
that calls the Back_Up subroutine once and the Turn_Left subroutine twice.
Write down the modifications you will have to make to the Main Routine section
of RoamingWithWhiskers.bs2
Projects
1. Modify RoamingWithWhiskers.bs2 so that the Boe-Bot makes a 4 kHz beep that
lasts 100 ms before executing the evasive maneuver. Make it beep twice if both
whisker contacts are detected during the same sample.
2. Modify RoamingWithWhiskers.bs2 so that the Boe-Bot roams in a 1 yard (or
meter) diameter circle. When you touch one whisker, it will cause the Boe-Bot
to travel in a tighter circle (smaller diameter). When you touch the other
whisker, it will cause the Boe-Bot to navigate in a wider diameter circle.
Solutions
Q1. A tactile switch.
Q2. Zero (0) volts, resulting in binary zero (0) at the input register.
IN8 = 0 when whisker is pressed.
IN8 = 1 when whisker is not pressed.
Q3. IN7 = 1 means the right whisker is not pressed.
IN7 = 0 means the right whisker is pressed.
IN5 = 1 means the left whisker is not pressed.
IN5 = 0 means the left whisker is pressed.
Q4. The GOSUB command performs the actual jump. The IF...THEN command is
used to decide which subroutine to jump to. That decision is based on conditions,
which are logical statements that evaluate to true or false.
Q5. The program can check for one condition, and if that condition is true, it can
check for another condition within the first condition.
E1. The key to solving this problem is to use a second CRSRXY command that will
place the right whisker state in the proper place on the screen. To line up with
the headings, the text should start on column 9 of row 3.
Tactile Navigation with Whiskers · Page 167
'
'
'
'
Robotics with the Boe-Bot - TestWhiskers_UpdateEaOnNewLine.bs2
Update each whisker state on a new line.
{$STAMP BS2}
' Stamp directive.
{$PBASIC 2.5}
' PBASIC directive.
DEBUG "WHISKER STATES", CR,
"Left
Right", CR,
"-----------"
DO
DEBUG CRSRXY, 0, 3, "P5 = ", BIN1 IN5
DEBUG CRSRXY, 9, 3, "P7 = ", BIN1 IN7
PAUSE 250
LOOP
' Print in Column 0,Row 3
' Print in Column 9,Row 3
' Change from 50 to 250
E2. Subroutine:
Turn_Away:
GOSUB Back_Up
GOSUB Turn_Left
GOSUB Turn_Left
RETURN
To modify the Main Routine, replace the three GOSUB commands under the first
IF condition with this single line:
GOSUB Turn_Away
P1. The key to solving this problem is to write a statement that makes a beep with
the required parameters:
FREQOUT 4, 100, 4000
' 4kHz beep for 100ms
This statement must be added to the Main Routine in the appropriate places, as
shown below. The rest of the program is unchanged.
' -----[ Main Routine ]---------------------------------------DO
IF (IN5 = 0) AND (IN7 = 0) THEN
FREQOUT 4, 100, 4000
FREQOUT 4, 100, 4000
GOSUB Back_Up
GOSUB Turn_Left
GOSUB Turn_Left
ELSEIF (IN5 = 0) THEN
FREQOUT 4, 100, 4000
GOSUB Back_Up
GOSUB Turn_Right
' Both whiskers detect
' 4 kHz beep for 100 ms
'
Repeat twice
' Back up & U-turn
' Left whisker contacts
' 4 kHz beep for 100 ms
' Back up & turn right
Page 168 · Robotics with the Boe-Bot
ELSEIF (IN7 = 0) THEN
FREQOUT 4, 100, 4000
GOSUB Back_Up
GOSUB Turn_Left
ELSE
GOSUB Forward_Pulse
ENDIF
LOOP
' Right whisker contacts
' 4 kHz beep for 100 ms
' Back up & turn left
'
'
'
'
Both whiskers 1, no
contacts
Apply a forward pulse
and check again
P2. We found from Chapter 4 Projects that a 1 yard circle can be achieved with
PULSOUT 13, 850 and PULSOUT 12, 716. Using these values as the 1 yard
circle, the radius can be adjusted by slightly increasing or decreasing the pulse
width from the starting value of 716. Each time a whisker is pressed the
program will add or subtract a bit from the right wheel's pulse width.
'
'
'
'
'
Robotics with the Boe-Bot - CirclingWithWhiskerInput.bs2
Move in 1 yard circle, increase/decrease radius in response
to whisker presses, one whisker increases, one decreases.
{$STAMP BS2}
' Stamp directive.
{$PBASIC 2.5}
' PBASIC directive.
DEBUG "Program Running!"
' -----[ Variables/Initialization ]-----------------------------------pulseWidth
VAR
Word
' Signal sent to servo
toneFreq
VAR
Word
' Frequency of beeping tone
pulseWidth = 716
' Found in Ch4 to make 1y circle
toneFreq = 4000
' Beginning tone is 4 kHz
' -----[ Main Routine ]-----------------------------------------------DO
PULSOUT 13, 850
PULSOUT 12, pulseWidth
PAUSE 20
IF (IN5 = 0) THEN
IF (pulseWidth <= 845) THEN
pulseWidth = pulseWidth + 5
toneFreq = toneFreq + 100
FREQOUT 4, 100, toneFreq
ENDIF
ELSEIF (IN7 = 0) THEN
IF (pulseWidth >= 655) THEN
pulseWidth = pulseWidth - 5
toneFreq = toneFreq - 100
FREQOUT 4, 100, toneFreq
ENDIF
ENDIF
LOOP
' Pulse servos in circular path
' 12 slower than 13 so it arcs
' Left whisker makes circle
' smaller, down to servo max
' pulseWidth of 850.
' Play tone as indicator.
' Right whisker makes circle
' larger, down to servo min
' pulseWidth of 650.
' Play tone as indicator.
Light-Sensitive Navigation with Phototransistors · Page 169
Chapter 6: Light-Sensitive Navigation with
Phototransistors
Should I save this chapter for last? Many classes skip to Chapter 7 and 8, and then
return here if time permits. Chapter 7 is the best “next step” after navigation with whiskers
because it introduces a sensor that the Boe-Bot can use to detect obstacles without
bumping into them. Chapter 8 uses that same sensor for distance detection and following
objects. That will complete your introduction to object detection and navigation. After that,
return here to make your Boe-Bot detect and respond to something entirely different and
somewhat more challenging—ambient light.
Download select example code: Some of the longer example programs in this chapter are
available for download from www.parallax.com/go/Boe-Bot. Look for the file
LightSensorExamples.zip.
Light has many applications in robotics and industrial control. Some examples include
sensing the edge of a roll of fabric in the textile industry, determining when to activate
streetlights at different times of the year, when to take a picture, or when to deliver water
to a crop of plants.
There are many different light sensors that serve unique functions. The light sensors in
your Boe-Bot kit respond to visible light along with an invisible type of light called
infrared. These sensors can be incorporated into a couple of different circuits, and the
BASIC Stamp can be programmed to interact with them to detect variations in light level.
With this information, your program can be expanded to make the Boe-Bot recognize
areas with light or dark perimeters, report overall brightness and darkness levels, and
seek out light sources such as flashlight beams and doorways that are letting light into
dark rooms.
INTRODUCING THE PHOTOTRANSISTOR
A transistor is like a valve that regulates the amount of electric current that passes
through two of its terminals. The third terminal of a transistor controls just how much
current passes through the other two. Depending on the type of transistor, the current
flow can be controlled by voltage, current, or in the case of the phototransistor, by light.
Page 170 · Robotics with the Boe-Bot
Figure 6-1 shows the schematic and part drawing of the phototransistor in your Boe-Bot
Robot kit. The brightness of the light shining on the phototransistor’s base (B) terminal
determines how much current it will allow to pass into its collector (C) terminal, and out
through its emitter (E) terminal. Brighter light results in more current; less-bright light
results in less current.
Light
B
Collector
C
Base
Flat spot and shorter
pin indicate the
emitter (E) terminal
B
E
Current
Figure 6-1
Phototransistor
Schematic Symbol and
Part Drawing
Emitter
E
C
Although the phototransistor and LED are different devices, they do have two
similarities. First, if you connect the phototransistor in the circuit backwards, it won’t
work right. Second, the phototransistor has two different length pins and a flat spot on its
plastic case for identifying its terminals. The longer of the two pins indicates the
phototransistor’s collector terminal, and the shorter pin indicates the emitter. The emitter
terminal also connects closer to a flat spot on the phototransistor’s clear plastic case,
which is useful for identifying the terminals if the leads have been trimmed.
 Check Figure 6-1 and find the emitter terminal’s flat spot and shorter pin.
In the ocean, you can measure the distance between the peaks of two adjacent waves in
feet or meters. With light, which also travels in waves, the distance between adjacent
peaks is measured in nanometers (nm) which are billionths of meters. Figure 6-2 shows
the wavelengths for colors of light we are familiar with along with some the human eye
cannot detect, such as ultraviolet and infrared.
The phototransistor in the Boe-Bot Parts Kit has its peak sensitivity at 850 nm, which
according to Figure 6-2, is in the infrared range. Infrared light is not visible to the human
eye, but many different light sources emit considerable amounts of it, including halogen
and incandescent lamps, and especially the sun. The phototransistor also responds to
visible light, thought it’s somewhat less sensitive, especially to wavelengths below
450 nm, which are left of blue in the figure.
Light-Sensitive Navigation with Phototransistors · Page 171
Figure 6-2 Wavelengths and their Corresponding Colors
For a better view,
download a full-color
PDF of this book from
www.parallax.com/go/
Boe-Bot.
Wavelength (nm) 10…380
Color
450
495
Violet
Ultraviolet
570 590 620
Green
Blue
Orange
Yellow
750…100,000
Infrared
Red
Circuit designs that use phototransistors for light detection can be adjusted to perform
better in certain light levels, and the phototransistor circuits in this chapter are designed
for indoor use. So if your robotics area has fluorescent, incandescent, or indirect halogen
indoor lighting, it should work great. Avoid sunlight streaming in through nearby
windows, because it’ll flood the phototransistors with too much infrared light. If your
work area is near windows that let sunlight in, it’s a good idea to draw the blinds before
getting started. Halogen lamps pointed directly at the course could also cause problems.
They should only provide indirect light, ideally directed upward so that the light is
reflected off the ceiling. For best results, set up your course in an area with bright
fluorescent lighting.
Illuminance is a scientific name for the measurement of incident light. One way to
understand incident light is to think about shining a flashlight at a wall. The focused beam
that you see is incident light. The unit of measurement of luminance is commonly the "footcandle" in the English system or the "lux" in the metric system. Boe-Bot phototransistor
measurements won’t be concerned with lux levels, just whether incident light coming from
certain directions is brighter or darker. The Boe-Bot’s program can then use differences
between right and left illuminance levels to make navigation decisions.
ACTIVITY #1: A SIMPLE BINARY LIGHT SENSOR
Imagine that your Boe-Bot is navigating a course, and that there’s a bright light at the
end. For example, it could be a bright light pointing down at a certain spot. Your BoeBot’s last task in the course could be to stop underneath that bright light. Incandescent
bulbs in desk lamps and flashlights make the best “bright light” sources. Compact
fluorescent and LED light sources are not as easy for the circuit in this activity to
recognize.
Page 172 · Robotics with the Boe-Bot
If stopping under a bright light is your Boe-Bot’s only light-seeking task, there’s a simple
circuit you can use that lets the BASIC Stamp know it detected bright light with a
binary-1, or ambient light with a binary-0.
Ambient According to Merriam Webster’s Dictionary, the word ambient means existing or
present on all sides. For the light level in a room, think about ambient light as the overall
level of brightness.
Parts List
(1) Phototransistor
(2) Jumper wires
(1) Resistor, 220 Ω (red-red-brown)
(1) Resistor, 470 Ω (yellow-violet-brown)
(1) Resistor, 1 kΩ (brown-black-red)
(1) Resistor, 2 kΩ (red-black-red)
(1) Resistor, 4.7 kΩ (yellow-violet-red)
(1) Resistor, 10 kΩ (brown-black-orange)
USE
THIS
ONE!
Phototransistor
Infrared LED
Flatter on
top
More Rounded
Dome
Figure 6-3: Phototransistors vs. IR LEDs
Building the Bright Light Detector
Figure 6-4 shows the schematic and wiring diagram of a voltage output phototransistor
circuit that the BASIC Stamp will use to get that binary 1 or 0 value. After some testing,
and depending on the light conditions in your robotics area, you might end up replacing
the 2 kΩ resistor with one of the other resistors in the parts list.
The circuit in Figure 6-4 is similar to ones you will find in lights that automatically turn on
at night, and certain conveyer belt detectors.
 Disconnect power from your board and servos.
 Build the circuit shown in Figure 6-4.
 The wiring diagram points out the phototransistor emitter pin, which is shorter
and closer to the flat spot on the plastic case. Double check your wiring using
the figure as a reference to make sure the phototransistor’s collector and emitter
are correctly connected in your light sensing circuits.
Light-Sensitive Navigation with Phototransistors · Page 173
Figure 6-4
Phototransistor Voltage Output
Circuit
Wiring Diagrams
Board of Education (left);
HomeWork Board (right)
Flat Spot,
Shorter Pin
Flat Spot,
Shorter Pin
Page 174 · Robotics with the Boe-Bot
Example Program: TestBinaryPhototransistor.bs2
This program should make the Debug Terminal display a value of 0 in a room with
fluorescent lights, and no direct sunlight. When you shine a bright light on the
phototransistor, the program should display a value of 1. Figure 6-5 shows an example.
Figure 6-5: Debug Terminal Displays TestBinaryPhototransistor.bs2 Messages
Ambient Fluorescent Light
Bright Light
 Make sure the phototransistor leads do not touch each other. Optionally wrap
the exposed portions of the leads with electrical tape.
 Reconnect power to your board.
 Enter, save, and run TestBinaryPhototransistor.bs2.
 Watch the value of IN6 in the Debug Terminal, and verify that it stores a 0 when
it’s not under the bright light and a 1 when it’s under bright light. Good sources
of bright light include incandescent flashlights (flashlights with bulbs, not
LEDs), incandescent desk lamps, and small halogen lamps.
 If the ambient light is brighter than just fluorescent lights, and you have a nice
bright lamp, you may need to replace the 2 kΩ resistor with a smaller value. Try
1 kΩ, 470 Ω, or even 220 Ω for really bright lights.
 If the ambient light is low, and you are using a fluorescent desk lamp bulb or an
LED flashlight for your bright light, you may need to change the 2 kΩ resistor to
4.7 kΩ, or even 10 kΩ.
Light-Sensitive Navigation with Phototransistors · Page 175
' Robotics with the Boe-Bot - TestBinaryPhototransistor.bs2
' Display 1 when the phototransistor circuit applies more than 1.4 V to P6
' or 0 when it applies less than 1.4 V.
' {$STAMP BS2}
' {$PBASIC 2.5}
PAUSE 1000
DEBUG CLS
DO
DEBUG HOME, "IN6 = ", BIN IN6
PAUSE 100
LOOP
Your Turn – Make the Boe-Bot Halt Under the Bright Light
HaltUnderBrightLight.bs2 will make the Boe-Bot go forward until the phototransistor
detects light that’s bright enough to make IN6 store a binary-1.
 Try starting the program with the Boe-Bot a few feet from the bright light.
 Point the Boe-Bot so that it will go straight under the bright light. How close did
the Boe-Bot get to stopping directly under the light?
 Try making adjustments to the code and resistor values to get the Boe-Bot to
park right underneath that bright light.
' Robotics with the Boe-Bot - HaltUnderBrightLight.bs2
' Full speed forward until bright light makes phototransistor circuit's
' output voltage exceed 1.4 V, resulting in IN6=1
' {$STAMP BS2}
' {$PBASIC 2.5}
FREQOUT 4, 2000, 3000
DEBUG "Program running... "
DO UNTIL IN6 = 1
PULSOUT 13, 850
PULSOUT 12, 650
PAUSE 20
LOOP
Page 176 · Robotics with the Boe-Bot
Advanced Topic: How the Phototransistor Voltage Output Circuit Works
The phototransistor circuit you built applies a voltage to I/O pin P6. The voltage labeled
VP6 in Figure 6-6 is the voltage output that the circuit applies to the I/O pin. This voltage
increases with more light and decreases with less light. Since P6 is set to input, this
voltage causes IN6 to store a binary-1 or a binary-0. If the voltage is above 1.4 V, IN6
stores a binary-1; if it’s below 1.4 V, IN6 stores a binary-0.
To P6
Figure 6-6
Phototransistor
Voltage Output
Circuit and IN6
Response to
VP6
A resistor “resists” the flow of current. Voltage in a circuit with a resistor can be likened
to water pressure. For a given amount of electric current, more voltage (pressure) is lost
across a larger resistor than a smaller resistor that has the same amount of current passing
through it. If you instead keep the resistance constant and vary the current, you can
measure a larger voltage (pressure drop) across the same resistor with more current or
less voltage with less current.
When a BASIC Stamp I/O pin is an input, the circuit behaves as though neither the I/O
pin nor the 220 Ω resistor is present. Figure 6-6 shows a circuit that’s equivalent to the
one you just built on the breadboard when the I/O pin is set to input. With Vdd (5 V) at
the top and ground (0 V) at the bottom of the circuit, 5 V of electrical pressure (voltage)
makes the supply of electrons in the Boe-Bot’s batteries want to flow through it.
Connected in Series When two or more elements are connected end-to-end, they are
connected in series. The phototransistor and resistor in Figure 6-6 are connected in series.
A logic threshold is a voltage that distinguishes a binary-1 from a binary-0. For a BASIC
Stamp I/O pin set to input, that threshold is 1.4 V.
Light-Sensitive Navigation with Phototransistors · Page 177
The reason the voltage at P6 changes with light is because the phototransistor lets more
current pass with more light, or less current pass with less light. That current, which is
labeled I in Figure 6-6, also has to pass though the resistor. When more current passes
through a resistor, the voltage across it will be higher. When less current passes, the
voltage will be lower. Since one end of the resistor is tied to Vss = 0 V, the voltage at the
VP6 end goes up with more current and down with less current.
If you replace the 2 kΩ resistor with a 1 kΩ resistor, VP6 will be smaller values for the
same currents. In fact, it will take twice as much current to get VP6 past the BASIC
Stamp I/O pin’s 1.4 V logic threshold, which means the light will have to be twice as
bright to make IN6 to store a 1. So, a smaller resistor in series with the phototransistor
makes the circuit less sensitive to light. If you instead replace the 2 kΩ resistor with a
10 kΩ resistor, VP6 will be 5 times larger with the same current, and it’ll only take 1/5th
the light to generate 1/5th the current to get VP6 increase to above 1.4 V to make IN6 store
a 1. So, a larger resistor makes the circuit more sensitive to light.
Ohm’s Law for Calculating Voltage, Current, and Resistance
Two properties affect the voltage at VP6: current and resistance, and Ohm’s Law explains
how it works. Ohm’s Law states that the voltage (V) across a resistor is equal to the
current (I) passing through it multiplied by its resistance (R). So, if you know two of
these values, you can use the Ohm’s Law equation to calculate the third:
V=I×R
In some textbooks, you will see E = I × R instead. E stands for electric potential.
Voltage V is measured in units of volts, which are abbreviated with an upper-case V.
Current I is measured in amps, which are abbreviated A, and resistance R is measured in
ohms which is abbreviated with the Greek letter omega (Ω). The current levels you are
likely to see through this circuit are in milliamps (mA). The lower-case m indicates that
it’s a measurement of thousandths of amps. Likewise, the lower-case k in kΩ indicates
that the measurement is in thousands of ohms.
Page 178 · Robotics with the Boe-Bot
Let’s use Ohm’s Law to calculate VP6 in with the phototransistor letting two different
amounts of current flow through the circuit: 1.75 mA, which might happen as a result of
fairly bright light, and 0.25 mA, which would happen with less bright light. Figure 6-8
shows the conditions and their solutions. When you try these calculations, make sure to
remember that milli (m) is thousandths and kilo (k) is thousands when you substitute the
numbers into Ohm’s Law.
Figure 6-7: VP6 Calculations for Two Different Phototransistor-Resistor Currents
VP6  I  R
 1.75 mA  2 k
1.75
A  2000 
1000
 1.75 A  2 

 3.5 A
 3.5V
VP6  I  R
 0.25 mA  2 k
0.25
A  2000 
1000
 0.25 A  2 

 0.5 A
 0.5V
Light-Sensitive Navigation with Phototransistors · Page 179
Your Turn – Ohm’s Law and Resistor Adjustments
Let’s say that the light in your room is twice as bright as the one in the room that resulted
in Vo = 3.5 V for bright light and 0.5 V for shade. Another situation that could cause
higher current is if the light is a stronger source of infrared. In either case, the
phototransistor could allow twice as much current to flow through the circuit, which
could lead to measurement difficulties.
Question: What could you do to bring the circuit’s voltage response back down
to 3.5 V for bright light and 0.5 V for dim?
Answer: Cut the resistor value in half; make it 1 kΩ instead of 2 kΩ.
Try repeating the Ohm’s Law calculations with R = 1 kΩ, and bright current I = 3.5 mA
and dim current I = 0.5 mA. Does it bring Vo back to 3.5 V for bright light and 0.5 V for
dim light with twice the current? (It should, if it didn’t for you, check your calculations.)
ACTIVITY #2: MEASURE LIGHT LEVELS WITH PHOTOTRANSISTORS
This activity introduces a circuit that the BASIC Stamp can use to measure the brightness
of light incident on the phototransistor’s base. The values of the measurements could
range from small numbers, indicating bright light, to large numbers, indicating low light.
Binary vs. Analog and Digital
A binary sensor can transmit two different states, typically to indicate the presence or
absence of something. For example, a whisker sends a high signal if it is not pressed, or a
low signal if it is pressed.
An analog sensor sends a continuous range of values that correspond to a continuous range
of measurements. The phototransistor circuits in this activity are examples of analog
sensors that provide continuous ranges of values that correspond to continuous ranges of
light levels.
A digital value is a number expressed by digits. Computers and microcontrollers store
analog measurements as digital values. The process of measuring an analog sensor and
storing that measurement as a digital value is called analog to digital conversion. The
measurement is called a digitized measurement. Analog to digital conversion documents
will also call them quantized measurements.
Page 180 · Robotics with the Boe-Bot
Parts List
In this activity, you will need two phototransistors and two 0.1 μF capacitors. Figure 6-8
shows drawings of both.
 Look carefully at Figure 6-8 and make note of the difference between a
phototransistor and an infrared LED.
Phototransistor
Flatter on
top
0.1 μF Capacitor Schematic
Symbol and Part Drawing
Infrared LED
More Rounded
Dome
Figure 6-8
Distinguishing
Phototransistors
from Infrared LEDs;
Identifying the
0.1 μF Capacitor
 Gather the parts listed below using Figure 6-8 as a guide for finding the
phototransistors and 0.1 μF capacitors in your parts kit.
(2)
(2)
(2)
(2)
Phototransistors
Capacitors, 0.1 μF (104)
Resistors, 1 kΩ (brown-black-red)
Jumper wires
Introducing the Capacitor
A capacitor is a device that stores charge, and it is a fundamental building block of many
circuits. Batteries are also devices that store charge, and for these activities, it will be
convenient to think of capacitors as tiny batteries that can be charged, discharged, and
recharged.
How much charge the capacitor tends to store is measured in farads (F). A farad is a very
large value that’s not practical for use with these Boe-Bot circuits. The capacitors you
will use in this activity store fractions of millionths of farads. A millionth of a farad is
called a microfarad, and it is abbreviated μF. The capacitor you will use in this exercise
stores one tenth of a millionth of a farad. That’s 0.1 μF.
Light-Sensitive Navigation with Phototransistors · Page 181
Common capacitance measurements are:
Microfarads:
Nanofarads:
Picofarads:
(millionths of a farad), abbreviated μF
(billionths of a farad), abbreviated nF
(trillionths of a farad), abbreviated pF
-6
1 μF = 1×10 F
-9
1 nF = 1×10 F
-12
1 pF = 1×10 F
The 104 on the 0.1 μF capacitor’s case is a measurement picofarads or (pF). In this
labeling system, 104 is the number 10 with four zeros added, so the capacitor is 100,000 pF,
which is 0.1 μF.
-12
(100,000) × (1 × 10 ) F
-9
= 100 × 10 F
= 0.1 μF.
=
=
3
-12
(100 × 10 ) × (1 × 10 ) F
-6
0.1 × 10 F
Building the Photosensitive Eyes
The BASIC Stamp can use the circuits in Figure 6-9 to measure the amount of light
incident on each phototransistor’s base. One phototransistor will be pointing forward and
to the left, and the other will be pointing forward and to the right. They will both also be
pointing upward at about 45°. Since they are pointing in different directions, the BASIC
Stamp will be able to use them to determine whether light is brighter on the Boe-Bot’s
left or right.
 Disconnect power from your board and servos.
 Remove all the phototransistor voltage output circuit parts from Figure 6-4,
including the wire that connected the phototransistor’s collector terminal to Vdd.
 Build the circuits shown in Figure 6-9.
 Double check your circuits against the wiring diagram to make sure your
phototransistors are not plugged in backwards. Use the “shorter pin” and “flat
spot” indicators as a guide.
Page 182 · Robotics with the Boe-Bot
Figure 6-9: Analog Phototransistor Circuit Schematics and Wiring Diagram
Flat Spots,
Shorter Pins
Flat Spots,
Shorter Pins
The roaming examples in this chapter will depend on the phototransistors being pointed
upward and outward to detect differences in incident light levels from different
directions.
 Make sure your phototransistors are pointing upward and outward as shown in
Figure 6-10.
Light-Sensitive Navigation with Phototransistors · Page 183
Figure 6-10: Point the Phototransistors Upward and Outward
About Charge Transfer and the Phototransistor Circuit
Each phototransistor/capacitor circuit is called a charge transfer circuit. The BASIC
Stamp will measure the rate at which each capacitor loses its charge through its
phototransistor by measuring how long it takes the capacitor’s voltage to decay. The
decay time corresponds to the brightness of the light incident on the phototransistor’s
base. Faster decay means more light, slower decay means less light.
QT Circuit: A common abbreviation for charge transfer is QT. The letter Q refers to
electrical charge (an accumulation of electrons), and T is for transfer.
Think of the capacitors in the Figure 6-11 circuit as tiny rechargeable batteries, and think
of the phototransistors as light controlled current valves. Each capacitor can be charged
to 5 V and then allowed to drain through its phototransistor. The rate that the capacitor
loses its charge depends on how much current the phototransistor (current valve) allows
to pass, which in turn depends on the brightness of the light shinning on the
phototransistor’s base. Again, brighter light results in more current, shadows result in
less current.
Page 184 · Robotics with the Boe-Bot
Connected in Parallel
The phototransistor and capacitor shown in Figure 6-11 are connected in parallel. For two
components to be connected in parallel, each of their leads must be connected to common
terminals (also called nodes). The phototransistor and the capacitor each have one lead
connected to Vss. They also each have one lead connected to the same 1 kΩ resistor lead.
So, they are connected in parallel.
Figure 6-11
QT Circuit Connected
to I/O Pin P6
The BASIC Stamp performs these steps to measure a light level with the phototransistor
charge transfer circuit in Figure 6-11:
1. Use the HIGH command to apply 5 V to the circuit and charge the capacitor (tiny
battery).
2. Use the PAUSE command to wait for the capacitor to charge.
3. Use the RCTIME command to set the I/O pin to input and measure the time it
takes for the capacitor’s voltage to decay to 1.4 V as it loses charge through the
phototransistor.
A longer decay time measurement in step 3 means less light; a shorter decay time means
more light.
The RCTIME command changes the Pin direction from output to input, and then waits for
the I/O pin’s state to change, which happens when the voltage the circuit applies to the
pin passes its 1.4 V logic threshold. The RCTIME command stores the time measurement
result in Variable. With the BASIC Stamp 2, this result is a number of 2 μs increments.
RCTIME Pin, State, Variable
Light-Sensitive Navigation with Phototransistors · Page 185
If the State argument is set to 1, RCTIME will wait for it to change to 0 indicating that the
voltage decayed down to 1.4 V. If State is set to 0, RCTIME will wait for the voltage to
rise to 1.4 V. In either case, the command stores the time measurement result in the
Variable argument, which is typically a word variable.
When the RCTIME command changes the pin direction from output to input, it stops
charging the capacitor and becomes invisible to the circuit. As soon as that happens, the
capacitor’s charge starts draining through the phototransistor. As an input, the I/O pin can
sense whether the circuit’s voltage is above or below 1.4 V.
The RC in RCTIME stands for resistor-capacitor, and the RCTIME command’s most common
use is with sensors that vary with either resistance or capacitance. For an example of using
this command to measure the position of a dial that controls resistance, see What’s a
Microcontroller?, Chapter 5. It’s a free download from www.parallax.com/go/WAM.
Test the Phototransistor Circuit
The TestP6LightSense.bs2 example program performs the three steps on the QT circuit
connected to P6 in Figure 6-11 and displays a time measurements that represent the light
level incident on the phototransistor’s base terminal. The QT circuit connected to P6 is
the Boe-Bot’s left light sensor, and the Debug Terminal will display the decay time as
tLeft, which is both the name of the variable that stores the result and an abbreviation of
time-left. The value it displays is the decay time, measured in 2 μs time increments. This
value will decrease with brighter light and increase with less bright light, like in Figure
6-12.
Figure 6-12: Two Different Light Levels Measured by Boe-Bot Robot’s Left Light Sensor
Normal Indoor Lighting
Shade Over Sensor
Page 186 · Robotics with the Boe-Bot
Is tLeft stuck at 0 or 1? A 0 could mean it’s way too dark, and a 1 could mean it’s way
too bright. Either one could also indicate a wiring error, so double check your circuits too.
 These sensor circuits are designed for indoor lighting. Make sure no direct
sunlight is shining in through the windows. If there is, close the blinds.
 Enter and run TestP6LightSense.bs2.
 Make a note of the value displayed in the Debug Terminal.
 Use your hand or a book to cast a shadow over the phototransistor in the circuit
connected to P6.
 Check the measurement in the Debug Terminal again. The value should be
larger than the first one. Make a note of it too.
 Move the object casting the shadow closer to the top of the phototransistor. Try
to make the shadow about twice as dark as the first shadow. Make a note of the
measurement.
 Experiment with progressively darker shadows, even cupping your hand over the
phototransistor. (If the light level gets low enough, the RCTIME command may
exceed its maximum result value of 65535, in which case, the command will
store a 0 in the tLeft variable, and the Debug Terminal will display
“tLeft = 00000.”)
' Robotics with the Boe-Bot - TestP6LightSense.bs2
' Test Boe-Bot's left photoresistor circuit.
' {$STAMP BS2}
' {$PBASIC 2.5}
tLeft
' Target module = BASIC Stamp 2
' Language = PBASIC 2.5
VAR
Word
' Stores left sensor decay time
PAUSE 1000
' Wait 1 s before any DEBUG
DO
' Main loop
HIGH 6
PAUSE 1
RCTIME 6, 1, tLeft
' 1 Set P6 high to start charging
' 2 Wait for cap to charge
' 3 P6->input, measure decay time
DEBUG HOME, "tLeft = ", DEC5 tLeft
PAUSE 100
' Display result
' Wait 0.1 seconds
LOOP
' Repeat main loop
Light-Sensitive Navigation with Phototransistors · Page 187
Your Turn – Test the Other Phototransistor Circuit
The Boe-Bot robot’s other phototransistor circuit is connected to P3. Before modifying
your program to test the other circuit, it’s always best to save the working program as-is
first.
 Save TestP6LightSense.bs2, then save a copy as TestP3LightSense.bs2.
 Change the Pin argument from 6 to 3 in the HIGH and RCTIME commands.
 Change the variable name from tLeft to tRight in the VAR declaration, and in
the RCTIME and DEBUG commands.
 Test and fix any typos or bugs.
 Update the comments at the beginning of the program.
 Save your modified program, and then run it.
It would also be nice to have a third program that tests both phototransistor circuits. As
before, save one of the working programs, and then save a copy of it under a new name,
like maybe TestP6P3LightSense.bs2. This program will need two variable declarations,
and two sets of HIGH-PAUSE-RCTIME commands in its main loop. The DEBUG commands
can be condensed into one. It might look something like this:
DEBUG HOME, "tLeft = ", DEC5 tLeft, "
", "tRight = ", DEC5 tRight
 Try TestP6P3LightSense.bs2. It’s in LightSensorExamples.zip, which is a free
download from www.parallax.com/go/Boe-Bot.
 See if you can rotate the Boe-Bot and detect which is pointing toward the
brightest light source in the room (lowest value) and which side is pointing away
from it (higher value).
Optional Advanced Topic: Voltage Decay Graphs
Figure 6-13 shows the Boe-Bot robot’s left and right QT circuit voltage responses while
TestP6P3LightSense.bs2 is running. The device that measures and graphs these voltage
responses over time is called an oscilloscope. The two lines that graph the two voltage
signals are called traces. The voltage scale for the upper trace is along the left, and the
voltage scale for the lower trace is along the right. The time scale for both traces is along
the bottom. Labels show when each command in TestP6P3LightSense.bs2 executes, so
that you can see how the voltage signals respond.
Page 188 · Robotics with the Boe-Bot
Figure 6-13: Oscilloscope View of Decay Times
HIGH 6
RCTIME 6, 1, tLeft
PAUSE 1
tLeft
≈5V
1.4 V
0V
PAUSE 1
HIGH 3
tRight
RCTIME 3, 1, tRight
≈5V
1.4 V
0V
The upper trace in Figure 6-13 plots the capacitor’s voltage in the QT circuit connected to
P6; that’s the Boe-Bot’s left light sensor circuit. In response to HIGH 6, the voltage rises
from 0 V to almost 5 V between about 0.5 ms and 1 ms. The signal stays at around 5 V
for the duration of PAUSE 1. Then, RCTIME causes the decay to start at about 2 ms into
the plot. The RCTIME command measures the time it takes the voltage to decay to 1.4 V
and stores it in the tLeft variable. In the plot, it looks like that decay took about 1.5 ms,
so the tLeft variable should store something in the neighborhood of 750 since
750 × 2 μs = 1.5 ms.
The lower trace in Figure 6-13 plots the other QT circuit’s capacitor voltage—the P3
sensor on the Boe-Bot’s right side. This measurement starts after the left side P6
measurement is done. The voltage varies in a manner similar to the upper trace, except
the decay time takes quite a bit longer, about 5 ms, and we would expect to see tRight
store a value in the 2500 neighborhood. This larger value corresponds to a slower decay,
which in turn corresponds to a lower light level.
Take your own oscilloscope measurements. You can measure and learn more about all
the signals in this chapter with the Understanding Signals with the PropScope book and kit.
To find out more, go to www.parallax.com/go/PropScope.
Light-Sensitive Navigation with Phototransistors · Page 189
ACTIVITY #3: LIGHT SENSITIVITY ADJUSTMENT
If these RCTIME light measurements are going to be used while the Boe-Bot is roaming,
they will have to share the BASIC Stamp’s processing time with PULSOUT commands for
servo control. There’s a 20 ms time window between each pair of PULSOUT commands
for RCTIME commands. Although 25 or 30 ms between servo pulses might not cause any
noticeable difference, delays of more than 50 ms or so will cause noticeable problems,
and larger delays will cause the servos to just twitch periodically instead of rotate.
A pair of phototransistor measurements in a really dark area might measure 50,000 each.
For both measurements, that would be 100,000 × 2 μs = 400 ms. All the Boe-Bot’s
servos would do with this delay between servo control pulses is twitch every 0.4 seconds.
In this activity, you will try a technique that can be used to reduce decay time
measurements in darker rooms. You will also test the effects of reduced light on servo
performance using both measurement techniques.
Fix the Problem by Charging the Capacitor to a Lower Voltage with PWM
How can a program make the measurements take less time? By charging the capacitors
to lower voltages before starting the decay measurements, the program can reduce the
time the decays take to reach 1.4 V. The PBASIC language has a command called PWM
that you can use to make the BASIC Stamp set the starting voltage across the capacitor to
a lower value. This command gives you 256 voltage levels to choose from in the 0 to
4.98 V range. The PWM command’s syntax is:
PWM Pin, Duty, Duration
The PWM command applies a rapid sequence of high/low signals to the I/O Pin for a
certain Duration in ms. The Duty is the number of 256ths the time the signal is high, and
it determines the number of 256ths of 5 V the capacitor gets charged to. For example, the
command PWM 6, 128, 1 sends a rapid sequence of high/low signals for 1 ms. They
are high for 128/256ths of the time—that’s half the time. So, it charges the capacitor to
128/256ths of 5 V. That’s half of 5 V, which is 2.5 V.
Page 190 · Robotics with the Boe-Bot
PWM stands for Pulse Width Modulation. In Chapter 2, you studied pulse width
modulation for servo control using PULSOUT. The PWM command makes the BASIC Stamp
create another form of pulse width modulation. This signal is a more rapid sequence of
pulses that’s especially useful for setting voltage across a capacitor through a resistor. The
proportion of high time to cycle time (high + low time) is what controls the capacitor voltage,
and it is called duty cycle. The PWM command’s Duty argument controls the PWM signals’
duty cycle.
Given a PWM command, you can calculate the voltage it establishes across the capacitor
by multiplying 5 V times the command’s Duty argument and then dividing by 256:
Vcap = 5 V × Duty ÷ 256
Here are two examples:
PWM 6, 128, 1
PWM 6, 96, 1
'
'
5 V × 128 ÷ 256 = 2.5 V.
5 V × 96 ÷ 256 = 1.875 V.
Let’s say you want to know what Duty value to use for a particular voltage. Just divide
both sides of the equation by 5 V, and multiply both sides by 256, and the result is:
Duty = Vcap × 256 ÷ 5 V
A useful duty value to know would be Vcap = 1.4 V. Values below that wouldn’t be any
good for voltage decay time measurements.
Duty = 1.4 V × 256 ÷ 5 V = 71.68
So, a value of 72 would be the smallest useful Duty argument in PWM 6, 72, 1.
Why is the PWM command’s Duration argument always 1 in these examples?
Because 1 ms is enough time to charge up a 0.1 μF capacitor through a 1 kΩ resistor. The
general rule is that you need at least 5×R×C seconds to charge a capacitor. With a 1 kΩ
resistor and 0.1 μF capacitor, the minimum would be 5 × 1000 × 0.0000001 = 0.0005 s =
0.5 ms. So, a 1 ms charge time is more than enough.
If the resistor or capacitor were larger, the PWM command’s Duration argument might have
to be larger. For example, if a 1 μF capacitor were used, the Duration argument would
have to be at least 5 for a 5 ms of charging time because 5×R×C = 5 × 1000 × 0.000001 =
0.005 s = 5 ms. R×C is called the RC time constant, and is often abbreviated with the Greek
letter tau τ. This letter is pronounced “taw.”
Light-Sensitive Navigation with Phototransistors · Page 191
Let’s start by reducing the decay times to half of the values you measured in the previous
activity. In practice, your program will need to reduce it by more to navigate in lower
light levels, but this shows the first step in getting there. To reduce decay times by ½,
you’ll have to use the PWM command to charge the capacitors to half way between 1.4 V
and 5 V. That corresponds to a PWM Duty value half way between 72 and 256, which is
(72 + 256) ÷ 2 = 184. So, you can replace HIGH 6 and PAUSE 1 with PWM 6, 184, 1
to reduce the decay times to half the values. Another way to think about this is that you
are using the PWM command to make the sensors half as sensitive to light because the
decay time measurements will take half as long for half the measured values.
 Enter, save and run HalfLightSensitivity.bs2.
 Try to make sure the ambient light is fairly close to the same level it was in the
previous activity.
 Verify that the light measurements are about half of what they were with
TestP6LightSense.bs2 from the previous activity. Precision is not important
here. Don’t worry if your measurements are not exactly one half of what they
were; in the general neighborhood of one half is fine.
 Try changing the PWM command’s Duty argument to 128 and verify that the
measurements are now in the neighborhood of a quarter of the values from
TestP6LightSense.bs2. Again, don’t worry about being precise.
'
'
'
'
'
Robotics with the Boe-Bot – HalfLightSensitivity.bs2
Test Boe-Bot's photoresistor circuits with the PWM command cutting
the phototransistor's light sensitivity in half.
{$STAMP BS2}
' Target module = BASIC Stamp 2
{$PBASIC 2.5}
' Language = PBASIC 2.5
tLeft
tRight
VAR
VAR
Word
Word
' Stores left sensor decay time
' Stores right sensor decay time
PAUSE 1000
' Wait 1 s before any DEBUG
DO
' Main loop
PWM 6, 184, 1
RCTIME 6, 1,tLeft
' Charge cap to 3.59 V
' P6->input, measure decay time
PWM 3, 184, 1
RCTIME 3, 1,tRight
' Charge cap to 3.59 V
' P3->input, measure decay time
DEBUG HOME, "tLeft = ", DEC5 tLeft, CR,
"tRight = ", DEC5 tRight
PAUSE 100
' Display results
LOOP
' Wait 0.1 seconds
' Repeat main loop
Page 192 · Robotics with the Boe-Bot
One Light Sensor, Two Different Measurements
HighVsPwmInRctime.bs2 demonstrates how the decay measurement for the sensor takes
less time when PWM charges it to a lower value.
 Enter and run HighVsPwmInRctime.bs2, and observe the two measurements of
the same light level in the Debug Terminal.
 Try varying the light level, the tRight2 measurement should always be
significantly smaller than tRight1.
'
'
'
'
Robotics with the Boe-Bot - HighVsPwmInRctime.bs2
Two decay measurements in a row. The first uses the HIGH, PAUSE, RCTIME
approach, and the second charges the capacitor to 2.5 V with PWM before
RCTIME.
' {$STAMP BS2}
' {$PBASIC 2.5}
tRight1
tRight2
VAR
VAR
' Target module = BASIC Stamp 2
' Language = PBASIC 2.5
Word
Word
' First right sensor decay time
' Second right sensor decay time
PAUSE 1000
' Wait 1 s before any DEBUG
DO
' Main loop
HIGH 3
PAUSE 1
RCTIME 3, 1, tRight1
' 1 Set P3 high to start charging
' 2 Wait for cap to charge
' 3 P3->input, measure decay time
PAUSE 1
' Separate measurements by 1 ms
PWM 3, 128, 1
RCTIME 3, 1,tRight2
' Charge P3 cap to 2.5 V
' P3->input, measure decay time
DEBUG HOME, "tRight1 = ", DEC5 tright1,
CR, "tRight2 = ", DEC5 tRight2
' Display results
PAUSE 100
' Wait 0.1 seconds
LOOP
' Repeat main loop
Figure 6-14 shows both a Debug Terminal and an oscilloscope measurement of the two
decays from HighVsPwmInRctime.bs2. In the oscilloscope trace, the first decay starts
from 5 V because of HIGH 3 and PAUSE 1 before RCTIME 3, 1, tRight1. The second
decay starts from 2.5 V because of PWM 3, 128, 1 before RCTIME 3, 1, tRight2.
Light-Sensitive Navigation with Phototransistors · Page 193
Figure 6-14: Debug Terminal and Oscilloscope Views of HighVsPwmInRctime.bs2
tRight1 = 1345
tRight2 = 264
≈5V
≈ 2.5 V
1.4 V
0V
th
th
Why is tRight2 more like 1/5 of tRight1? Isn’t it supposed to be 1/4 ? In the first
decay, the I/O pin was output-high right up until the time RCTIME changes it to input. In the
second decay, PWM changes the I/O pin to input when it is done, and then there is a brief
delay between the end of the PWM command and the start of the RCTIME command. The
voltage starts decaying at the end of the PWM command, so it is a little lower than 2.5 V by
the time the RCTIME starts measuring the decay time.
This reduction in measurement value could be corrected with some testing, but won’t matter
because it will be the same for both right and left sensors. When Boe-Bot programs
compare the two sensor values to determine which side is brighter or dimmer, both
measurements will be lower by a small, fixed amount. So, if one sensor detects less light
than the other, its measurement will still be larger, and that’s what the program will need for
navigation decisions.
Your Turn – Test Measurement Time’s Impact on Servo Control
You can add a couple of PULSOUT commands that make the Boe-Bot go full speed
forward to test the effect of measurement time on servo control. The first step would be a
test to find out how low the light level has to get before the servos stop functioning
properly with the HIGH-PAUSE-RCTIME approach. Then, use the PWM commands in place
Page 194 · Robotics with the Boe-Bot
of HIGH and PAUSE, with a Duty of 80, and test to find out how much darker it can get
before the servo stop functioning properly.
 Run TestMaxDarkWithHighPause.bs2.
 Gradually increase the shade until the servos start twitching. (A shoebox would
work well for this.)
 Repeat with TestMaxDarkWithPwm.bs2. The servos should still start twitching
at some point, but it should be darker before it happens.
Figure 6-15 The Program on the Right Should Allow the Servos to Work in Lower Light
' TestMaxDarkWithHighPause.bs2
' TestMaxDarkWithPwm.bs2
' {$STAMP BS2}
' {$PBASIC 2.5}
' {$STAMP BS2}
' {$PBASIC 2.5}
tLeft
tRight
VAR
VAR
Word
Word
tLeft
tRight
VAR
VAR
Word
Word
PAUSE 1000
DEBUG "Program running... "
PAUSE 1000
DEBUG "Program running... "
DO
DO
HIGH 6
PAUSE 1
RCTIME 6, 1,tLeft
HIGH 3
PAUSE 1
RCTIME 3, 1,tRight
PULSOUT 13, 850
PULSOUT 12, 650
PWM 6, 80, 1
RCTIME 6, 1,tLeft
PWM 3, 80, 1
RCTIME 3, 1,tRight
PULSOUT 13, 850
PULSOUT 12, 650
LOOP
LOOP
ACTIVITY #4: LIGHT MEASUREMENTS FOR ROAMING
This activity features a program that automatically adjusts to the light conditions in the
room and provides information about:



How bright it is in the room
Which of the two light sensors sees more shade
How strong the light/dark contrast is between the two sensors
Light-Sensitive Navigation with Phototransistors · Page 195
The Boe-Bot will be able to use this information for tasks like navigating toward or away
from light.
The first example program looks pretty long, but don’t worry. You’ll be downloading it
from www.parallax.com/go/Boe-Bot instead of hand-entering it.
Test LightSensorValues.bs2
LightSensorValues.bs2 utilizes several subroutines that condense the light measurements
into two values: light, and ndShade. The light variable stores the ambient light level
detected by the Boe-Bot. The ndShade variable stores a normalized, differential shade
measurement. Normalized means that the measurements were fit to a certain scale, -500
to 500 in the case of ndShade. Differential means that the number corresponds to a
difference between the two sensor measurements. In the case of ndShade, the value
indicates the difference between the level of shade each sensor detects.
The program’s light variable is useful for detecting overall light levels. The variable’s
scale is 1 to 324, with 1 being the darkest condition the system can measure and report
and 324 being the brightest. This measurement is useful if a goal or waypoint in a
navigation contest involves detecting when the robot passes under a bright light. In that
situation, the light variable might store a larger value than elsewhere in the robot
course, and the program could use an IF…THEN statement to detect that condition and take
action.
The program’s ndShade variable indicates how much more shade one light sensor detects
over the other. The variable’s scale is -500 (shade much darker on left) to 500 (shade
much darker on right). If the value of ndShade is 0, it means the light levels are about
the same on both phototransistors. The measurement can be useful for code that makes
the Boe-Bot roam either toward or away from light sources. For example, to make the
Boe-Bot roam toward light, a routine simply has to make the Boe-Bot turn away if it
detects shade on one side or the other.
Figure 6-16 shows examples of two different light/shade conditions measured with
LightSensorsValues.bs2. The Debug Terminal on the left is an example of the Boe-Bot
facing the main light source in a room. The light variable reports 230/324, which is in
the normal range of indoor lighting. The ndShade variable reports 0, which means both
phototransistors detect light levels that are very close to each other. The Debug Terminal
on the right side of the figure shows a measurement with a shadow cast over the left light
Page 196 · Robotics with the Boe-Bot
sensor. The value of ndShade is -279, which indicates shade over the left sensor, and the
light value has dropped because shade over one sensor also reduces the total light
measurement.
Figure 6-16: Example Shadow Tests with LightSeekingDisplay.bs2
Facing a Light Source
Shade over Boe-Bot’s Left Light Sensor
 Download LightSensorExamples.zip from www.parallax.com/go/Boe-Bot.
 Make sure there is no direct sunlight streaming in nearby windows. Indoor
lighting is good, but direct sunlight will blind the sensors.
 Unzip to a folder, and then open LightSensorValues.bs2.
 Open the program with your BASIC Stamp Editor and load it into the BASIC
Stamp.
 For best results, adjust ambient lighting in the room so that the light variable is
in the 125 to 275 range with no shade over the sensors.
 Verify that when you cast shade over the Boe-Bot’s left sensor, it results in
negative values, with darker shade resulting in larger negative values.
 Verify that when you cast shade over the Boe-Bot’s right sensor, it results in
positive values, with darker shade resulting in larger positive values.
 Verify that when both sensors see about the same level of light or shade, that
ndShade reports values close to 0.
 Verify that the light variable drops with increased shade and rises with more
light.
Light-Sensitive Navigation with Phototransistors · Page 197
'-----[ Title ]--------------------------------------------------------------' Robotics with the Boe-Bot - LightSensorValues.bs2
' Displays conditioned ambient light level and differential shade on a scale
' of -500 to 500.
' {$STAMP BS2}
' {$PBASIC 2.5}
' Stamp directive.
' PBASIC directive.
'-----[ Constants/Variables ]------------------------------------------------Negative
CON
1
' For negative numbers
' Application Variables
light
VAR
Word
ndShade
VAR
Word
' Brightness/darkness indicator
' Normalized differential shade
' Subroutine Variables
tLeft
VAR
Word
tRight
VAR
Word
n
VAR
Word
d
VAR
Word
q
VAR
Word
sumDiff
VAR
Word
duty
VAR
Byte
i
VAR
Nib
temp
VAR
Nib
sign
VAR
Bit
'
'
'
'
'
'
'
'
'
'
Stores left RCTIME measurement
Stores right RCTIME measurement
Numerator
Denominator
Quotient
For sum and difference calcs
PWM duty argument variable
Index counting variable
Temp storage for calcs
Var.BIT15 = 1 if neg, 0 if pos
'-----[ Initialization ]-----------------------------------------------------FREQOUT 4, 2000, 3000
' Start beep
DEBUG CLS
' Clear Debug Terminal
'-----[ Main Routine ]-------------------------------------------------------DO
GOSUB Light_Shade_Info
' Main Loop.
' Get light & ndShade
DEBUG HOME, "light = ", DEC3 light,
"/324", CLREOL, CR,
"ndShade = ", SDEC3 ndShade, CLREOL
' Display light & ndShade
PAUSE 100
' Delay 0.1 seconds
LOOP
' Repeat main loop
'-----[ Subroutine - Light_Shade_Info ]--------------------------------------' Uses tLeft and tRight (RCTIME measurements) and pwm var to calculate:
'
o light
- Ambient light level on a scale of 0 to 324
'
o ndShade - Normalized differential shade on a scale of -500 to + 500
Page 198 · Robotics with the Boe-Bot
'
'
(-500 -> dark shade over left, 0 -> equal shade,
+500 -> dark shade over right)
Light_Shade_Info:
GOSUB Light_Sensors
sumdiff = (tLeft + tRight) MAX 65535
IF duty <= 70 THEN
light=duty-(sumdiff/905) MIN 1
IF sumdiff = 0 THEN light = 0
ELSEIF duty = 255 THEN
light=duty+((1800-(sumdiff))/26)
ELSE
light = duty
ENDIF
GOSUB Duty_Auto_Adjust
n = tLeft
d = tLeft + tRight
GOSUB Fraction_Thousandths
ndShade = 500-q
RETURN
'
'
'
'
'
'
'
'
'
'
'
'
'
Subroutine label
Get raw RC light measurements
Start light level with sum
If duty at min
Find how much darker
If timeout, max darkness
If duty at max
Find how much brighter
If duty in range
light = duty
Done with light level
Adjust PWM duty for next loop
Set up tLeft/(tLeft+tRight)
' Divide (returns thousandths)
' Normalized differential shade
' Return from subroutine
'-----[ Subroutine - Light_Sensors ]-----------------------------------------' Measure P6 and P3 light sensor circuits. Duty variable must be in 70...255.
' Stores results in tLeft and tRight.
Light_Sensors:
PWM 6, duty,
RCTIME 6, 1,
PWM 3, duty,
RCTIME 3, 1,
RETURN
1
tLeft
1
tRight
'
'
'
'
'
'
Subroutine label
Charge cap in P6 circuit
Measure P6 decay
Charge cap in P3 circuit
Measure decay
Return from subroutine
'-----[ Subroutine - Duty_Auto_Adjust ]--------------------------------------' Adjust duty variable to keep tLeft + tRight in the 1800 to 2200 range.
' Requires sumdiff word variable for calculations.
Duty_Auto_Adjust:
sumDiff = (tLeft + tRight) MAX 4000
IF sumDiff = 0 THEN sumDiff = 4000
IF (sumDiff<=1800) OR (sumDiff>=2200) THEN
sumDiff = 2000 - sumDiff
sign = sumDiff.BIT15
sumDiff = ABS(sumDiff) / 10
sumDiff = sumDiff MAX ((duty-68)/2)
sumDiff = sumDiff MAX ((257-duty)/2)
IF sign=NEGATIVE THEN sumDiff=-sumDiff
duty = duty + sumDiff MIN 70 MAX 255
ENDIF
RETURN
'
'
'
'
'
'
'
'
'
'
'
'
'
Subroutine label
Limit max ambient value
If 0 (timeout) then 4000
If outside 1800 to 2200
Find excursion from target val
Pos/neg if .BIT15 = 0/1
Max sumDiff will be +/- 10
Reduce adjustment increments
near ends of the range
Restore sign
Limit duty to 70 to 255
End of if outside 1800 to 2200
Return from subroutine
Light-Sensitive Navigation with Phototransistors · Page 199
'-----[ Subroutine - Fraction_Thousandths ]----------------------------------' Calculate q = n/d as a number of thousandths.
' n and d should be unsigned and n < d. Requires Nib size temp & i variables.
Fraction_Thousandths:
q = 0
IF n > 6500 THEN
temp = n / 6500
n = n / temp
d = d / temp
ENDIF
FOR i = 0 TO 3
n = n // d * 10
q = q * 10 + (n/d)
NEXT
IF q//10>=5 THEN q=q/10+1 ELSE q=q/10
RETURN
' Subroutine label
' Clear quotient
' If n > 6500
'
scale n into 0..6500
'
scale d with n
' Long division ten thousandths
' Multiply remainder by 10
' Add next digit to quotient
' Round q to nearest thousandth
' Return from subroutine
Optional: How LightSensorValues.bs2 Works
The phototransistor circuits and RCTIME measurements pose two problems for Boe-Bot
navigation. First, an RCTIME measurement for a shadow in a darker room will be a larger
value than the same object casting the same shadow in a brighter room. Second, in
darker rooms, the RCTIME measurements can end up taking a lot longer than the 20 ms of
free time a program has between servo pulses.
The Main Routine in LightSensorValues.bs2 doesn’t have to worry about either of those
problems because the Light_Shade_Info subroutine solves them. The Main Routine
just makes a single call to the Light_Shade_Info subroutine, and then checks the values
of the light and ndShade variables for the two values it needs for navigation with a pair
of light sensors. Again, the light variable indicates the overall light level on a scale of 0
to 324, and the ndShade variable indicates the light/shade difference between sensors on
a scale of -500 to 500.
More detail about the subroutines: This section only focuses on what the subroutines do,
not how they do it. Some more advanced activities that chronicle the development of the
subroutines are available for download from www.parallax.com/go/Boe-Bot. Look for the
Advanced Light Sensing section.
The Light_Shade_Info subroutine starts by calling the Light_Sensors subroutine to
get the tLeft and tRight variables that store the RCTIME measurements. Notice that the
PWM commands in the Light_Sensors subroutine rely on a variable named duty to set
their sensitivity, which in turn controls how long the commands take to get their light
Page 200 · Robotics with the Boe-Bot
measurements. The program has a subroutine named Duty_Auto_Adjust that
automatically adjusts the duty variable to help prevent rooms that are too dark from
disabling the Boe-Bot’s servos and rooms that are bright from blinding the light sensors.
After calling the Light_Sensors subroutine, the Light_Shade_Info subroutine does
some math on tLeft, tRight, and the duty variable to calculate the light variable’s
value, which again indicates the overall light level. Next, it calls the Duty_Auto_Adjust
subroutine, which adjusts the duty variable to try to keep the sum of the RCTIME
measurements in the 1800 to 2200 range. Really dark rooms will still cause the servos to
make the wheels twitch instead of turn, and direct sunlight will still blind the Boe-Bot,
but Duty_Auto_Adjust significantly extends the range of light conditions that the
Boe-Bot can automatically adjust to and navigate in.
Next, the Light_Shade_Info subroutine normalizes the difference between the two
sensors by calculating how much of the total light (measured by both sensors) a single
sensor sees. It does that by solving this equation:


tLeft

ndShade  500  1000 
tLeft  tRight 

This equation solves the problem of shade having different values in rooms with different
light levels. It simply divides one measurement into the sum of both measurements for a
fractional result that could range from 0 to 1. It then multiplies this by 1000 for a result
that could range from 0 to 1000. It then subtracts all that from 500, for the ndShade
variable value, which ranges from -500 to 500.
Let’s say that tLeft is 1500 and tRight is 500. That means there’s shade over the
Boe-Bot’s left light sensor. If you plug the values into the equation, the result will be
-250. Now, in a darker room, that same shade condition might cause tLeft to be 3600
and tRight to be 1200. Those values still result in an ndShade value of -250.
 Use the ndShade equation to calculate both pairs of values discussed in the
paragraph above.
You might have also noticed a new and different feature in the Constants/Variables
section: Negative CON 1. This is a constant declaration, and it allows you to use a
name in place of a number in your program. Instead of using the number 1 at a certain
point in the program to check to find out if a number is negative, the program uses the
Light-Sensitive Navigation with Phototransistors · Page 201
Negative constant instead. So, down in the Duty_Auto_Adjust subroutine, the
statement IF sign=Negative THEN sumDiff = -sumDiff checks to find out if the
sign variable contains a 1, indicating that a value tested as negative earlier in the
subroutine. This line would still work if it was rewritten IF sign=1 THEN sumDiff =
-sumDiff.
Constants can be useful for helping commands with numbers in them be more self
explanatory, and are also useful if you have a number that is used several places in a
program. By updating one CON directive, all the code that uses the constant’s name will use
the updated value. Chapter 8 will utilize this feature of constants for calibrating a program
that makes the Boe-Bot follow objects within a certain range of its infrared object sensors.
Light Measurement Graphic Display
Figure 6-17 shows an example of a graphical display of the ndShade variable. The
asterisk will be in the center of the -500 to 500 scale if the light or shade is the same over
both sensors. If the shade is darker over the Boe-Bot’s left sensor, the asterisk will
position to the left in the scale. If it’s darker over the right, the asterisk will position
toward the right. A larger shade/light contrast (like darker shade over one of the sensors)
will result in the asterisk positioning further from the center.
Figure 6-17: Graphic Display of ndShade Variable
Same light/shade on both sides
Shade darker over left side
Asterisk indicator in center of scale
Asterisk indicator about half way between
center and far left
Page 202 · Robotics with the Boe-Bot
All you need for this display is some small modifications to LightSensorValues.bs2’s
Initialization and Main Routine sections. Below is an example. It makes use of some
new DEBUG formatters, like REP and CRSRX.
The REP formatter repeats a character a certain number of times. So DEBUG CLS, REP
CR\5 clears the screen, and then prints 5 carriage returns, which sends the cursor down 5
lines.
The CRSRX formatter positions the cursor a certain number of spaces to the right of the
Debug Terminal’s left margin. For example, DEBUG HOME, CRSRX 8 sends the cursor to
the Debug Terminal’s top-left character position, then it moves the cursor eight spaces to
the right.
CLREOL is another new formatter that erases everything to the right of the cursor on a
given line. This can be useful when you don’t necessarily know how many digits will be
displayed. If a measurement displays fewer digits than the one before it, the CLREOL
formatter erases any phantom digits that might be left over to the right.
' Excerpt from LightSensorDisplay.bs2
'-----[ Initialization ]-----------------------------------------------------FREQOUT 4, 2000, 3000
' Start beep
DEBUG CLS, REP CR\5,
"
-+---------+---------+-",
CR, "
-500
500"
' Shade level graphical view
'-----[ Main Routine ]-------------------------------------------------------DO
' Main Loop.
GOSUB Light_Shade_Info
DEBUG HOME, CRSRX, 8, "light = ",
DEC3 light, "/324",CR, CR,
CRSRX, 13, "ndShade", CR,
CRSRX,15, SDEC3 ndShade, CLREOL, CR,
CLREOL, CRSRX,
6+((ndShade+500)/50), "*"
' Get light & ndShade
' Display
' light variable name at x=8
' light variable value
' shade heading at x = 13
' ndShade value at x = 15
' display asterisk at ndShade
'
x-position
PAUSE 100
' Delay 0.1 seconds
LOOP
' Repeat main loop
Light-Sensitive Navigation with Phototransistors · Page 203
 LightSensorDisplay.bs2 was another example in LightSensorExamples.zip.
Open it with the BASIC Stamp Editor.
 If you would prefer to save LightSensorValues.bs2 as LightSensorDisplay.bs2
and hand enter the changes, make sure to leave five spaces between the quotation
marks and the first characters in each scale. For example, there are 5 spaces
-500…
between the quotation marks and the first dash in CR, "
 Remember, for best results, make sure to adjust the area lighting so that the
Debug Terminal displays light values in the 125 to 275 range with no shadows
over the phototransistors.
 Run the program and try casting different levels of shade over each light sensor,
and watch how the asterisk in Figure 6-17 responds. Remember that if you cast
equal shade over both sensors, the asterisk should still be in the middle, it only
indicates which sensor sees more shade if there’s a difference between them.
ACTIVITY #5: ROUTINE FOR ROAMING TOWARD LIGHT
One approach toward making the Boe-Bot roam toward light sources is to make it turn
away from shade. You can even use the ndShade variable to make the Boe-Bot turn
more or less when the contrast between the light detected on each side is more or less.
First, we need a couple variables to store pulse duration variables for the servos.
' Application Variables
pulseLeft
VAR
Word
pulseRight
VAR
Word
Next, we need some code to set those pulse values. The code below sets pulseLeft and
pulseRight to keep the wheel under shade going full speed and slow down or reverse
the other wheel. When the contrast between light and shade measurements is small, the
wheel that’s not under shade only slows down somewhat for a gradual turn. When the
contrast is larger, the wheel on the other side from the shade may slow down more, or
even start turning backwards so that the Boe-Bot executes a sharper turn away from that
darker shadow.
' Navigation Routine
IF (ndShade + 500) > 500 THEN
pulseLeft = 900 - ndShade MIN 650 MAX 850
pulseRight = 650
ELSE
pulseLeft = 850
pulseRight= 600 - ndShade MIN 650 MAX 850
ENDIF
Page 204 · Robotics with the Boe-Bot
The routine that sets pulseLeft and pulseRight values starts by deciding if the shade
is over the right or left sensor, by comparing (ndShade + 500) to 500. The > (greater
than), >= (greater than or equal to), < (less than), and <= (less than or equal to) operators
only compare two positive numbers. Since the smallest ndShade could be is -500, the
code in the IF condition adds 500 to ndShade and then compares it to 500. It’s the
PBASIC equivalent to IF ndShade > 0.
Let’s say that ndShade is 125, which means there’s definitely some shade over the right
light sensor. IF ndShade + 500 > 500 THEN checks if 625 is greater than 500, which
it is. Figure 6-18 shows what happens next as the code slows down the left wheel with
pulseLeft = 900 – ndShade MIN 650 MAX 850, and sets the right wheel to full
speed forward with pulseRight = 650. Since ndShade is 125 in this example, 900 –
125 = 775, which would cause a PULSOUT command to slow down the left wheel.
Figure 6-18
LightSeekingBoeBot.bs2’s
Reaction to
Shade on Right
In this example,
an ndShade
measurement of
125 gets
subtracted from
900, and the
result of 775
slows down the
left servo’s
speed.
Light-Sensitive Navigation with Phototransistors · Page 205
If ndShade is larger, like maybe 190, which means the shade over the right sensor is
darker, pulseLeft ends up with a value of 710, which would make the left wheel turn
backwards for a much sharper turn. For values of ndShade that are greater than 250, the
expression 900 - ndShade might result in values smaller than 650. Likewise, for values
of ndShade between 1 and 49, the expression might result in values above 850. So, the
code uses MIN and MAX operators to keep the result in the 650 to 850 range even though
900 - ndShade might have intermediate results outside that range.
The MIN operator takes a result below the specified value and increases it to that value,
but leaves results above the MIN value alone. So, if the result of 600 - ndShade is
anything below 650, the MIN operator stores 650 in pulseLeft.
For example, if ndShade were 350, the intermediate result of 900 - ndShade would be
550, but MIN 650 would change that to 650. Similarly, the MAX operator takes a result
above the specified value and decreases it to that value, but leaves results below the MAX
value alone. So, even though values from 0 to 49 would yield intermediate 900 ndShade results in the 900 to 851 range, the MAX 850 part of the expression sets any
result in that range to 850.
For ndShade values of zero or less, it means shade is over the left sensor, and the right
wheel needs to slow down. The code in the ELSE block does that by setting the left wheel
to full speed with pulseLeft = 850 to make the Boe-Bot’s left wheel go full speed
forward, and pulseRight = 600 - ndShade MIN 650 MAX 850 to slow down or even
reverse the direction of the Boe-Bot’s right wheel, depending on how dark the shade is
over the left light sensor.
Page 206 · Robotics with the Boe-Bot
Test Navigation Routine with Debug Terminal
Figure 6-19 shows some Debug Terminal display examples from the next example
program, LightSeekingDisplay.bs2. Checklist instructions will prompt you to run the
program next, but first, just take a look at the Debug Terminal displays in the figure.
These screen captures demonstrate how the navigation routine adjusts the pulseLeft and
pulseRight variables in response to different ndShade values. The program makes the
Debug Terminal display a top-view of the Boe-Bot with pulseLeft and pulseRight
labels and their values next to each wheel. The program also positions the left > and right
< wheel speed indicators to show how fast and in which direction each wheel would be
turning.
For example, in the upper-left Debug Terminal, both wheel speed indicators are level
with the forward label, which means the Boe-Bot would be going full speed forward.
In the upper-right Debug Terminal, the right speed indicator is half way between the
forward and backward labels which means that wheel would stop.
In the lower-left Debug Terminal, the left wheel speed indicator is level with the reverse
label, which means the left wheel would be turning full speed in reverse.
In the lower-right, the light variable is lower. Since ndShade is close to zero, the level
of shade is about the same, so there must be shade over both sensors. Since the Boe-Bot
only responds to differences in shade, the same shade over both sensors means that both
wheels would be turning full speed forward again.
Light-Sensitive Navigation with Phototransistors · Page 207
Figure 6-19: Shade and Wheel Speed Indicator Examples
Equal light – full speed forward
Left speed
indicator
Shade over left – slow down right wheel
Right speed
indicator
Right wheel
stopped
Dark shade over right – left wheel full speed reverse
Left wheel full
speed reverse
Equal shade – back to full speed forward
Page 208 · Robotics with the Boe-Bot
LightSeekingDisplay.bs2 is another example from the LightSensorExamples.zip archive.
It expands on LightSensorDisplay.bs2 with these features:





Word-size variable declarations for pulseLeft and pulseRight
A DEBUG command that displays a top view of the Boe-Bot
The light seeking routine IF…THEN…ELSE…ENDIF code block
Debug commands that display the pulseLeft and pulseRight variable values
next to each wheel.
DEBUG commands that position the left wheel speed indicator > and right wheel
speed indicator < to show the speed and direction of each wheel.
LightSeekingDisplay.bs2 is a great program for observing the pulseLeft and
pulseRight variable responses to shadows over each sensor and how those values affect
wheel speed.
 Open LightSeekingDisplay.bs2 with the BASIC Stamp Editor and download it to
the BASIC Stamp.
 Again, for best results, adjust ambient lighting in the room so that the Debug
Terminal displays a light variable value in the 125 to 275 range with no shade
over the sensors.
 Experiment with more and less shade over each sensor, and pay careful attention
to how that affects the ndShade value, which in turn affects the pulseLeft and
pulseRight variables and the wheel speeds that they would set if used in
PULSOUT commands.
' Excerpts from LightSeekingDisplay.bs2
'
...
(three dots indicate code omitted)
' Application Variables
pulseLeft
VAR
Word
pulseRight
VAR
Word
'
...
' Left servo pulse duration
' Right servo pulse duration
Light-Sensitive Navigation with Phototransistors · Page 209
'-----[ Initialization ]-----------------------------------------------------FREQOUT 4, 2000, 3000
' Start beep
DEBUG CLS, REP CR\5,
"
-+---------+---------+-",
' Shade level graphical view
CR, "
-500
500",
CR, "
Boe-Bot
forward",
CR, "
--------",
' Boe-Bot top view with pulse
CR, "
|| |[] ---- | ||
",
' labels
CR, "
|| |[]| -- | | ||
",
CR, "pulse
||=|
---- |=||
pulse ",
CR, "Left
|| |
==== | ||
Right ",
CR, "
|| ||| ___ | ||
",
CR, "
||| |___| |
",
CR, "
|||
|
reverse",
CR, "
---\O/--"
'
0123456789
+10
0123456789
+30 ' Cursor positions to help if you
'
+0
0123456789
+20
01234567' are hand entering the code.
'-----[ Main Routine ]-------------------------------------------------------DO
' Main Loop.
GOSUB Light_Shade_Info
DEBUG HOME, CRSRX, 8, "light = ",
DEC3 light, "/324",CR, CR,
CRSRX, 13, "ndShade", CR,
CRSRX,15, SDEC3 ndShade, CLREOL, CR,
CLREOL, CRSRX,
6+((ndShade+500)/50), "*"
' Get light & ndShade
' Display
' light variable name at x=8
' light variable value
' shade heading at x = 13
' ndShade value at x = 15
' display asterisk at ndShade
'
x-position
' Navigation Routine
IF (ndShade + 500) > 500 THEN
' If more shade on right...
pulseLeft = 900 - ndShade MIN 650 MAX 850' Slow left wheel w/ right shade
pulseRight = 650
' Right wheel full speed forward
ELSE
' If more shade on left...
pulseLeft = 850
' Left wheel full speed forward
pulseRight= 600 - ndShade MIN 650 MAX 850' Slow right wheel w/ left shade
ENDIF
DEBUG CRSRXY, 1, 10, DEC3 pulseLeft,
CRSRX, 29, DEC3 pulseRight
' Display pulse variable values
' above variable names
FOR i = 7 TO 15
DEBUG CRSRXY, 6, i, " ", CRSRX, 26, " "
NEXT
' Clear areas where > and <
' wheel speed and direction
' indicators might be placed
DEBUG CRSRXY,6,15-((pulseLeft-650)/25),">" ' Place new wheel speed and
DEBUG CRSRXY,26,7+((pulseRight-650)/25),"<"' direction indicators
LOOP
' ...
' Repeat main loop
Page 210 · Robotics with the Boe-Bot
LightSeekingDisplay.bs2 utilizes another DEBUG formatter, CRSRXY, to position the cursor
to display each wheel speed indicator. CRXRXY should be followed by two numbers. For
example, DEBUG CRXRXY, 6, 11, ">" would display the > character at six spaces from
the Debug Terminal’s left margin, and 11 carriage returns down from the top. The
program actually uses an expression to set the number of carriage returns for positioning
the cursor. For example, the command:
DEBUG CRSRXY,6,15-((pulseLeft-650)/25),">"
...positions the cursor 6 spaces from the Debug Terminal’s far left, but it uses an
expression to choose the number of carriage returns from the Debug Terminal’s top line.
Let’s say that pulseLeft is 750. Then, the y position would be 11 because 15 – ((750 –
650)/25 = 15 – 4 = 11. So in that case, CRSRXY positions the cursor at 6, 11, and then
prints the ">" character.
Your Turn – Save Lots of RAM
You might want to add other features to your Boe-Bot on top of light seeking, but that
might be difficult if your application runs out of RAM just because you try to declare a
few more variables. The left side of Figure 6-20 shows the problem, the application has
slightly less than 2 words left. The right side of the figure shows how much space you
can save by using a simple technique called variable aliasing.
 To see the RAM Map for LightSeekingDisplay.bs2, click the Memory Map
button; it’s just to the left of the Run button. You can also display it by clicking
Run → Memory Map, or by pressing the CTRL + M keys. Your RAM Map should
resemble the one on the left side of Figure 6-20.
According to the BASIC Stamp Help, an alias is “an alternative name for an existing
variable.”
 In the BASIC Stamp Editor, click Help and select BASIC Stamp Help.
 Click PBASIC Language Reference and then click Variables to display the
Variables page. Find the alias explanation, read it and examine the example
variable declaration that uses aliases.
Not all of the variables in LightSeekingDisplay.bs2 are used all the time. For example,
the program is done with tLeft and tRight after the Light_Shade_Info subroutine is
done. Further, it never uses those two variables at the same time that it uses pulseLeft
Light-Sensitive Navigation with Phototransistors · Page 211
and pulseRight. So tLeft can be declared as an alias of pulseLeft and tRight can
be declared as an alias for pulseRight. Now, pulseLeft and tLeft use the same piece
of memory, and likewise with pulseRight and tRight, and your application just
recovered two words of RAM.
With the modifications in LightSeekingDisplayBetterRAM.bs2, the program reduces
RAM usage from “almost all” to “less than half.”
Figure 6-20: Variable Aliases Save almost Half of the BASIC Stamp’s RAM
 Save LightSeekingDisplay.bs2 as LightSeekingDisplayBetterRAM.bs2.
 Update the variable declarations in LightSeekingDisplay.bs2 so that they match
the right side of Figure 6-21.
 Recheck your Memory Map. It should now resemble the one on the right side of
Figure 6-20.
Page 212 · Robotics with the Boe-Bot
Figure 6-21 Saving Space with Variable Aliases
' Application Variables
pulseLeft
VAR
Word
pulseRight
VAR
Word
light
VAR
Word
ndShade
VAR
Word
' Application Variables
pulseLeft
VAR
Word
pulseRight
VAR
Word
light
VAR
Word
ndShade
VAR
Word
' Subroutine Variables
tLeft
VAR
Word
tRight
VAR
Word
n
VAR
Word
d
VAR
Word
q
VAR
Word
sumDiff
VAR
Word
duty
VAR
Byte
i
VAR
Nib
temp
VAR
Nib
sign
VAR
Bit
' Subroutine Variables
tLeft
VAR
pulseLeft
tRight
VAR
pulseRight
n
VAR
tLeft
d
VAR
Word
q
VAR
ndShade
sumDiff
VAR
d
duty
VAR
Byte
i
VAR
Nib
temp
VAR
i
sign
VAR
Bit
CAUTION: Be careful how you use variable aliases. If the program needs two variables
at the same time, one variable cannot be an alias for the other. Likewise, if the program
needs to check the previous value of a variable in the next iteration of a loop, giving it an
alias and using it for another purpose would erase a value your program needs later.
For example, if you tried to make pulseLeft an alias for pulseRight, both your servo
speeds would always be the same. They could not be the two independent values your
code needs for servo control.
ACTIVITY #6: TEST NAVIGATION ROUTINE WITH THE BOE-BOT
This code excerpt from LightSeekingBoeBot.bs2 has a navigation-only version of the
Initialization and Main Routines from LightSeekingDisplayBetterRAM.bs2. All the
DEBUG commands have been removed along with that 100 ms PAUSE command. They
all got replaced with PULSOUT commands that use the pulseLeft and pulseRight
variables to control the servos.
Light-Sensitive Navigation with Phototransistors · Page 213
'-----[ Initialization ]-----------------------------------------------------FREQOUT 4, 2000, 3000
' Start beep
DEBUG "Program running..."
' Display program running message
'-----[ Main Routine ]-------------------------------------------------------DO
' Main Loop.
GOSUB Light_Shade_Info
' Get light & ndShade
' Navigation Routine
IF (ndShade + 500) > 500 THEN
' If more shade on right...
pulseLeft = 900 - ndShade MIN 650 MAX 850' Slow left wheel w/ right shade
pulseRight = 650
' Right wheel full speed forward
ELSE
' If more shade on left...
pulseLeft = 850
' Left wheel full speed forward
pulseRight= 600 - ndShade MIN 650 MAX 850' Slow right wheel w/ left shade
ENDIF
PULSOUT 13, pulseLeft
PULSOUT 12, pulseRight
LOOP
' Left servo control pulse
' Right servo control pulse
' Repeat main loop
 Open LightSeekingBoeBot.bs2 with the BASIC Stamp Editor and load it into
your BASIC Stamp.
 Or, save LightSeekingDisplayBetterRAM.bs2 as LightSeekingBoeBot.bs2 and
update the Initialization and Main Routine so that they match the code snippet
above.
 Run the program.
 If you have a Board of Education, set the 3-position switch to 2 after you have
disconnected the Boe-Bot from its programming cable and set it where you want
it to start roaming.
 Your Boe-Bot can now roam toward the light.
 Try casting shadows over your Boe-Bot’s light sensors as it roams. It should
turn away from the shadows.
 Try sending your Boe-Bot toward a dark shadow cast by a desk. Make sure its
approach is at an angle instead of making it drive straight into the shadow. It
should veer away.
 Try taking the Boe-Bot into a low light room with bright light streaming in the
doorway. Can it find its way out?
Page 214 · Robotics with the Boe-Bot
Troubleshooting
If the Boe-Bot robot seems a little less sensitive to light on one side, try correcting it by
following the instructions in the next section (Your Turn – Light/Shade Sensitivity
Adjustments). The same applies if you want to the Boe-Bot to either be more or less
sensitive to shade.
If the Boe-Bot does not respond to shadows by turning away from them, or if it turns in place
instead of roaming, follow these steps:

If you hand-entered your code, try the LightSeekingBoeBot.bs2 code example in
LightSensorExamples.zip, a free download from www.parallax.com/go/Boe-Bot.
This will rule out coding errors before taking a closer look at your circuit.

If code from the Parallax web site doesn’t fix the problem, you’ll need to check the
circuit next. Start by carefully verifying all your circuit connections against the
schematic and wiring diagram in Activity #2. Double check the resistor color
codes (brown-black-red), capacitor numbers (104), the phototransistor pin lengths,
and make sure that all the leads are connected as shown in the wiring diagram.
Also make sure to verify that you selected phototransistors and not infrared LEDs
with the help of Figure 6-8 on page 180. Verify that the phototransistors are
pointing upwards and outwards, like in Figure 6-10 on page 183. Sometimes,
pointing them a little further outwards improves responses to shade. As you
adjust the directions the phototransistors point, make sure that their leads do not
touch each other. Repeat the Debug Terminal tests, and make sure to do it for
both sensors. Each one should respond similarly to light and shade.

Next, repeat the tests in Activity #5. Use the Debug Terminal to verify that the
light levels are in the 125 to 275 range. Also, shade over a given sensor should
slow down the servo on the other side. Similar shade over the other sensor
should result in a similar motor speed adjustment of the other wheel. Also verify
that the same light or shade level over both sensors results in full-speed-forward.

If the Debug Terminal displays a value of ndShade that’s way off from zero, even
when the sensors see about the same light level, there are some additional tests
you can try in the Phototransistor Matching activity. It’s part of the advanced light
sensing activities available from www.parallax.com/go/Boe-Bot. If the Debug
Terminal instead indicates that the sensors and servos are both responding to
shade and light correctly, try LightSeekingBoeBot.bs2 again. If it still doesn’t
respond correctly to shadows, it’s time to check your servo motors. Repeat
Chapter 2, Activity #6. For a closer look, make the graphs in Chapter 3, Activity
#4 for both servos.
For more troubleshooting help, try the www.parallax.com/support resources.
Light-Sensitive Navigation with Phototransistors · Page 215
Your Turn – Light/Shade Sensitivity Adjustments
You can change the 900 and 600 in these lines to make the Boe-Bot more or less
sensitive to shade:
pulseLeft = 900 - ndShade MIN 650 MAX 850
...and:
pulseRight= 600 - ndShade MIN 650 MAX 850
For example, you can increase the Boe-Bot’s light/shade sensitivity by changing 900 to
875 and changing 600 to 625. You can make it even more sensitive by changing the 900
to 850 and the 600 to 650. Likewise, you could make the whole system less sensitive by
changing the 900 to 925, and the 600 to 575, and so on…
 Try it.
You can also adjust one of the values to make either the left or right sensor more
sensitive. Changing 900 to another value would change the Boe-Bot’s sensitivity to
shade on the right, while changing 600 to another value would change the Boe-Bot’s
sensitivity to shadows on the right.
 Try that too.
Other things you can do with minimal adjustments to the Main Routine include:




Following shade instead of light with ndShade = -ndShade right after the
Main Routine’s Light_Shade_Info subroutine call.
Ending roaming under a bright light or in a dark cubby by detecting very bright
or dark conditions with the light variable with an IF…THEN statement.
Functioning as a light compass by remaining stationary and turning toward
bright light sources.
Incorporating whiskers into the roaming toward light so that the Boe-Bot can
detect and navigate around objects in its way.
Page 216 · Robotics with the Boe-Bot
SUMMARY
A phototransistor is a light–controlled current valve. It lets more current through with
brighter incident light and less current through with less bright light. This chapter
utilized two different phototransistor circuits to detect light: a voltage output circuit and a
charge transfer circuit.
The phototransistor voltage output circuit in this chapter was connected to an I/O pin set
to input for a binary value that indicated bright or ambient light. When the
phototransistor lets more current through, the voltage across the resistor is larger. When
it lets less current through, the voltage across the resistor is smaller. By choosing the
right resistor for the lighting conditions, the circuit can be monitored by an I/O pin
because its voltage will go above 1.4 V in bright light, and below 1.4 V in ambient light.
The I/O pin’s input register stores a 1 when the voltage is above 1.4 V and a 0 when it’s
below 1.4 V.
A pair of charge transfer circuits were used for measuring differences in light intensity
between the left and right phototransistor, and the Boe-Bot was programmed to detect
and act on those differences. The charge transfer circuit consisted of a parallel capacitor
and phototransistor connected to an I/O pin with a resistor. In this circuit, the BASIC
Stamp used an I/O pin to charge the capacitor. Then, it switched the I/O pin to input and
measured the time it took the capacitor’s voltage to decay as it lost its charge through the
phototransistor. This decay time measurement turns out to be smaller with bright light
and larger in shade.
A HIGH command followed by a PAUSE can charge the capacitor, and then the RCTIME
command changes the I/O pin to input and measures the time it takes for the capacitor’s
voltage to decay to 1.4 V as it loses its charge through the phototransistor. The
measurement’s time can be reduced by using the PWM command in place of HIGH and
PAUSE. The PWM command can charge the capacitor to values less than 5 V before the
RCTIME command, so the capacitor has fewer volts to decay before it reaches 1.4 V, and
this takes less time. The time reduction helps keep the delay between PULSOUT
commands from getting so large that it makes the servos twitch instead of turn.
Light-Sensitive Navigation with Phototransistors · Page 217
Activity #4 through Activity #6 utilize a collection of subroutines that supply the Main
Routine with values of overall light levels along with the light/shade contrast between the
two sensors. This light/shade contrast between the sensors is called a differential
measurement, and the subroutines also normalize the measurement to a scale of -500 to
500. The actual decay measurements may vary depending on ambient light levels, so the
normalized values keep these measurements on a scale that is useful for servo control and
Boe-Bot navigation toward light sources.
Questions
1. What does a transistor regulate?
2. Which phototransistor terminals have leads?
3. How can you use the flat spot on the phototransistor’s plastic case to identify its
terminals?
4. Which color would the phototransistor be more sensitive to: red or green?
5. How does VP6 in Figure 6-6 respond if the light gets brighter?
6. What does the phototransistor in Figure 6-6 do that causes VP6 to increase or
decrease?
7. How can the circuit in Figure 6-6 be modified to make it more sensitive to light?
8. What happens when the voltage applied to an I/O pin that has been set to input is
above or below the threshold voltage?
9. If the amount of charge a capacitor stores decreases, what happens to the voltage
at its terminals?
Exercises
1. Solve for VP6 if I = 1 mA in Figure 6-6.
2. Calculate the current through the resistor if VP6 in Figure 6-6 is 4.5 V.
3. Assume that the threshold between light and dark needed for your application
occurs when VP6 = 2.8 V. Calculate the resistor value you would need for the
BASIC Stamp to detect this threshold.
4. Calculate the value of a capacitor that has been stamped 105.
5. Write an RCTIME command that measures decay time with I/O pin P7 and stores
the result in a variable named tDecay.
6. Write a PWM command that charges the capacitor in Figure 6-11 to about
1.625 V to prepare the circuit for a decay measurement.
7. Calculate what the ndShade measurement would be if the BASIC Stamp
measures decay values of 1001 on both sides.
8. Write a DEBUG command that displays fifty equal sign characters.
Page 218 · Robotics with the Boe-Bot
9. Write a DEBUG command that positions the character “#” eight spaces to the
right of the Debug Terminal’s left margin and 10 carriage returns down from the
top.
Projects
1. In Activity #1, the circuit along with the example code in the Your Turn section
made the Boe-Bot stop under a light at the end of the course. What if you will
only have a limited time at the course before the competition, and you don’t
know the lighting conditions in advance? You might need to calibrate your BoeBot on site. A program that makes the piezospeaker beep repeatedly when the
Boe-Bot detects bright light and stay quiet when it detects ambient light could be
useful for this task. Write and test the program that works with the circuit in
Figure 6-4 on page 173.
2. Develop an application that makes the Boe-Bot roam and search for darkness
instead of light. This application should utilize the charge transfer circuits in
Figure 6-9 on page 182.
3. Develop an application that makes the Boe-Bot roam toward a bright
incandescent desk lamp in a room where the only other light sources are
fluorescent ceiling lights. The Boe-Bot should be able to roam toward the desk
lamp and play a tone when it’s under it. This application should utilize the
charge transfer circuits in Figure 6-9 on page 182.
Solutions
Q1. The amount of current it allows to pass into its collector and out through it’s
base.
Q2. The phototransistor’s collector and emitter terminals are connected to pins.
Q3. The pin that’s closer to the flat spot is the emitter. The pin that’s further away
from the flat spot is the collector.
Q4. The wavelength of red is closer to the wavelength of infrared, so it should be
more sensitive to red.
Q5. VP6 increases with more light.
Q6. It supplies the resistor with more or less current.
Q7. Change the 2 kΩ resistor to a higher value.
Q8. If the applied voltage is above the threshold voltage, the input register bit for that
pin stores a 1. If it’s below threshold voltage, the input register bit stores a 0.
Q9. The voltage decreases.
E1. V = I × R = 0.001 A × 2000 Ω = 2 V.
Light-Sensitive Navigation with Phototransistors · Page 219
E2. V = I × R → I = V ÷ R = 4.5 ÷ 2000 = 0.00225 A = 2.25 mA.
E3. The BASIC Stamp’s threshold voltage is 1.4 V, but the light threshold occurs at
2.8 V. So, the phototransistor delivers a certain current that results in a 2.8 V
measurement, in terms of V = I × R, that’s 2.8 V = I × 2000 Ω. We need to
figure out the resistance to make the voltage 1.4 V for that same current, that’s
1.4 V = I × R. To figure out R, rearrange the first equation to determine I; that’s
I = 2.8 V ÷ 2000 Ω. Then, substitute 2.8 V ÷ 2000 Ω for I in the second equation
and solve for R. That’s 1.4 V = I × R → 1.4 V = (2.8 V ÷ 2000 Ω) × R → R =
1.4 V ÷ (2.8 V ÷ 2000 Ω ) = 2000 Ω × (1.4 V ÷ 2.8 V) = 1000 Ω = 1 kΩ.
E4. 105 → 10 with 5 zeros appended and multiplied by 1 pF. 1,000,000 × 1 pF = (1
× 106) × (1 × 10–12) F = 1 × 10–6 F = 1 μF.
E5. It would be RCTIME 7, 1, tDecay
E6. 1.625 × 256 ÷ 5 = 83.2, take 83. Answer: PWM 6, 83, 1
E7. ndShade = 500 – (1000 × tLeft ÷ (tLeft + tRight)) = 500 – (1000 × 1001 ÷
(1001 + 1001)) = 500 – 1000/2 = 500 – 500 = 0.
E8. It would be DEBUG REP "="\50
E9. It would be DEBUG CRSRXY 8, 10, "#"
P1.
'
'
'
'
Robotics with the Boe-Bot - CH6P1.bs2
Chirp periodically if bright light. Otherwise, stay silent.
{$STAMP BS2}
{$PBASIC 2.5}
PAUSE 1000
DEBUG "Program running..."
DO
IF IN6 = 1 THEN FREQOUT 4, 20, 4000
PAUSE 100
LOOP
P2. The solution for this one is to make a copy of LightSeekingBoeBot.bs2, and add
one command to the Main Routine: ndShade = -ndShade. Add it right after
the call to the Light_Shade_Info subroutine. Then, instead of indicating shade
to turn away from, it indicates light to turn away from.
P3. Below is a modified Main Routine from LightSeekingBoeBot.bs2 that roams
toward the light and stops when it gets under an incandescent lamp. The key to
this one is very simple because LightSeekingBoeBot.bs2 has a light variable
that reaches higher values under bright light. With each repetition of the
DO…LOOP, the IF…THEN statement checks for values above 320.
Page 220 · Robotics with the Boe-Bot
For lower light areas and weaker flashlights, you may need to adjust IF light
> 320 THEN... so that it compares the light variable to a lower value, for
example: IF light > 250 THEN… Decreasing the value the IF…THEN
statement compares to the light variable to makes it more sensitive; increasing
it makes it less sensitive. The value 324 is the highest possible value so don’t
increase your comparison value above 323.
TIP: Use LightSensorValues.bs2 to test and find a value that’s between ambient
light and the flashlight beam.
DO
GOSUB Light_Shade_Info
IF light > 320 THEN
FREQOUT 4, 500, 3000
PAUSE 500
FREQOUT 4, 500, 3500
PAUSE 500
END
ENDIF
IF (ndShade + 500) > 500 THEN
pulseLeft = 900 - ndShade MIN 650 MAX 850
pulseRight = 650
ELSE
pulseLeft = 850
pulseRight= 600 - ndShade MIN 650 MAX 850
ENDIF
PULSOUT 13, pulseLeft
PULSOUT 12, pulseRight
LOOP
Navigating with Infrared Headlights · Page 221
Chapter 7: Navigating with Infrared Headlights
The Boe-Bot can already use whiskers to get around objects it detects when it bumps into
them, but wouldn’t it be better if the Boe-Bot could just “see” objects and then decide
what to do about them? Well, that’s exactly what the Boe-Bot can do with infrared
headlights and eyes like the ones in Figure 7-1. The infrared headlight is an infrared
LED inside a light shield that directs its light forward just like a flashlight. The infrared
eye is an infrared receiver that sends the BASIC Stamp high/low signals to indicate
whether it detects the infrared LED’s light reflected off an object.
Figure 7-1: Infrared Object Detection
Infrared Receiver
Infrared LED
Left: Infrared reflected, obstacle detected.
Infrared Receiver
Infrared LED
Right: Infrared not reflected, no obstacle detected.
INFRARED LIGHT
Infrared is abbreviated IR and you can think about it as a form of light the human eye
cannot detect. For a refresher, take a look at Figure 6-2 on page 171. Devices like the IR
LED introduced in this chapter emit infrared light, and devices like the phototransistor
from the previous chapter and the infrared receiver from this chapter detect infrared light.
Figure 7-2 shows how the infrared LED the Boe-Bot uses as a tiny flashlight is actually
the same one you can find in just about any TV remote. The TV remote sends IR
messages to your TV, and the microcontroller in your TV picks up those messages with
an infrared receiver like the one your Boe-Bot will use to detect IR reflected off of
objects.
Page 222 · Robotics with the Boe-Bot
Figure 7-2: IR LED and Receiver in Your Home
IR on/off at ≈38 kHz for
certain periods of time
IR Receiver
IR LED
Note that the TV remote sends messages describing which button you press by rapidly
flashing its IR LED on/off at a rate in the 38 kHz neighborhood. That’s around 38,000
times per second. The IR receiver only responds to infrared if it’s flashing on/off at this
rate. This prevents infrared from sources like the sun and incandescent lights from being
misinterpreted as messages from the remote. So, to send signals the IR receiver can
detect, your BASIC Stamp will have to send signals in the 38 kHz range too. The
PBASIC language makes short work of that task, with just one line of code to transmit
the signal, and a second line to check the IR receiver.
Some fluorescent lights do generate signals that can be detected by the IR receivers.
These lights can cause problems for your Boe-Bot’s infrared headlights. One of the things
you will do in this chapter is develop an infrared interference “sniffer” that you can use to test
the fluorescent lights near your Boe-Bot courses.
The light color sensors inside most digital cameras, including cell phones and webcams,
can all detect infrared light, which gives us a way to “see” it even if the eye cannot detect
it. Figure 7-3 shows an example with a digital camera and a TV remote. When you press
Navigating with Infrared Headlights · Page 223
and hold a button on the remote and point its IR LED into the digital camera’s lens, it
displays the infrared LED as a flashing, bright white light.
Figure 7-3: IR LED in a TV Remote Viewed through a Digital Camera
With a button pressed and held, the IR LED
doesn’t look any different.
Through a digital camera display, the IR LED
appears as a flashing, bright white light.
The pixel sensors inside the digital camera detect red, green, and blue light levels, and the
processor adds up those levels to determine each pixel’s color and brightness. Regardless
of whether a pixel sensor detects red, green, or blue, it detects infrared. Since all three
pixel color sensors detect infrared, the digital camera display mixes all the colors
together, which results in white.
Infra means below, so infrared means below red. The name refers to the fact that the
frequency of infrared light waves is less than the frequency of red light waves.
IR wavelengths and their uses: The wavelength our IR LED transmits is 980 nm, and
that’s the same wavelength our IR receiver detects. This wavelength is in the near-infrared
range. The far-infrared range is 2000 to 10,000 nm, and certain wavelengths in this range
are used for night-vision goggles and IR temperature sensing.
ACTIVITY #1: BUILDING AND TESTING THE IR OBJECT DETECTORS
In this activity, you will build and test infrared object detectors for the Boe-Bot robot.
 Gather the parts in the Parts List using Figure 7-4 to help identify the infrared
receivers, LEDs, and shield assembly parts.
Page 224 · Robotics with the Boe-Bot
Parts List:
Figure 7-4
New Parts
Used in this
Chapter
(2)
(2)
(2)
(2)
IR receivers
IR LEDs (clear case)
IR LED shield assemblies
Resistors, 220 Ω
(red-red-brown)
(2) Resistors, 1 kΩ
(brown-black-red)
IR receiver
(top)
IR LED
(middle)
IR LED shield
assembly
(bottom)
 Check Figure 7-5 to make sure you have selected infrared LEDs and not
phototransistors. The infrared LED has a taller and more rounded plastic dome,
and is shown on the right side of Figure 7-5.
More Rounded
Dome
Flatter on
top
Phototransistor
Infrared LED
Figure 7-5
Distinguishing
Phototransistors from
Infrared LEDs
Make sure you have two
infrared LEDs.
USE THIS ONE!
Building the IR Headlights
 Insert the infrared LED into the LED standoff (the larger of the two shield
assembly pieces) as shown in Figure 7-6.
 Make sure the IR LED snaps into the LED standoff.
 Slip the LED shield (the smaller half of the LED shield assembly) over the IR
LED’s clear plastic case. The ring on one end of the LED shield should fit right
into the LED standoff.
 Use a small piece of clear tape to make sure the two halves of the shield
assembly don’t separate during use.
Navigating with Infrared Headlights · Page 225
IR LED will snap in.
+
Figure 7-6
Snapping the IR LED into the Shield
Assembly
-
IR Object Detection Circuit
Figure 7-7 shows the IR object detection circuit schematic and Figure 7-8 shows a wiring
diagram of the circuit. In the wiring diagram, one IR object detector (IR LED and
receiver) is mounted on each corner of the breadboard closest to the very front of the
Boe-Bot.
 Disconnect power from your board and servos.
 Build the circuit in the Figure 7-7 schematic using the Figure 7-8 wiring diagram
as a reference for parts placement.
Vdd
P2
1 k
IR
LED
P9
220 
Vss
Vss
Vdd
P8
1 k
IR
LED
P0
220 
Vss
Vss
Figure 7-7
Left and Right IR
Object Detectors
Page 226 · Robotics with the Boe-Bot
Watch your IR LED anodes and cathodes!
The anode lead is the longer lead on an IR LED by convention. The cathode lead is shorter
and mounted in the plastic case closer to its flat spot. The same conventions applied to the
red LEDs in Chapter 2.
In Figure 7-8, the anode lead of each IR LED connects to a 1 kΩ resistor. The cathode lead
plugs into the same breadboard row as an IR detector’s center pin, and that row is
connected to Vss with a jumper wire.
Figure 7-8: Wiring Diagrams for Infrared Emitter and Receiver Circuits
To Servos
To Servos
15 14 Vdd 13 12
(916) 624-8333
Rev B
www.parallax.com
www.stampsinclass.com
Red
Black
X4
Vdd
X5
Vin
Vdd
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Left
anode
leads
+
Board of Education
Rev C
Vin
Vss
X3
X3
© 2000-2003
Right
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Left
anode
leads
+
HomeWork Board
Right
Object Detection Test Code
Your Boe-Bot’s infrared receivers are designed to detect infrared light with a 980 nm
wavelength that’s either flashing on/off or varying in brightness at a rate in the 38 kHz
neighborhood. The infrared LED emits 980 nm IR, so that’s taken care of. All we need
is to make the LED’s brightness vary, brighter and then dimmer, at a rate of about 38
kHz. We can do this with the same command we’ve been using to make the Boe-Bot’s
speaker play a tone at the beginning of each program—the FREQOUT command.
Navigating with Infrared Headlights · Page 227
It takes two lines of code to test for the presence or absence of an object using an IR
object detection circuit. Here is an example that tests to find out if an object is in front of
the Boe-Bot robot’s left IR detection circuit.
FREQOUT 8, 1, 38500
irDetectLeft = IN9
The command FREQOUT 8, 1, 38500 makes the IR LED’s brightness vary, getting
brighter and dimmer 38500 times per second. It does this for 1 ms; then, irDetectLeft
= IN9 stores the IR receiver’s output in a variable. The detector’s output will be high if
it does not detect 38.5 kHz IR reflected off an object, or low if it does. So the value of
IN9 that gets copied to the irDetectLeft variable will be 1 if no object is detected, or 0
if an object is detected.
Always use irDetectLeft = IN9 right after FREQOUT 8, 1, 38500.
The BASIC Stamp only has a brief time window to copy the binary signal it gets from the IR
receiver to a variable. The IR receiver sends a low signal while it detects 38.5 kHz IR
reflected off an object, which causes IN9 to store 0. When the BASIC Stamp finishes
transmitting its FREQOUT signal and moves on to the next command, it stops sending that
38.5 kHz signal. So the program has to use irDetectLeft = IN9 to catch that zero
value before the IR receiver realizes the 38.5 kHz signal stopped. It only takes a fraction of
a millisecond for the IR receiver to realize the signal stopped, and after that, its output
rebounds to high, and IN9 stores 1 again.
Example Program: TestLeftIr.bs2
 Reconnect power to your board.
 Enter, save, and run TestLeftIr.bs2.
' Robotics with the Boe-Bot - TestLeftIr.bs2
' Test IR object detection circuits, IR LED to P8 and detector to P9.
' {$STAMP BS2}
' {$PBASIC 2.5}
irDetectLeft
VAR
Bit
DO
FREQOUT 8, 1, 38500
irDetectLeft = IN9
DEBUG HOME, "irDetectLeft = ", BIN1 irDetectLeft
PAUSE 100
LOOP
Page 228 · Robotics with the Boe-Bot
 Leave the Boe-Bot connected to its programming cable, because you will be
using the Debug Terminal to test your IR object detector.
 Place an object, such as your hand or a sheet of paper, about an inch from the left
IR object detector, in the manner shown in Figure 7-1 on page 221.
 Verify that the Debug Terminal displays a 0 when you place an object a few
inches in front of the IR object detector.
 Verify that it displays 1 when you remove the object.
 If the Debug Terminal displays the expected values for object not detected (1)
and object detected (0), move on to the Your Turn section.
 If the Debug Terminal does not display the expected values, try the steps in the
Troubleshooting section.
Troubleshooting
If the Debug Terminal does not display the expected values, try this checklist:
 Check for circuit and program entry errors. One common error is to use a 10 kΩ
resistor (brown-black-orange) instead of 1 kΩ (brown-black-red).
 Keep the Boe-Bot out of direct sunlight.
 If you are always getting 0, even when an object is not placed in front of the
Boe-Bot, there may be a nearby object that is reflecting the infrared. The surface
of the table in front of the Boe-Bot is a common culprit. Move the Boe-Bot so
that the IR LED and detector cannot possibly be reflecting off any nearby object.
 If the reading is 1 most of the time when there is no object in front of the BoeBot, but flickers to 0 occasionally, it may mean you have interference from a
nearby fluorescent light. Turn off any nearby fluorescent lights and repeat your
tests. Also try closing the blinds if you are near a window.
 If the reading is 1 all of the time, even when an object is placed in front of the
Boe-Bot: Although it’s not a common mistake, manufacturers occasionally make
a batch of LEDs with the longer and shorter leads reversed. If you have already
double-checked your wiring and program, try disconnecting the IR LED and
reversing its polarity, so that the shorter lead is connected to the 1 kΩ resistor
and the longer lead is connected to Vss.
 One final test you can try is to connect your IR LED circuit to a different I/O pin
and adjust your program accordingly. Start with the correct anode/cathode
orientation, and if it doesn’t work, try reversing it again.
Navigating with Infrared Headlights · Page 229
Your Turn – Test the Right IR Object Detector
 Save TestLeftIr.bs2 as TestRightIr.bs2.
 Change the DEBUG command, program title and comments to refer to the right IR
object detector.
 Change the variable name from irDetectLeft to irDetectRight. You will
need to do this in four places in the program.
 Change the FREQOUT command’s Pin argument from 8 to 2.
 Change the input register monitored by the irDetectRight variable from IN9
to IN0.
 Repeat the testing steps in this activity for the Boe-Bot’s right IR object detector.
Sine Waves Synthesized by FREQOUT
The FREQOUT command transmits a rapid sequence of on/off signals that
digitally synthesize voltages to create a sine wave pattern. Sine waves sound
much more natural that square waves when played by a speaker. Square
waves make more of a buzzing noise.
Sine Wave
Square Wave
A FREQOUT signal contains two sine wave components with two different frequencies. One
component’s frequency is Freq1. The second component’s frequency 65536 – Freq1.
When the FREQOUT command is used to play audible tones, the signal’s second frequency
is always well above 20 kHz, which is typically the highest pitch that the human ear can
detect.
Example 1: FREQOUT 4, 2000, 3000 plays a 3 kHz sine wave tone on the piezospeaker
because Freq1 is 3000. The signal contains a second component with a frequency of
65536 – 3000 = 62536 Hz, but the human ear cannot detect it. Since 65536 – 62536 =
3000, you could play the same tone with FREQOUT 4, 2000, 62536. Although Freq1 is
now well outside the human ear’s range, the second signal is 3 kHz, so you’ll get the same
tone out of your piezospeaker.
Example 2: FREQOUT 8, 1, 38500 makes the IR LED’s brightness vary at a rate of
38500 Hz so that the IR receiver can detect it. The signal it creates also contains a second
sine wave with a frequency of 65536 – 38500 = 27036 Hz, but that signal has no effect on
the IR receiver.
Page 230 · Robotics with the Boe-Bot
ACTIVITY #2: FIELD TESTING FOR OBJECT DETECTION AND
INFRARED INTERFERENCE
In this activity, you will build and test indicator LEDs that will tell you if an object is
detected without the help of the Debug Terminal. This is handy if you are not near a PC
or laptop, and you need to trouble-shoot your IR detector circuits. You will also write a
program to “sniff” for infrared interference from fluorescent lights. Some fluorescent
lights send signals that resemble the signal sent by your infrared LEDs. The device
inside a fluorescent light fixture that controls voltage for the lamp is called the ballast.
Some ballasts operate in the same frequency range of your IR detector, 38.5 kHz, which
in turn causes the lamp to emit a signal at this frequency. When you integrate IR object
detection with navigation, this interference can cause some bizarre Boe-Bot behavior!
Rebuilding the LED Indicator Circuits
These are the same LED indicator circuits that you used with the whiskers.
Parts List:
(2) Red LEDs
(2) Resistors, 220 Ω (red-red-brown)
 Disconnect power from your board and servos.
 Build the circuit shown in Figure 7-9 using Figure 7-10 as a reference.
P1
P10
220 
220 
Red
LED
Figure 7-9
Left and Right
Indicator LEDs
Red
LED
Vss
Vss
Navigating with Infrared Headlights · Page 231
Figure 7-10: Wiring Diagrams for Infrared Emitter and Receiver Circuits
To Servos
To Servos
15 14 Vdd 13 12
anode
lead
(916) 624-8333
Rev B
www.parallax.com
www.stampsinclass.com
Red
Black
X4
Vdd
Vdd
X5
Vin
Vss
X3
anode
lead
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
anode
lead
+
Board of Education
Rev C
Vin
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
anode
lead
+
HomeWork Board
© 2000-2003
Board of Education (left) and HomeWork Board (right)
Testing the System
There are quite a few components involved in this system, and this increases the
likelihood of a wiring error. That’s why it’s important to have a test program that shows
you what the infrared detectors are sensing. You can use this program to verify that all
the circuits are working before unplugging the Boe-Bot from its programming cable and
testing other objects.
Example Program – TestBothIrAndIndicators.bs2
 Reconnect power to your board.
 Enter, save, and run TestBothIrAndIndicators.bs2.
 Verify that the speaker makes a clear, audible tone while the Debug Terminal
displays “Testing piezospeaker…”
 Use the Debug Terminal to verify that the BASIC Stamp still receives a zero
from each IR detector when an object is placed in front of it.
Page 232 · Robotics with the Boe-Bot
 Verify that the LED next to each detector emits light when the detector detects
an object. If one or both of the LEDs appear not to work, check your wiring and
your program.
' Robotics with the Boe-Bot - TestBothIrAndIndicators.bs2
' Test IR object detection circuits.
' {$STAMP BS2}
' {$PBASIC 2.5}
' Stamp directive.
' PBASIC directive.
' -----[ Variables ]---------------------------------------------------------irDetectLeft
irDetectRight
VAR
VAR
Bit
Bit
' -----[ Initialization ]----------------------------------------------------DEBUG "Testing piezospeaker..."
FREQOUT 4, 2000, 3000
DEBUG CLS,
"IR DETECTORS", CR,
"Left
Right", CR,
"----- -----"
' -----[ Main Routine ]------------------------------------------------------DO
FREQOUT 8, 1, 38500
irDetectLeft = IN9
FREQOUT 2, 1, 38500
irDetectRight = IN0
IF (irDetectLeft = 0) THEN
HIGH 10
ELSE
LOW 10
ENDIF
IF (irDetectRight = 0) THEN
HIGH 1
ELSE
LOW 1
ENDIF
DEBUG CRSRXY, 2, 3, BIN1 irDetectLeft,
CRSRXY, 9, 3, BIN1 irDetectRight
PAUSE 100
LOOP
Navigating with Infrared Headlights · Page 233
Your Turn – Remote Testing and Range Testing
You can now use your LED detectors to take your Boe-Bot and test your IR detectors on
objects that might not otherwise be in reach of your computer’s programming cable.
 Unplug your Boe-Bot from the programming cable, and take your Boe-Bot to a
variety of objects and test the range of the IR detectors.
 Try the detection range of different colored objects. What color is detected at the
furthest range? What color is detected at the closest range?
Sniffing for IR Interference
If you happened to notice that your Boe-Bot let you know it detected something even
though nothing was in range, it may mean that a nearby light is generating some IR light
at a frequency close to 38.5 kHz. If you try to have a Boe-Bot contest or demonstration
under one of these lights, your infrared systems might end up performing very poorly.
The last thing anybody wants is to have their robot not perform as advertised during a
public demonstration, so make sure to check any prospective demo area with this IR
interference “sniffer” program beforehand.
The concept behind this program is simple: don’t transmit any IR through the IR LEDs,
just monitor to see if any IR is detected. If IR is detected, sound the alarm using the
piezospeaker.
You can use a handheld remote for just about any piece of equipment to generate IR
interference. TVs, VCRs, CD/DVD players, and projectors all use the same type of IR
detectors you have on your Boe-Bot right now. Likewise, the remotes you use to control
these devices all use the same kind of IR LED that's on your Boe-Bot to transmit messages
to the IR detector in your TV, VCR, CD/DVD player, etc.
Example Program – IrInterferenceSniffer.bs2
 Enter, save, and run IrInterferenceSniffer.bs2.
 Test to make sure the Boe-Bot sounds the alarm when it detects IR interference.
If you are in a classroom, you can do this with a separate Boe-Bot that’s running
TestBothIrAndIndicators.bs2. If you don’t have a second Boe-Bot, just use a
handheld remote for a TV, VCR, CD/DVD player, or projector. Simply point
the remote at the Boe-Bot and press a button. If the Boe-Bot responds by
sounding the alarm, you know your IR interference sniffer is working.
Page 234 · Robotics with the Boe-Bot
' Robotics with the Boe-Bot – IrInterferenceSniffer.bs2
' Test fluorescent lights, infrared remotes, and other sources
' of 38.5 kHz IR interference.
' {$STAMP BS2}
' {$PBASIC 2.5}
counter
' Stamp directive.
' PBASIC directive.
VAR
Nib
DEBUG "IR interference not detected, yet...", CR
DO
IF (IN0 = 0) OR (IN9 = 0) THEN
DEBUG "IR Interference detected!!!", CR
FOR counter = 1 TO 5
HIGH 1
HIGH 10
FREQOUT 4, 50, 4000
LOW 1
LOW 10
PAUSE 20
NEXT
ENDIF
LOOP
Your Turn – Testing for Fluorescent Lights that Interfere
 Disconnect your Boe-Bot from its programming cable, and point it at any
fluorescent light near where you plan to operate it. Especially if you get frequent
alarms, turn off that fluorescent light before trying to use IR object detection
under it.
Always use this IrInterferenceSniffer.bs2 to make sure that any area where you are
using the Boe-Bot is free of infrared interference.
ACTIVITY #3: INFRARED DETECTION RANGE ADJUSTMENTS
You may have noticed that brighter car headlights (or a brighter flashlight) can be used to
see objects that are further away when it’s dark. By making the Boe-Bot’s infrared LED
headlights brighter, you can also increase its detection range. By resisting electric current
less, a smaller resistor allows more current to flow through an LED. More current
through an LED is what causes it to glow more brightly. In this activity, you will
examine the effect of different resistance values with both the red and infrared LEDs.
Navigating with Infrared Headlights · Page 235
Parts List:
You will need some extra parts for this activity.
(2)
(2)
(2)
(2)
Resistors, 470 Ω (yellow-violet-brown)
Resistors, 220 Ω (red-red-brown)
Resistors, 2 kΩ (red-black-red)
Resistors, 4.7 kΩ (yellow-violet-red)
Series Resistance and LED Brightness
First, let’s use one of the red LEDs to “see” the difference that a resistor makes in how
brightly an LED glows. All we need to test the LED is a program that sends a high signal
to the LED.
Example Program – P1LedHigh.bs2
 Enter, save and run P1LedHigh.bs2.
 Run the program and verify that the LED in the circuit connected to P1 emits
light.
' Robotics with the Boe-Bot - P1LedHigh.bs2
' Set P1 high to test for LED brightness testing with each of
' these resistor values in turn: 220 ohm , 470 ohm, 1 k ohm.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
HIGH 1
STOP
The command STOP is used here rather than END, since END would put the BASIC Stamp
into low power mode.
Your Turn – Testing LED Brightness
Remember to disconnect power before you make changes to a circuit. Remember
also that the same program will run again when you reconnect power, so you can
pick up right where you left off with each test.
 Note how brightly the P1 LED circuit is glowing with the 220 Ω resistor.
Page 236 · Robotics with the Boe-Bot
 Replace the 220 Ω resistor connected to P1 and the right LED’s cathode with a
470 Ω resistor. Note now how brightly the LED glows.
 Repeat for a 2 kΩ resistor.
 Repeat once more with a 4.7 kΩ resistor.
 Replace the 4.7 kΩ resistor with the 220 Ω resistor before moving on to the next
portion of this activity.
 Explain in your own words the relationship between LED brightness and series
resistance.
Series Resistance and IR Detection Range
We now know that less series resistance will make an LED glow more brightly. A
reasonable hypothesis would be that brighter IR LEDs can make it possible to detect
objects that are further away.
 Open and run TestBothIrAndIndicators.bs2 (from page 244).
 Verify that both detectors are working properly.
Your Turn – Testing IR LED Range
 With a ruler, measure the furthest distance from the IR LED that a sheet of paper
can be detected, using 1 kΩ resistor, and record your data in Table 7-1.
 Replace the 1 kΩ resistors that connect P2 and P8 to the IR LED anodes with
4.7 kΩ resistors.
 Determine the furthest distance at which the same sheet of paper is detected, and
record your data.
 Repeat with 2 kΩ resistors, 470 Ω resistors, and 220 Ω resistors.
Table 7-1: Detection Distances vs. Resistance
IRELD Series Resistance (Ω)
Maximum Detection Distance
4700
2000
1000
470
220
 Before moving on to the next activity, restore your IR object detectors to their
original configuration (with 1 kΩ resistors in series with each IR LED).
Navigating with Infrared Headlights · Page 237
 Also, before moving on, make sure to test this last change with
TestBothIrAndIndicators.bs2 to verify that both IR object detectors are working
properly.
ACTIVITY #4: OBJECT DETECTION AND AVOIDANCE
An interesting thing about the IR detectors is that their outputs are just like the whiskers.
When no object is detected, the output is high; when an object is detected, the output is
low. In this activity, RoamingWithWhiskers.bs2 from page 178 is modified so that it
works with the IR detectors.
Converting the Whiskers Program for IR Object Detection/Avoidance
This next example program started as RoamingWithWhiskers.bs2. Aside from adjusting
the name and description, two bit variables were added to store the states of the IR
detectors.
irDetectLeft VAR
irDetectRight VAR
Bit
Bit
A routine was also added to read the IR object detectors.
FREQOUT 8, 1, 38500
irDetectLeft = IN9
FREQOUT 2, 1, 38500
irDetectRight = IN0
The IF…THEN statements were modified so that they look at the variables that store the IR
object detections instead of the whisker inputs.
IF (irDetectLeft = 0) AND (irDetectRight = 0) THEN
GOSUB Back_Up
GOSUB Turn_Left
GOSUB Turn_Left
ELSEIF (irDetectLeft = 0) THEN
GOSUB Back_Up
GOSUB Turn_Right
ELSEIF (irDetectRight = 0) THEN
GOSUB Back_Up
GOSUB Turn_Left
ELSE
GOSUB Forward_Pulse
ENDIF
Page 238 · Robotics with the Boe-Bot
Example Program – RoamingWithIr.bs2





'
'
'
'
'
Open RoamingWithWhiskers.bs2
Modify it so that it matches the program below.
Reconnect power to your board and servos.
Save and run it.
Verify that, aside from the fact that there’s no contact required, it behaves like
RoamingWithWhiskers.bs2.
-----[ Title ]-------------------------------------------------------------Robotics with the Boe-Bot - RoamingWithIr.bs2
Adapt RoamingWithWhiskers.bs2 for use with IR object detectors.
{$STAMP BS2}
' Stamp directive.
{$PBASIC 2.5}
' PBASIC directive.
DEBUG "Program Running!"
' -----[ Variables ]---------------------------------------------------------irDetectLeft VAR
irDetectRight VAR
pulseCount
VAR
Bit
Bit
Byte
' -----[ Initialization ]----------------------------------------------------FREQOUT 4, 2000, 3000
' Signal program start/reset.
' -----[ Main Routine ]------------------------------------------------------DO
FREQOUT 8, 1, 38500
irDetectLeft = IN9
' Store IR detection values in
' bit variables.
FREQOUT 2, 1, 38500
irDetectRight = IN0
IF (irDetectLeft = 0) AND (irDetectRight = 0) THEN
GOSUB Back_Up
' Both detect obstacle
GOSUB Turn_Left
' Back up & U-turn (left twice)
GOSUB Turn_Left
ELSEIF (irDetectLeft = 0) THEN
' Left detects
GOSUB Back_Up
' Back up & turn right
GOSUB Turn_Right
ELSEIF (irDetectRight = 0) THEN
' Right detects
GOSUB Back_Up
' Back up & turn left
GOSUB Turn_Left
ELSE
' None detect
GOSUB Forward_Pulse
' Apply a forward pulse
ENDIF
' and check again
LOOP
Navigating with Infrared Headlights · Page 239
' -----[ Subroutines ]-------------------------------------------------------Forward_Pulse:
PULSOUT 13,850
PULSOUT 12,650
PAUSE 20
RETURN
' Send a single forward pulse.
Turn_Left:
FOR pulseCount = 0 TO 20
PULSOUT 13, 650
PULSOUT 12, 650
PAUSE 20
NEXT
RETURN
' Left turn, about 90-degrees.
Turn_Right:
FOR pulseCount = 0 TO 20
PULSOUT 13, 850
PULSOUT 12, 850
PAUSE 20
NEXT
RETURN
Back_Up:
FOR pulseCount = 0 TO 40
PULSOUT 13, 650
PULSOUT 12, 850
PAUSE 20
NEXT
RETURN
' Right turn, about 90-degrees.
' Back up.
Your Turn
 Modify RoamingWithIr.bs2 so that the IR object detectors are checked in a
subroutine.
ACTIVITY #5: HIGH-PERFORMANCE IR NAVIGATION
The style of pre-programmed maneuvers that were used in the previous activity were fine
for whiskers, but are unnecessarily slow when using the IR LEDs and detectors. You can
greatly improve the Boe-Bot’s roaming performance by checking for obstacles before
delivering each set of pulses to the servos. The program can use the sensor inputs to
select the best maneuver for each moment of navigation. That way, the Boe-Bot never
turns further than it has to, and it can neatly find its way around obstacles and
successfully navigate more complex courses.
Page 240 · Robotics with the Boe-Bot
Sampling Between Every Pulse to Avoid Collisions
The great thing about detecting an obstacle before bumping into it is that it gives the BoeBot some room to navigate around it. The Boe-Bot can apply a pulse to turn away from
an object, check again and if the object is still there, apply another pulse to avoid it. The
Boe-Bot can keep applying pulses and checking, until it steers clear of the obstacle.
Then, it can resume forward pulses. After experimenting with this next example
program, you’ll likely agree that it’s a much better way for the Boe-Bot to roam.
Example Program – FastIrRoaming.bs2
 Enter, save, and run FastIrRoaming.bs2.
' Robotics with the Boe-Bot - FastIrRoaming.bs2
' Higher performance IR object detection assisted navigation
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
irDetectLeft
irDetectRight
pulseLeft
pulseRight
VAR
VAR
VAR
VAR
Bit
Bit
Word
Word
' Variable Declarations
FREQOUT 4, 2000, 3000
' Signal program start/reset.
DO
' Main Routine
FREQOUT 8, 1, 38500
irDetectLeft = IN9
FREQOUT 2, 1, 38500
irDetectRight = IN0
' Check IR Detectors
' Decide how to navigate.
IF (irDetectLeft = 0) AND (irDetectRight = 0) THEN
pulseLeft = 650
pulseRight = 850
ELSEIF (irDetectLeft = 0) THEN
pulseLeft = 850
pulseRight = 850
ELSEIF (irDetectRight = 0) THEN
pulseLeft = 650
pulseRight = 650
ELSE
pulseLeft = 850
pulseRight = 650
ENDIF
Navigating with Infrared Headlights · Page 241
PULSOUT 13,pulseLeft
PULSOUT 12,pulseRight
PAUSE 15
' Apply the pulse.
LOOP
' Repeat Main Routine
How FastIrRoaming.bs2 Works
This program takes a slightly different approach to applying pulses. Aside from the two
bits used to store the IR detector outputs, it uses two word variables to set the pulse
durations delivered by the PULSOUT command.
irDetectLeft
irDetectRight
pulseLeft
pulseRight
VAR
VAR
VAR
VAR
Bit
Bit
Word
Word
Inside the DO…LOOP, the FREQOUT commands are used to send a 38.5 kHz IR signal to
each IR LED. Immediately after the 1 ms burst of IR is sent, a bit variable stores the
output state of the IR detector. This is necessary, because if you wait any longer than a
command’s worth of time, the IR detector will return to the not detected (1 state),
regardless of whether or not it detected an object.
FREQOUT 8, 1, 38500
irDetectLeft = IN9
FREQOUT 2, 1, 38500
irDetectRight = IN0
In the IF…THEN statements, instead of delivering pulses or calling navigation routines,
this program sets variable values that will be used in PULSOUT commands’ Duration
arguments.
IF (irDetectLeft = 0) AND (irDetectRight = 0) THEN
pulseLeft = 650
pulseRight = 850
ELSEIF (irDetectLeft = 0) THEN
pulseLeft = 850
pulseRight = 850
ELSEIF (irDetectRight = 0) THEN
pulseLeft = 650
pulseRight = 650
ELSE
pulseLeft = 850
pulseRight = 650
ENDIF
Page 242 · Robotics with the Boe-Bot
Before the DO…LOOP repeats, the last thing to do is to deliver pulses to the servos. Notice
that the PAUSE command is no longer 20. Instead, it’s 15 since roughly 5 ms is taken
checking the IR LEDs.
PULSOUT 13,pulseLeft
PULSOUT 12,pulseRight
PAUSE 15
' Apply the pulse.
Your Turn
 Save FastIrRoaming.bs2 as FastIrRoamingYourTurn.bs2.
 Use the LEDs to broadcast that the Boe-Bot has detected an object.
 Try modifying the values that pulseLeft and pulseRight are set to so that the
Boe-Bot does everything at half speed.
ACTIVITY #6: THE DROP-OFF DETECTOR
Up until now, the Boe-Bot has mainly been programmed to take evasive maneuvers when
an object is detected. There are also applications where the Boe-Bot must take evasive
action when an object is not detected. For example, if the Boe-Bot is roaming on a table,
its IR detectors might be looking down at the table surface as shown in Figure 7-11. The
program should make it continue forward so long as both IR detectors can “see” the
surface of the table.
To Servos
15 14 Vdd 13 12
Red
Black
X4
Vdd
X5
Vin
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Top view (left);
side view
(right).
+
Board of Education
Rev C
Figure 7-11
IR Object
detectors
Directed
Downwards to
Scan for a
Drop-Off
© 2000-2003
Navigating with Infrared Headlights · Page 243
 Disconnect power from your board and servos.
 Point your IR object detectors downward and outward as shown in Figure 7-11.
Recommended Materials:
(1) Roll of black vinyl electrical tape, ¾″ (19 mm) wide.
(1) Sheet of white poster board, 22 x 28 in (56 x 71 cm).
Simulating a Drop-Off with Electrical Tape
A sheet of white poster board with a border made of electrical tape makes for a handy
way to simulate the drop-off presented by a table edge, with much less risk to your BoeBot.
 Build a course similar to the electrical tape delimited course shown in Figure
7-12. Use at least three strips of electrical tape, edge to edge with no paper
visible between the strips.
 Replace your 1 kΩ resistors with 2 kΩ resistors (red-black-red) to connect P2 to
its IR LED and P8 to its IR LED. We want the Boe-Bot to be nearsighted for
this activity.
 Reconnect power to your board.
 Run the program IrInterferenceSniffer.bs2 (page 234) to make sure that nearby
fluorescent lighting will not interfere with your Boe-Bot’s IR detectors.
 Use the TestBothIrAndIndicators.bs2 (page 232) to make sure that the Boe-Bot
detects the poster board but does not detect the electrical tape.
If the Boe-Bot still "sees" the electrical tape too clearly, here are a few remedies:




Try adjusting the IR detectors and LEDs downward at various angles.
Try a different brand of vinyl electrical tape.
Try replacing the 2 kΩ resistors with 4.7 kΩ (yellow-violet-red) resistors to make
the Boe-Bot more nearsighted.
Adjust the FREQOUT command with different Freq1 arguments. Here are some
arguments that will make the Boe-Bot more nearsighted: 38250, 39500, 40500
If you are using older IR LEDs, the Boe-Bot might actually be having problems with being
too nearsighted. Here are some remedies that will increase the Boe-Bot's sensitivity to
objects and make it more far sighted:

Try 1 kΩ (brown-black-red) or 470 Ω (yellow-violet-brown) or even 220 Ω (red-redbrown) resistors in series with the IR LEDs instead of 2 kΩ.
Page 244 · Robotics with the Boe-Bot
22” (56 cm)
22” (56 cm)
Figure 7-12
Electrical Tape Outline
Simulates Tabletop Edge
If you try a tabletop after success with the electrical tape course:

Remember to follow the same steps you followed before running the Boe-Bot in
the electrical tape delimited course!
Make sure to be the spotter for your Boe-Bot. Be ready as your Boe-Bot roams the tabletop:


Always be ready to pick your Boe-Bot up from above as it approaches the edge of
the table it’s navigating. If the Boe-Bot tries to drive off the edge, pick it up before
it takes the plunge. Otherwise, your Boe-Bot might become a Not-Bot!
Your Boe-Bot may detect you if you are standing in its line of sight. Its current
program has no way to differentiate you from the table below it, so it might try to
continue forward and off the edge of the table. So, stay out of its detector’s line
of sight as you spot.
Programming for Drop-Off Detection
For the most part, programming your Boe-Bot to navigate around a table top without
going over the edge is a matter of adjusting the IF...THEN statements from
FastIrNavigation.bs2. The main adjustment is that the servos should be directed to make
the Boe-Bot roll forward when irDetectLeft and irDetectRight are both 0,
indicating that an object (the table’s surface) has been detected. The Boe-Bot also has to
turn away from a detector that indicates it has not detected an object. For example, if
irDetectLeft is 1, the Boe-Bot had better turn right.
A second feature of a program for turning away from drop-offs is adjustable distance.
You may want your Boe-Bot to only take one pulse forward between checking the
Navigating with Infrared Headlights · Page 245
detectors, but as soon as a drop-off is detected, you may want your Boe-Bot to take
several pulses worth of turn before checking the detectors again.
Just because you are taking multiple pulses in an evasive maneuver, it doesn’t mean you
have to return to whiskers-style navigation. Instead, you can add a pulseCount variable
that you can use to set to the number of pulses to deliver for a maneuver. The PULSOUT
command can be placed inside a FOR…NEXT loop that executes FOR 1 TO pulseCount
pulses. For one pulse forward, pulseCount can be 1, for ten pulses left, pulseCount
can be set to 10, and so on.
Example Program – AvoidTableEdge.bs2
 Open FastIrNavigation.bs2 and save it as AvoidTableEdge.bs2.
 Modify the program so that it matches the example program. This will involve
adding variables, modifying the IF…THEN statements, and nesting the PULSOUT
commands inside a FOR…NEXT loop. Be careful to make sure that all the
pulseLeft and pulseRight variable values inside the IF…THEN statement are
properly adjusted.
Their values are different from the ones in
FastIrNavigation.bs2 because the rules of the course are different.
 Reconnect your board and servos.
 Test the program on your electrical tape delimited course.
 If you decide to try a tabletop, remember to follow the testing and spotting tips
discussed earlier.
' Robotics with the Boe-Bot - AvoidTableEdge.bs2
' IR detects object edge and navigates to avoid drop-off.
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
irDetectLeft
irDetectRight
pulseLeft
pulseRight
loopCount
pulseCount
VAR
VAR
VAR
VAR
VAR
VAR
FREQOUT 4, 2000, 3000
Bit
Bit
Word
Word
Byte
Byte
' Variable declarations.
' Signal program start/reset.
Page 246 · Robotics with the Boe-Bot
DO
' Main Routine.
FREQOUT 8, 1, 38500
irDetectLeft = IN9
FREQOUT 2, 1, 38500
irDetectRight = IN0
' Check IR detectors.
' Decide navigation.
IF (irDetectLeft = 0) AND (irDetectRight = 0) THEN
pulseCount = 1
' Both detected,
pulseLeft = 850
' one pulse forward.
pulseRight = 650
ELSEIF (irDetectRight = 1) THEN
' Right not detected,
pulseCount = 10
' 10 pulses left.
pulseLeft = 650
pulseRight = 650
ELSEIF (irDetectLeft = 1) THEN
' Left not detected,
pulseCount = 10
' 10 pulses right.
pulseLeft = 850
pulseRight = 850
ELSE
' Neither detected,
pulseCount = 15
' back up and try again.
pulseLeft = 650
pulseRight = 850
ENDIF
FOR loopCount = 1 TO pulseCount
PULSOUT 13,pulseLeft
PULSOUT 12,pulseRight
PAUSE 20
NEXT
' Send pulseCount pulses
LOOP
How AvoidTableEdge.bs2 Works
Since this program is a modified version of FastIrRoaming.bs2, only changes to the
program are discussed here.
A FOR…NEXT loop is added to the program to control how many pulses are delivered each
time through the main (DO…LOOP) routine. Two variables are added, loopCount
functions as an index for a FOR…NEXT loop and pulseCount is used as the EndValue
argument.
loopCount
pulseCount
VAR
VAR
Byte
Byte
Navigating with Infrared Headlights · Page 247
The IF…THEN statements now set the value of pulseCount as well as pulseRight and
pulseLeft. If both detectors can see the table, take one cautious pulse forward.
IF (irDetectLeft = 0) AND (irDetectRight = 0) THEN
pulseCount = 1
pulseLeft = 850
pulseRight = 650
Else, if the right IR detector does not see the tabletop, rotate left 10 pulses.
ELSEIF (irDetectRight = 1) THEN
pulseCount = 10
pulseLeft = 650
pulseRight = 650
Else, if the left IR detector does not see the tabletop, rotate right 10 pulses.
ELSEIF (irDetectLeft = 1) THEN
pulseCount = 10
pulseLeft = 850
pulseRight = 850
Else, if neither detector can see the table top, back up 15 pulses and try again, hoping that
one of the detectors will see the drop-off before the other.
ELSE
pulseCount = 15
pulseLeft = 650
pulseRight = 850
ENDIF
Now that the value of pulseCount, pulseLeft, and pulseRight are set, this FOR…NEXT
loop delivers the specified number of pulses for the maneuver determined by the
pulseLeft and pulseRight variables.
FOR loopCount = 1 TO pulseCount
PULSOUT 13,pulseLeft
PULSOUT 12,pulseRight
PAUSE 20
NEXT
Your Turn
You can experiment with setting different pulseLeft, pulseRight, and pulseCount
values inside the IF…THEN statement. For example, if the Boe-Bot doesn’t turn as far, it
Page 248 · Robotics with the Boe-Bot
may actually track the edge of the electrical tape delimited course. Pivoting backward
instead of rotating in place may also lead to some interesting behaviors.
 Modify AvoidTableEdge.bs2 so that it follows the edge of the electrical tape
delimited course by adjusting the pulseCount values so that the Boe-Bot
doesn’t turn too far away from the edge.
 Experiment with pivoting as a way to make the Boe-Bot roam inside the
perimeter instead of following the edge.
SUMMARY
This chapter covered a unique technique for infrared object detection that uses the
infrared LED found in common handheld remotes, and the infrared detector found in
TVs, CD/DVD players, and other appliances that are controlled by these remotes. By
shining infrared into the Boe-Bot’s path and looking for its reflection, object detection
can be accomplished without physically contacting the object. Infrared LED circuits are
used to send a 38.5 kHz signal with the help of a property of the FREQOUT command
called a harmonic, which is inherent to digitally synthesized signals.
An infrared detection indicator program was introduced for remote (not connected to the
PC) testing of the IR LED/detector pairs. An infrared interference sniffer program was
also introduced to help detect interference that can be generated by some fluorescent light
fixtures. Since the signals sent by the IR detectors are so similar to the signals sent by the
whiskers, RoamingWithWhiskers.bs2 was adapted to the infrared detectors. A program
that checks the IR detectors between each servo pulse was introduced to demonstrate a
higher performance way of roaming without colliding into objects. This program was
then modified to avoid the edge of an electrical tape delimited area. Since electrical tape
absorbs infrared, framing a large sheet of construction paper emulates the drop-off that is
seen at a table edge without the danger to the actual Boe-Bot.
Questions
1. What is the frequency of the signal sent by FREQOUT 2, 1, 38500? What is
the value of the second frequency sent by that command? How long are these
signals sent for? What I/O pin does the IR LED circuit have to be connected to
in order to broadcast this signal?
2. What command has to immediately follow the FREQOUT command in order to
accurately determine whether or not an object has been detected?
Navigating with Infrared Headlights · Page 249
3. What does it mean if the IR detector sends a low signal? What does it mean
when the detector sends a high signal?
4. What happens if you change the value of a resistor in series with a red LED?
What happens if you change the value of a resistor in series with an infrared
LED??
Exercises
1. Modify a line of code in IrInterferenceSniffer.bs2 so that it only monitors one of
the IR detectors.
2. Explain the function of pulseCount in AvoidTableEdge.bs2.
Projects
1. Design a Boe-Bot application that sits still until you wave your hand in front of
it, then it starts roaming.
2. Design a Boe-Bot application that slowly rotates in place until it detects an
object. As soon as it detects an object, it locks onto and chases the object. This
is a classic SumoBot behavior.
3. Design a Boe-Bot application that roams, but if it detects infrared interference, it
sounds the alarm briefly, and then continues roaming. This alarm should be
different from the low battery alarm.
Solutions
Q1. 38.5 kHz is the frequency of the signal. The second frequency = 65536 – 38500
= 27036 Hz. The signals are sent for 1 millisecond, and the IR LED must be
connected to I/O Pin 2.
Q2. The command which stores the detector's output in a variable. For example,
irDetectLeft = IN9.
Q3. A low signal means IR at 38.5 kHz was detected, thus, an object was detected.
A high signal means no IR at 38.5kHz was detected, so, no object.
Q4. Electrically speaking, for both red and infrared LEDs, a smaller resistor will
cause the LED to glow more brightly. A bigger resistor results in dimmer LEDs.
In terms of results, brighter IR LEDs make it possible to detect objects that are
farther away.
E1. Change the IF…THEN to read.
IF (IN0 = 0) THEN
Page 250 · Robotics with the Boe-Bot
E2. The program sets this variable to 1 when it’s taking a forward pulse. That way,
as the Boe-Bot moves forward, it checks for a drop-off between each pulse.
When it detects a drop-off, it executes a turn for a certain number of pulses,
which is also determined by the value of the pulseCount variable.
P1. The FastIrRoaming.bs2 program can be combined with a DO…LOOP UNTIL loop
that does nothing until it detects an object. A sample solution is shown below.
' -----[ Title ]------------------------------------------------------' Robotics with the Boe-Bot - MotionActivatedBoeBot.bs2
' Boe-Bot starts roaming when hand is waved in front of IR detectors.
' {$STAMP BS2}
' {$PBASIC 2.5}
' -----[ Variables ]--------------------------------------------------irDetectLeft
irDetectRight
pulseLeft
pulseRight
VAR
VAR
VAR
VAR
Bit
Bit
Word
Word
' Variable Declarations
' -----[ Initialization ]---------------------------------------------DEBUG "Program Running!"
FREQOUT 4, 2000, 3000
start/reset.
' Signal program
' -----[ Main Routine ]-----------------------------------------------Main:
' Loop until something is detected
DO
GOSUB Check_IRs
LOOP UNTIL (irDetectLeft = 0) OR (irDetectRight = 0)
' Now start roaming -- this code from FastIrRoaming.bs2
DO
IF (irDetectLeft = 0) AND (irDetectRight = 0) THEN
pulseLeft = 650
' Both detect
pulseRight = 850
' Back up
ELSEIF (irDetectLeft = 0) THEN
' Left detect
pulseLeft = 850
' Turn right
pulseRight = 850
ELSEIF (irDetectRight = 0) THEN
' Right detect
pulseLeft = 650
' Turn left
pulseRight = 650
ELSE
' Nothing detected
pulseLeft = 850
' Go forward
pulseRight = 650
Navigating with Infrared Headlights · Page 251
ENDIF
PULSOUT 13, pulseLeft
PULSOUT 12, pulseRight
PAUSE 15
GOSUB Check_IRs
LOOP
' Apply the pulse.
' Check IRs again
' -----[ Subroutines ] -----------------------------------------------Check_IRs:
FREQOUT 8, 1, 38500
irDetectLeft = IN9
FREQOUT 2, 1, 38500
IrDetectRight = IN0
RETURN
' Check IR Detectors
P2. This behavior is in many ways the opposite of the roaming behavior covered in
this chapter. Instead of avoiding objects, the Boe-Bot tries to go toward the
objects.
For this reason, the main code can be derived from
"FastIrRoaming.bs2", with a bit added that spins the Boe-Bot slowly until an
object is detected. In the solution below, once the Boe-Bot has spied an object, it
will continue forward even if the detectors both read "no object" (1) for a few
loops. This is because, as the Boe-Bot is maneuvering toward the object,
sometimes the detectors read "no object" for brief moments, but this is not
reason enough to give up the chase.
'
'
'
'
Robotics with the Boe-Bot - SumoBoeBot.bs2
Search for object, lock onto it and push it.
{$STAMP BS2}
{$PBASIC 2.5}
irDetectLeft
irDetectRight
pulseLeft
pulseRight
VAR
VAR
VAR
VAR
Bit
Bit
Word
Word
' Left IR reading
' Right IR reading
' pulse values for servos
' -----[ Initialization ]---------------------------------------------DEBUG "Program Running!"
FREQOUT 4, 2000, 3000
' Signal start/reset.
' -----[ Main Routine ]-----------------------------------------------Main:
' Spin around slowly until an object is spotted
Page 252 · Robotics with the Boe-Bot
DO
PULSOUT 13, 790
' Rotate slowly
PULSOUT 12, 790
PAUSE 15
' 5 ms for detectors
GOSUB Check_IRs
' While looking for object
LOOP UNTIL (irDetectLeft = 0) OR (irDetectRight = 0)
' Now figure out exactly where the object is and go toward it
DO
' Object in both detectors -- go forward
IF (irDetectLeft = 0) AND (irDetectRight = 0) THEN
pulseLeft = 850
' Forward
pulseRight = 650
' Object on left - go left
ELSEIF (irDetectLeft = 0) THEN
pulseLeft = 650
' Left toward object
pulseRight = 650
' Object on right - go right
ELSEIF (irDetectRight = 0) THEN
pulseLeft = 850
' Right toward object
pulseRight = 850
' No object -- go forward anyway, because the detectors will
ELSE
' momentarily show
pulseLeft = 850
' "no object" as the
pulseRight = 650
' Boe-Bot is adjusting
ENDIF
' its position.
PULSOUT 13,pulseLeft
PULSOUT 12,pulseRight
PAUSE 15
' Apply the pulse.
' 5 ms for detectors
' Check IRs again in case object is moving
GOSUB Check_IRs
LOOP
' -----[ Subroutines ] -----------------------------------------------Check_IRs:
FREQOUT 8, 1, 38500
irDetectLeft = IN9
FREQOUT 2, 1, 38500
IrDetectRight = IN0
RETURN
' Check IR Detectors
P3. The key to solving this problem is to combine "FastIrRoaming.bs2" and
"IrInterferenceSniffer.bs2" in a single program.
' -----[ Title ]------------------------------------------------------' Robotics with the Boe-Bot - RoamAndSniffBoeBot.bs2
' Boe-Bot roams around and sounds alarm when IR detected.
Navigating with Infrared Headlights · Page 253
' {$STAMP BS2}
' {$PBASIC 2.5}
' -----[ Variables ]--------------------------------------------------irDetectLeft
irDetectRight
pulseLeft
pulseRight
counter
VAR
VAR
VAR
VAR
VAR
Bit
Bit
Word
Word
Nib
' Left IR sensor reading
' Right IR sensor reading
' Pulses sent to servos
' Loop counter
' -----[ Initialization ]---------------------------------------------DEBUG "Program Running!"
FREQOUT 4, 2000, 3000
start/reset.
' Signal program
' -----[ Main Routine ]-----------------------------------------------Main:
DO
GOSUB Roam
GOSUB Sniff
LOOP
' -----[ Subroutines ] -----------------------------------------------Sniff:
IF (IN0 = 0) OR (IN9 = 0) THEN
FOR counter = 1 TO 5
HIGH 1
HIGH 10
FREQOUT 4, 50, 4000
LOW 1
LOW 10
PAUSE 20
NEXT
ENDIF
RETURN
Roam:
FREQOUT 8, 1, 38500
irDetectLeft = IN9
FREQOUT 2, 1, 38500
irDetectRight = IN0
' From IrInterferenceSniffer.bs2
' Beep 5 times
' and flash LEDs
' From FastIrRoaming.bs2
' Check IR Detectors
' Decide how to navigate.
IF (irDetectLeft = 0) AND (irDetectRight = 0) THEN
pulseLeft = 650
pulseRight = 850
ELSEIF (irDetectLeft = 0) THEN
pulseLeft = 850
pulseRight = 850
Page 254 · Robotics with the Boe-Bot
ELSEIF (irDetectRight = 0) THEN
pulseLeft = 650
pulseRight = 650
ELSE
pulseLeft = 850
pulseRight = 650
ENDIF
PULSOUT 13,pulseLeft
PULSOUT 12,pulseRight
PAUSE 15
RETURN
' Apply the pulse.
Robot Control with Distance Detection · Page 255
Chapter 8: Robot Control with Distance Detection
In Chapter 7, we used the infrared LEDs and receivers to detect whether an object is in
the Boe-Bot’s way without actually touching it. Wouldn’t it be nice to also know how far
away the object is? This is usually a task for sonar, which sends a pulse of sound out and
records how long it takes for the echo to come back. The time it takes for the echo to
come back can then be used to calculate how far away the object is. There is, however, a
way to accomplish distance detection with the very same circuit you used in the previous
chapter. With your Boe-Bot able to determine the distance of an object, it can be
programmed to follow a moving object without colliding into it. The Boe-Bot can also
be programmed to follow black tracks on a white background.
DETERMINING DISTANCE WITH THE SAME IR LED/DETECTOR CIRCUIT
You will use the same circuit from the previous chapter to detect distance.
 If the circuit is still built on your Boe-Bot, make sure your IR LED’s have 1 kΩ
resistors in series.
 If you already disassembled the circuit from the previous chapter, repeat the
steps in Chapter 7, Activity #1, on page 223.
Recommended Equipment and Materials:
(1) Ruler
(1) Sheet of paper
ACTIVITY #1: TESTING THE FREQUENCY SWEEP
Figure 8-1 shows an excerpt from one specific brand of IR detector’s datasheet
(Panasonic PNA4602M; a different brand may have been used in your kit). This excerpt
is a graph that shows how much less sensitive this IR detector becomes if the IR signal it
receives flashes on/off at a frequency other than 38.5 kHz. For example, if you send it IR
flashed on/off at 40 kHz, it’s only 50% as sensitive as it would be at 38.5 kHz. If the IR
is flashed on/off at 42 kHz, the detector is only 20% as sensitive. Especially for
frequencies that make the detector less sensitive, the object has to be closer to make the
reflected IR brighter for the detector to detect it.
Page 256 · Robotics with the Boe-Bot
Figure 8-1
Filter Sensitivity Depends on
Carrier Frequency
Another way to think about it is that the most sensitive frequency will detect the objects
that are the farthest away, while less sensitive frequencies can only be used to detect
closer objects. This makes distance detection simple. Pick 5 frequencies, then test them
from most sensitive to least sensitive. Try at the most sensitive frequency first. If an
object is detected, check and see if the next most sensitive frequency detects it.
Depending on which frequency makes the reflected infrared no longer visible to the IR
detector, you can infer the distance.
Frequency Sweep is the technique of testing a circuit’s output using a variety of input
frequencies.
Programming Frequency Sweep for Distance Detection
Figure 8-2 shows an example of how the Boe-Bot can test for distance using frequency.
In this example, the object is in Zone 3. That means that the object can be detected when
37500 and 38250 Hz is transmitted, but it cannot be detected with 39500, 40500, and
41500 Hz. If you were to move the object into Zone 2, then the object can be detected
when 37500, 38250, and 39500 Hz are transmitted, but not when 40500 and 41500 Hz
are transmitted.
Robot Control with Distance Detection · Page 257
Figure 8-2 Distance Detection Frequencies and Zones for the Boe-Bot
Object
15 14
Vd d
13 12
Red
Black
X4
Vdd
X5
Vin
Vs s
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Zone 0
41500 Hz
+
Zone 1
40500 Hz
Zone 2
Zone 3
39500 Hz 38250 Hz
Zone 4
37500 Hz
Boar d of Education
© 20 00 -2 00 3
Zone 5
No Detection
at any
Frequency
You might be wondering why the value of zone 4 is 37.5 kHz and not 38.5 kHz. The
reason they are not the values that you would expect based on the % sensitivity graph is
because the FREQOUT command transmits a slightly more powerful signal at 37.5 kHz than
it does at 38.5 kHz. The frequencies listed in Figure 8-2 are frequencies you will program
the BASIC Stamp to use to determine the distance of an object.
In order to test the IR detector at each frequency, you will need to use FREQOUT to send
five different frequencies and test at each frequency to find out whether the IR detector
could see the object. The steps between each frequency are not quite even enough to use
the FOR…NEXT loop’s STEP option. You could use DATA and READ, but that would be
cumbersome. You could use five different FREQOUT commands, but that would be a
waste of code space. Instead, the best approach for storing a short list of values that you
want to use in sequence is a command called LOOKUP. The syntax for the LOOKUP
command is:
LOOKUP Index, [Value0, Value1, …ValueN], Variable
If the Index argument is 0, Value0 from the list inside the square braces will be placed in
Variable. If Index is 1, Value1 from the list will be placed in Variable. There could be up
to 256 values in the list, but for the next example program, we will only need 5. Here is
how it will be used:
FOR freqSelect = 0 TO 4
LOOKUP freqSelect,[37500,38250,39500,40500,41500],irFrequency
FREQOUT 8,1, irFrequency
irDetect = IN9
' Commands not shown...
NEXT
Page 258 · Robotics with the Boe-Bot
The first time through the FOR…NEXT loop, freqSelect is 0, so the LOOKUP command
places the value 37500 in the irFrequency variable. Since irFrequency contains
37500 after the LOOKUP command, the FREQOUT command sends that frequency to the IR
LED connected to P8. As in the previous chapter, the value of IN9 is then saved in the
irDetect variable. The second time through the FOR…NEXT loop, the value of
freqSelect is now 1, which means the LOOKUP command places 38250 into the
irFrequency variable, and the process is repeated for this higher frequency. The third
time through, it’s repeated again with 39500, and so on. The result is remarkable,
especially considering you are using parts that were designed for a completely different
purpose, to make IR communication between a handheld remote and a television
possible.
Example Program – TestLeftFrequencySweep.bs2
TestLeftFrequencySweep.bs2 does two things. First, it tests the left IR object detector
(connected to P8 and P9) to make sure it is functioning properly for distance detection.
However, it also demonstrates how the frequency sweep illustrated in Figure 8-2 is
accomplished.
When you run the program, the Debug Terminal will display your zone measurement.
There are many possible yes-no patterns that can be generated; two are shown in Figure
8-3. The test patterns will vary depending on the characteristics of the filter inside the IR
detector.
The program determines which zone the detected object is in by counting the number of
“No” occurrences. Notice that even though the two Debug Terminal test patterns in
Figure 8-3 are different, they both have three “Yes” and two “No” occurrences.
Therefore, “Zone 2” is the location of the object detected in both examples.
 Enter, save, and run TestLeftFrequencySweep.bs2.
 Use a sheet of paper or card facing the IR LED/detector to test the distance
detection.
 Start with the sheet very close to the IR LED, perhaps ¼ in (or 1 cm) away from
the IR LED. Your Zone in the Debug Terminal should either be 0 or 1.
 Gradually move the sheet of paper away from the IR LED and make a note of
each distance that causes the zone value to get larger.
Robot Control with Distance Detection · Page 259
Figure 8-3
Testing
Distance
Detection
Output
Examples
Keep in mind that these distance measurements are relative and not necessarily
precise or evenly spaced. However, they will give the Boe-Bot a good enough sense of
object distance for following, tracking, and other activities.
Zones 1-4 typically fall in the range of 6 to 12 in (15 to 30 cm) for the shielded LEDs with a 1
kΩ resistor. As long as objects can be detected up to 4 in (10 cm) away, the experiments in
this chapter will work. If the distance detector range is less than that, try reducing your
series resistance from 1 kΩ to 470 Ω or 220 Ω.
' -----[ Title ]-------------------------------------------------------------' Robotics with the Boe-Bot - TestLeftFrequencySweep.bs2
' Test IR detector distance responses to frequency sweep.
' {$STAMP BS2}
' {$PBASIC 2.5}
' Stamp directive.
' PBASIC directive.
' -----[ Variables ]---------------------------------------------------------freqSelect
irFrequency
irDetect
distance
VAR
VAR
VAR
VAR
Nib
Word
Bit
Nib
' -----[ Initialization ]----------------------------------------------------DEBUG CLS,
"
"FREQUENCY
"---------
OBJECT", CR,
DETECTED", CR,
--------"
Page 260 · Robotics with the Boe-Bot
' -----[ Main Routine ]------------------------------------------------------DO
distance = 0
FOR freqSelect = 0 TO 4
LOOKUP freqSelect,[37500,38250,39500,40500,41500], irFrequency
FREQOUT 8,1, irFrequency
irDetect = IN9
distance = distance + irDetect
DEBUG CRSRXY, 4, (freqSelect + 3), DEC5 irFrequency
DEBUG CRSRXY, 11, freqSelect + 3
IF (irDetect = 0) THEN DEBUG "Yes" ELSE DEBUG "No "
PAUSE 100
NEXT
DEBUG CR,
"--------"Zone
LOOP
--------", CR,
", DEC1 distance
Your Turn – Testing the Right IR LED/Detector Object Detector
Although there’s some labeling involved, you can modify this program to test the right IR
LED and detector by changing these two lines:
FREQOUT 8,1, irFrequency
irDetect = IN9
...so that they read:
FREQOUT 2,1, irFrequency
irDetect = IN0
 Modify TestLeftFrequencySweep.bs2 for testing the distance measurement of
the right IR object detector.
 Run the program and verify that it can measure a similar distance.
Robot Control with Distance Detection · Page 261
Displaying Both Distances
It’s useful at times to have a quick program you can run to test both the Boe-Bot’s
distance detectors at the same time. This program is organized into subroutines, which
can be handy for copying and pasting into other programs that require distance detection.
Example Program – DisplayBothDistances.bs2
 Enter, save, and run DisplayBothDistances.bs2.
 Repeat the distance measurement exercise with a sheet of paper on each LED,
then on both LEDs at the same time.
'
'
'
'
-----[ Title ]-------------------------------------------------------------Robotics with the Boe-Bot - DisplayBothDistances.bs2
Test IR detector distance responses of both IR object detectors to
frequency sweep.
' {$STAMP BS2}
' {$PBASIC 2.5}
' Stamp directive.
' PBASIC directive.
' -----[ Variables ]---------------------------------------------------------freqSelect
irFrequency
irDetectLeft
irDetectRight
distanceLeft
distanceRight
VAR
VAR
VAR
VAR
VAR
VAR
Nib
Word
Bit
Bit
Nib
Nib
' -----[ Initialization ]----------------------------------------------------DEBUG CLS,
"IR OBJECT ZONE", CR,
"Left
Right", CR,
"----- -----"
' -----[ Main Routine ]------------------------------------------------------DO
GOSUB Get_Distances
GOSUB Display_Distances
LOOP
Page 262 · Robotics with the Boe-Bot
' -----[ Subroutine – Get_Distances ]----------------------------------------Get_Distances:
distanceLeft = 0
distanceRight = 0
FOR freqSelect = 0 TO 4
LOOKUP freqSelect,[37500,38250,39500,40500,41500], irFrequency
FREQOUT 8,1,irFrequency
irDetectLeft = IN9
distanceLeft = distanceLeft + irDetectLeft
FREQOUT 2,1,irFrequency
irDetectRight = IN0
distanceRight = distanceRight + irDetectRight
PAUSE 100
NEXT
RETURN
' -----[ Subroutine – Display_Distances ]------------------------------------Display_Distances:
DEBUG CRSRXY,2,3, DEC1 distanceLeft,
CRSRXY,9,3, DEC1 distanceRight
RETURN
Your Turn – More Distance Tests
 Try measuring the distance of different objects and find out if the color and/or
texture make any difference to the distance measurement.
ACTIVITY #2: BOE-BOT SHADOW VEHICLE
For one Boe-Bot to follow another, the Boe-Bot that follows, a.k.a. the shadow vehicle,
has to know how far ahead the lead vehicle is. If the shadow vehicle is lagging behind, it
has to detect this and speed up. If the shadow vehicle is too close to the lead vehicle, it
has to detect this as well and slow down. If it’s the right distance, it can wait until the
measurements indicate it’s too far or too close again.
Distance is just one kind of value that robots and other automated machinery are
responsible for. When a machine is designed to automatically maintain a value, such as
Robot Control with Distance Detection · Page 263
distance, pressure, or fluid level, it generally involves a control system. These systems
sometimes consist of sensors and valves, or sensors and motors, or, in the case of the
Boe-Bot, sensors and continuous rotation servos. There is also some kind of processor
that takes the sensor measurements and converts them to mechanical action. The
processor has to be programmed to make decisions based on the sensor inputs, and then
control the mechanical outputs accordingly. In the case of the Boe-Bot, the processor is
the BASIC Stamp 2.
Closed loop control is a common method of maintaining levels, and it works very well
for helping the Boe-Bot maintain its distance from an object. There are lots of different
kinds of closed loop control. Some of the most common are hysteresis, proportional,
integral, and derivative control. All of these types of control are introduced in detail in
the Stamps in Class text Process Control, listed in the Preface.
Most control techniques can be implemented with just a few lines of code in PBASIC. In
fact, the majority of the proportional control loop shown in Figure 8-4 reduces to just one
line of PBASIC code. This diagram is called a block diagram, and it describes the steps
of the proportional control process that the Boe-Bot will use to measure distance with its
right IR LED and detector and adjust position to maintain distance with its right servo.
Center pulse width
750
Error = -2
+
-
Kp X error
35 X -2
Output
adjust
-70
+
+
Right servo
output
680
Measured right
distance = 4
Figure 8-4
Proportional
Control Block
Diagram for
Right Servo and
IR Object
Detector
System
Let’s take a closer look at the numbers in Figure 8-4 to learn how proportional control
works. This particular example is for the right IR LED/detector and right servo. The set
point is 2, which means we want the Boe-Bot to maintain a distance of 2 between itself
and any object it detects. The measured distance is 4, which is too far away. The error is
the set point minus the measured distance which is 2 – 4 = -2. This is indicated by the
symbols inside the circle on the left. This circle is called a summing junction. Next, the
Page 264 · Robotics with the Boe-Bot
error feeds into an operator block. This block shows that error will be multiplied by a
value called a proportional constant (Kp). The value of Kp is 35. The block’s output
shows the result of -2 × 35 = -70, which is called the output adjust. This output adjust
goes into another summing junction, and this time it is added to the servo’s center pulse
width of 750. The result is a 680 pulse width that will make the servo turn about ¾ speed
clockwise. That makes the Boe-Bot’s right wheel roll forward, toward the object. This
correction goes into the overall system, which consists of the Boe-Bot, and the object,
that was at a measured distance of 4.
The next time through the loop, the measured distance might change, but that’s OK
because regardless of the measured distance, this control loop will calculate a value that
will cause the servo to move to correct any error. The correction is always proportional
to the error, which is the difference between the set point and measured distances.
A control loop always has a set of equations that govern the system. The block diagram
in Figure 8-4 is a way of visually describing this set of equations. Here are the equations
that can be taken from this block diagram, along with solutions.
Error
Output adjust
Right servo output
=
=
=
=
=
=
=
=
Right distance set point – Measured right distance
2–4
error  Kp
–2  35
– 70
Output adjust + Center pulse width
– 70 + 750
680
By making some substitutions, the three equations above can be reduced to this one,
which will give you the same result.
Right servo output
=
(Right distance set point – Measured right distance)  Kp
+ Center pulse width
By substituting the values from the example, we can see that the equation still works:
=
=
((2 – 4)  35) + 750
680
The left servo and IR object detector have a similar algorithm shown in Figure 8-5. The
difference is that Kp is -35 instead of +35. Assuming the same measured value at the
Robot Control with Distance Detection · Page 265
right IR object detector, the output adjust results is a pulse width of 820. Here is the
equation and calculations for this block diagram:
Left servo output
=
=
=
(Left distance set point – Measured left distance)  Kp
+ Center pulse width
((2 – 4)  –35) + 750
820
The result of this control loop is a pulse width that makes the left servo turn about ¾ of
full speed counterclockwise. This is also a forward pulse for the left wheel. The idea of
feedback is that the system’s output is re-sampled, by the shadow Boe-Bot taking another
distance measurement. Then the control loop repeats itself again and again and
again…roughly 40 times per second.
Center pulse width
750
Error = -2
Kp X error
-35 X -2
+
-
Output
adjust
+70
+
+
Left servo
output
820
Measured left
distance = 4
Figure 8-5
Proportional
Control Block
Diagram for Left
Servo and IR
Object Detector
System
Programming the Boe-Bot Shadow Vehicle
Remember that the equation for the right servo’s output was:
Right servo output
=
(Right distance set point – Measured right distance)  Kp
+ Center pulse width
Here is an example of solving this same equation in PBASIC. The right distance set
point is 2, the measured distance is a variable named distanceRight that will store the
IR distance measurement, Kp is 35, and the center pulse width is 750:
pulseRight = 2 - distanceRight * 35 + 750
Page 266 · Robotics with the Boe-Bot
Remember that in PBASIC math expressions are executed from left to right. First,
distanceRight is subtracted from 2. The result of this subtraction is then multiplied by
Kpr, which is 35, and after that, the product is added to the center pulse width of 750.
You can use parentheses to force a calculation that is further to the right in a line of PBASIC
code to be completed first. Recall this example: you can rewrite this line of PBASIC code:
pulseRight = 2 - distanceRight * 35 + 750
...like this:
pulseRight = 35 * (2 – distanceRight) + 750
In this expression, 35 is multiplied by the result of (2 – distanceRight), then the product is
added to 750.
The left servo is different because Kp for that system is -35
pulseLeft = 2 - distanceLeft * (-35) + 750
Since the values -35, 35, 2, and 750 all have names, it’s definitely a good place for some
constant declarations.
Kpl
Kpr
SetPoint
CenterPulse
CON
CON
CON
CON
-35
35
2
750
With these constant declarations in the program, you can use the name Kpl in place of
-35, Kpr in place of 35, SetPoint in place of 2, and CenterPulse in place of 750. After
these constant declarations, the proportional control calculations now look like this:
pulseLeft = SetPoint - distanceLeft * Kpl + CenterPulse
pulseRight = SetPoint - distanceRight * Kpr + CenterPulse
The convenient thing about declaring constants for these values is that you can change
them in one place, at the beginning of the program. The changes you make at the
beginning of the program will be reflected everywhere these constants are used. For
example, by changing the Kpl CON directive from -35 to -40, every instance of Kpl in the
entire program changes from -35 to -40. This is exceedingly useful for experimenting
with and tuning the right and left proportional control loops.
Robot Control with Distance Detection · Page 267
Example Program – FollowingBoeBot.bs2
FollowingBoeBot.bs2 repeats the proportional control loop just discussed with every
servo pulse. In other words, before each pulse, the distance is measured and the error
signal is determined. Then the error is multiplied by Kp, and the resulting value is
added/subtracted to/from the pulse widths that are sent to the left/right servos.
 Enter, save, and run FollowingBoeBot.bs2.
 Point the Boe-Bot at an 8 ½ x 11” sheet of paper held in front of it as though it’s
a wall-obstacle. The Boe-Bot should maintain a fixed distance between itself
and the sheet of paper.
 Try rotating the sheet of paper slightly. The Boe-Bot should rotate with it.
 Try using the sheet of paper to lead the Boe-Bot around. The Boe-Bot should
follow it.
 Move the sheet of paper too close to the Boe-Bot, and it should back up, away
from the paper.
' -----[ Title ]-------------------------------------------------------------' Robotics with the Boe-Bot - FollowingBoeBot.bs2
' Boe-Bot adjusts its position to keep objects it detects in zone 2.
' {$STAMP BS2}
' {$PBASIC 2.5}
' Stamp directive.
' PBASIC directive.
DEBUG "Program Running!"
' -----[ Constants ]---------------------------------------------------------Kpl
Kpr
SetPoint
CenterPulse
CON
CON
CON
CON
-35
35
2
750
' -----[ Variables ]---------------------------------------------------------freqSelect
irFrequency
irDetectLeft
irDetectRight
distanceLeft
distanceRight
pulseLeft
pulseRight
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
Nib
Word
Bit
Bit
Nib
Nib
Word
Word
' -----[ Initialization ]----------------------------------------------------FREQOUT 4, 2000, 3000
Page 268 · Robotics with the Boe-Bot
' -----[ Main Routine ]------------------------------------------------------DO
GOSUB Get_Ir_Distances
' Calculate proportional output.
pulseLeft = SetPoint - distanceLeft * Kpl + CenterPulse
pulseRight = SetPoint - distanceRight * Kpr + CenterPulse
GOSUB Send_Pulse
LOOP
' -----[ Subroutine - Get IR Distances ]-------------------------------------Get_Ir_Distances:
distanceLeft = 0
distanceRight = 0
FOR freqSelect = 0 TO 4
LOOKUP freqSelect,[37500,38250,39500,40500,41500], irFrequency
FREQOUT 8,1,irFrequency
irDetectLeft = IN9
distanceLeft = distanceLeft + irDetectLeft
FREQOUT 2,1,irFrequency
irDetectRight = IN0
distanceRight = distanceRight + irDetectRight
NEXT
RETURN
' -----[ Subroutine – Get Pulse ]--------------------------------------------Send_Pulse:
PULSOUT 13,pulseLeft
PULSOUT 12,pulseRight
PAUSE 5
RETURN
How FollowingBoeBot.bs2 Works
FollowingBoeBot.bs2 declares four constants using the CON directive: Kpr, Kpl,
SetPoint, and CenterPulse. Everywhere you see SetPoint, it’s actually the number 2
(a constant). Likewise, everywhere you see Kpl, it’s actually the number -35. Kpr is
actually 35, and CenterPulse is 750.
Robot Control with Distance Detection · Page 269
Kpl
Kpr
SetPoint
CenterPulse
CON
CON
CON
CON
-35
35
2
750
The first thing the Main Routine does is call the Get_Ir_Distances subroutine. After
the Get_Ir_Distances subroutine is finished, distanceLeft and distanceRight each
contain a number corresponding to the zone in which an object was detected for both the
left and right IR object detectors.
DO
GOSUB Get_Ir_Distances
The next two lines of code implement the proportional control calculations for each
servo.
' Calculate proportional output.
pulseLeft = SetPoint - distanceLeft * Kpl + CenterPulse
pulseRight = SetPoint - distanceRight * Kpr + CenterPulse
Now that the pulseLeft and pulseRight calculations are done, the Send_Pulse
subroutine can be called.
GOSUB Send_Pulse
The LOOP portion of the DO…LOOP sends the program back to the command immediately
following the DO at the beginning of the main loop.
LOOP
Page 270 · Robotics with the Boe-Bot
Your Turn
Figure 8-6 shows a lead Boe-Bot followed by a shadow Boe-Bot. The lead Boe-Bot is
running a modified version of FastIrRoaming.bs2, and the shadow Boe-Bot is running
FollowingBoeBot.bs2. Proportional control makes the shadow Boe-Bot a very faithful
follower. One lead Boe-Bot can string along a chain of 6 or 7 shadow Boe-Bots. Just
add the lead Boe-Bot’s side panels and tailgate to the rest of the shadow Boe-Bots in the
chain.
Figure 8-6
Lead Boe-Bot (left) and
Shadow Boe-Bot (right)
 If you are part of a class, mount paper panels on the tail and both sides of the
lead Boe-Bot as shown in Figure 8-6.
 If you are not part of a class (and only have one Boe-Bot) the shadow vehicle
will follow a piece of paper or your hand just as well as it follows a lead BoeBot.
 Replace the 1 kΩ resistors that connect the lead Boe-Bot’s P2 and P8 to the IR
LEDs with 470 Ω or 220 Ω resistors.
 Program the lead Boe-Bot for object avoidance using a modified version of
FastIrRoaming.bs2, renamed SlowerIrRoamingForLeadBoeBot.bs2.
 Make these modifications to SlowerIrRoamingForLeadBoeBot.bs2:
o Increase all PULSOUT Duration arguments that are now 650 to 710.
o Reduce all PULSOUT Duration arguments that are now 850 to 790.
 The shadow Boe-Bot should be running FollowingBoeBot.bs2 without any
modifications.
Robot Control with Distance Detection · Page 271
 With both Boe-Bots running their respective programs, place the shadow BoeBot behind the lead Boe-Bot. The shadow Boe-Bot should follow at a fixed
distance, so long as it is not distracted by another object such as a hand or a
nearby wall.
You can adjust the set points and proportionality constants to change the shadow BoeBot’s behavior. Use your hand or a piece of paper to lead the shadow Boe-Bot while
doing these exercises:
 Try running FollowingBoeBot.bs2 using values of Kpr and Kpl constants,
ranging from 15 to 50. Note the difference in how responsive the Boe-Bot is
when following an object.
 Try making adjustments to the value of the SetPoint constant. Try values from 0
to 4.
ACTIVITY #3: FOLLOWING A STRIPE
Figure 8-7 shows an example of a course you can build and program your Boe-Bot to
follow. Each stripe in this course is three long pieces of ¾ in (19 mm) vinyl electrical
tape placed edge to edge on white poster board. No paper should be visible between the
strips of electrical tape.
15 1 4 V dd 13 1 2
6- 9VD C
9 V dc
B a t t ery
R ed
Bl ack
X4
X5
V dd
V ni
VVssss
V ss
X3
P1
P 3 P 15
P 5 P 14
P 7 P 13
P 9 P 12
P 11 P 11
P 13 P 10
P 15 P 9
V ni P 8
P7
P6
P5
P4
P3
+
P2
R eset
P1
P0
X2
0 1 2
B o a rd o f E d uc a t i o n
Pw r
R ev C
© 2 000 -2 003
22” (56 cm)
Start
P 11
P 14
P 13
P 15
Vn
i
V ss
TM
X3
V ss
X3
P1
P 3 P1
P 155
P 5 P1
P 144
P 133
P 7 P1
P 122
P 9 P1
P 111
P 11 P1
P 1 3 P1
P 100
P 1 5 P9
P9
P88
Vin P
P7
P7
P6
P6
P5
P5
P4
P4
P3
P3
P2
P2
P1
P1
P0
P0
X2
R ese t
P 12
V dd
R st
© 200 0- 200 3
R ev C
0 1 2
P9
P8
P 10
ST
AMP
S
i nC
LA
SS
X1
Vs s
P0
P2
P4
P6
P8
P1 0
P1 2
P1 4
Vd d
Pw r
B o a rd o f E d u ca t i o n
+
Vd d
V ni
X4
V ss
X5
Ba
lc k
9 Vd c
B a t e ry
R ed
6- 9V DC
1514
V dd
1312
P8
w w w . st a mp si nc la ss .c om
U1
P7
1
P9
ATN
P 11
P 10
P0
P 13
P 12
P1
P2
U1
X1
P3
P2
P3
P4
P5
P6
V ss
P0
P2
P4
P6
P8
P 10
P 12
P 14
V dd
P4
P 14
P5
V dd
P 15
P6
P7
R st
w w w .s t am ps ni cl a ss. c om
Vn
i
V ss
S out
Sn
i
TM
V ss
S TA
n
i C MPS
LAS
S
1
S out
Sn
i
AT N
V ss
P0
P1
Finish
28” (71 cm)
Figure 8-7
Stripe Following
Course
Page 272 · Robotics with the Boe-Bot
Building and Testing the Course
For successful navigation of this course, some testing and Boe-Bot adjustment will be
required.
Materials Required
(1) Sheet of poster board, approximate dimensions: 22 X 28 in (56 X 71 cm)
(1) Roll of black vinyl electrical tape, ¾” (19 mm) wide
 On your poster board, use the electrical tape to lay out a course as shown in
Figure 8-7.
Testing the Stripe
 Point your IR object detectors downward and outward as shown in Figure 8-8
(Figure 7-11 from page 242 repeated here for convenience).
X4
Vdd
X5
Vin
Vss
Figure 8-8
IR Object
Detectors
Directed
Downwards to
Scan for the
Stripe
+
Board of Education
Rev C
© 2000-2003
Top View
Side View
 Make sure your electrical tape course is free of fluorescent light interference.
See Sniffing for IR Interference on page 233.
 Replace the 1 kΩ resistors in series with the IR LEDs with 2 kΩ resistors to
make the Boe-Bot more nearsighted.
 Run DisplayBothDistances.bs2 from page 275. Keep your Boe-Bot connected to
its programming cable so that you can see the displayed distances.
 Start by placing your Boe-Bot so that it is looking directly at the white
background of your poster board as shown in Figure 8-9.
Robot Control with Distance Detection · Page 273
 Verify that your zone readings indicate that an object is detected in a very close
zone. Both sensors should give you a 1 or 0 reading.
15 1 4
6- 9VD C
V dd
R ed
Bl ack
S
i nTA MPS
C LAS
S
TM
1
S out
V in
S in
V ss
AT N
V ss
R st
V dd
P 15
P0
P1
P2
P3
P4
P5
Figure 8-9
Test for Low Zone
Number – Top View
13 1 2
9 V dc
B a t t ery
U1
P 14
P 13
P 12
P 11
P 10
P6
P9
P7
P8
w w w . st a mps i nc al ss .c om
X4
X5
Pw r
V dd
V in
V ss
X3
V ss
P1
P 3 P 15
P 5 P 14
P 7 P 13
P 9 P 12
P 11 P 11
P 13 P 10
P 15 P 9
V in P 8
P7
P6
P5
P4
P3
+
R eset
P2
P1
P0
X2
0 1 2
B o a rd o f E d uc a t i o n
V ss
P0
P2
P4
P6
P8
P 10
P 12
P 14
V dd
X1
R ev C
© 2 000 -2 003
 Place your Boe-Bot so that both IR object detectors are focused directly at the
center of your electrical tape stripe (see Figure 8-10 and Figure 8-11).
 Then, adjust your Boe-Bot’s position (toward and away from the tape) until both
zone values reach the 4 or 5 level indicating that either a far away object is
detected, or no object is detected.
 If you are having difficulties getting the higher readings with your electrical tape
course, see Trouble Shooting the Electrical Tape Course on page 289.
15 1 4
6- 9VD C
V dd
13 1 2
9 V dc
B a t t ery
R ed
Bl ack
S
i nTA MPS
C LAS
S
TM
1
S out
S in
AT N
V ss
P0
P1
P2
P3
P4
P5
P6
P7
U1
V in
V ss
R st
V dd
P 15
P 14
P 13
P 12
P 11
P 10
P9
P8
w w w . st a mps i nc al ss .c om
X4
X5
Pw r
V dd
V in
V ss
X3
V ss
P1
P 3 P 15
P 5 P 14
P 7 P 13
P 9 P 12
P 11 P 11
P 13 P 10
P 15 P 9
V in P 8
P7
P6
P5
P4
P3
+
R eset
P2
P1
P0
X2
0 1 2
B o a rd o f E d uc a t i o n
V ss
P0
P2
P4
P6
P8
P 10
P 12
P 14
V dd
X1
R ev C
© 2 000 -2 003
Figure 8-10
Test for High Zone
Number – Top View
Page 274 · Robotics with the Boe-Bot
Figure 8-11
Test for High Zone
Number – Side View
Electrical Tape
Troubleshooting the Electrical Tape Course
If you are unable to get a high zone value when the IR detectors are focused on the
electrical tape, take a separate piece of paper, and make a stripe that’s four strips wide
instead of three. If the zone numbers are still low, make sure that you are using 2 kΩ
resistors (red-black-red) in series with your IR LEDs. You can also try a 4.7 kΩ resistor to
make the Boe-Bot more nearsighted. If none of this works, try a different brand of black vinyl
electrical tape. Adjusting the IR LED/detector so that it is focused closer to or further from
the front of the Boe-Bot (see Figure 8-11) may also help.
If you are having trouble with low zone measurements when reading the white surface, try
pointing the IR LEDs and detectors further downward and toward the front of the Boe-Bot,
but be careful not to cause reflection off the chassis. You can also try a lower-value resistor
like 1 kΩ (brown-black-red).
 Now, place the Boe-Bot on the course so that its wheels straddle the black line.
The IR detectors should be facing slightly outward. See close-up in Figure 8-12.
Verify that the distance reading for both IR object detectors is 0 or 1 again. If
the readings are higher, it means they need to be pointed slightly further
outward, away from the edge of the stripe.
When you move the Boe-Bot in either direction indicated by the double-arrow, one or the
other IR object detector will become focused on the electrical tape. When you do this,
the readings for the object detector that is now over the electrical tape should increase to
4 or 5. Keep in mind that if you move the Boe-Bot toward its left, the right detectors
should increase in value, and if you move the Boe-Bot toward its right, the left detectors
should show the higher value.
Robot Control with Distance Detection · Page 275
 Adjust your IR object detectors until the Boe-Bot passes this last test. Then you
will be ready to try following the stripe.
Figure 8-12: Stripe Scan Test
15 1 4
6- 9VD C
X4
Vdd
X5
Vin
V dd
13 1 2
9 V dc
B a t t ery
R ed
Bl ack
X4
X5
V dd
V in
V ss
X3
V ss
P1
P 3 P 15
P 5 P 14
P 7 P 13
P 9 P 12
P 11 P 11
P 13 P 10
P 15 P 9
V in P 8
P7
P6
P5
P4
P3
+
R eset
P2
P
P 11
P0
X2
0 1 2
B o a rd o f E d uc a t i o n
Pw r
S TAMP
S
n
i
C LAS
S
Vss
TM
1
S out
V in
S in
V ss
AT N
V ss
R st
V dd
P0
P1
P2
P3
P4
P5
P 15
U1
P 14
P 13
P 12
P 11
P 10
P6
P9
P7
P8
w w w . st a mps i nc al ss .c om
V ss
P0
P2
P4
P6
P8
P 10
P 12
P 14
V dd
X1
R ev C
© 2 000 -2 003
+
Board of Education
Rev C
© 2000-2003
IR Object Detectors
close-up
Top view of Boe-Bot straddling the stripe
Programming for Stripe Following
You will only need to make a few small adjustments to FollowingBoeBot.bs2 to make it
work for following a stripe. First, the Boe-Bot should move toward objects closer than
the SetPoint and away from objects further from the SetPoint. This is the opposite of
how FollowingBoeBot.bs2 behaved. To reverse the direction the Boe-Bot moves when it
senses that the object is not at the SetPoint distance, simply change the signs of Kpl and
Kpr. In other words, change Kpl from -35 to 35, and change Kpr from 35 to -35. You
will need to experiment with your SetPoint. Values from 2 to 4 tend to work best. This
next example program will use a SetPoint of 3.
Example Program: StripeFollowingBoeBot.bs2





Open FollowingBoeBot.bs2 and save it as StripeFollowingBoeBot.bs2.
Change the SetPoint declaration from SetPoint CON 2 to SetPoint CON 3.
Change Kpl from -35 to 35.
Change Kpr from 35 to -35.
Run the program.
Page 276 · Robotics with the Boe-Bot
 Place your Boe-Bot at the “Start” location shown in Figure 8-13. The Boe-Bot
should wait there until you place your hand in front of its IR object detectors. It
will then roll forward. When it clears the starting stripe, take your hand away,
and it should start tracking the stripe. When it sees the “Finish” stripe, it should
stop and wait there.
 Assuming that you can get distance readings of 5 from the electrical tape and 0
from the poster board, SetPoint constant values of 2, 3, and 4 should work.
Try different SetPoint values and make notes of your Boe-Bot’s performance
on the track.
15 1 4 V dd 13 1 2
6- 9VD C
9 V dc
B a t t ery
R ed
Bl ack
X4
X5
V dd
V in
VVssss
X3
V ss
P1
P 3 P 15
P 5 P 14
P 7 P 13
P 9 P 12
P 11 P 11
P 13 P 10
P 15 P 9
V in P 8
P7
P6
P5
P4
P3
+
R eset
P2
P1
P0
X2
0 1 2
B o a rd o f E d uc a t i o n
Pw r
S
n
i TAMPS
C LAS
S
TM
1
S out
V in
S in
V ss
AT N
V ss
P0
R st
V dd
P 15
P1
P2
P3
P4
P5
U1
P 14
P 13
P 12
P 11
P 10
P6
P9
P7
P8
w w w . st a mp si nc al ss .c om
V ss
P0
P2
P4
P6
P8
P 10
P 12
P 14
V dd
X1
R ev C
© 2 000 -2 003
22” (56 cm)
Start
Figure 8-13
Stripe Following
Course.
w w w .s t am ps ni cl a ss. c om
P9
P8
P5
P 10
P4
P 11
P3
U1
P 14
P 13
P 15
P0
V dd
R st
ATN
Vn
i
V ss
1
TM
ST
AMP
S
in
C LA
SS
X3
V ss
X3
P1
P 3 P1
P 155
P 5 P1
P 144
P 133
P 7 P1
P 122
P 9 P1
P 111
P 11 P1
P 1 3 P1
P 100
P1 5 P
P99
P8
V i n P8
P7
P7
P6
P6
P5
P5
P4
P4
P3
P3
P2
P2
P1
P1
P0
P0
X2
R ese t
P 12
P1
P2
S out
Sn
i
© 200 0- 200 3
R ev C
0 1 2
P6
P7
V ss
X1
Vs s
P0
P2
P4
P6
P8
P1 0
P1 2
P1 4
Vd d
Pw r
B o a rd o f E d u ca t i o n
+
Vd d
V ni
X4
V ss
X5
Ba
lc k
9 Vd c
B a t e ry
R ed
6- 9V DC
1514
V dd
1312
Finish
28” (71 cm)
'
'
'
'
'
'
'
-----[ Title ]-------------------------------------------------------------Robotics with the Boe-Bot - StripeFollowingBoeBot.bs2
Boe-Bot adjusts its position to move toward objects that are closer than
zone 3 and away from objects further than zone 3. Useful for following a
2.25 inch wide vinyl electrical tape stripe.
{$STAMP BS2}
' Stamp directive.
{$PBASIC 2.5}
' PBASIC directive.
DEBUG "Program Running!"
' -----[ Constants ]---------------------------------------------------------Kpl
Kpr
SetPoint
CenterPulse
CON
CON
CON
CON
35
-35
3
750
' Change from -35 to 35
' Change from 35 to -35
' Change from 2 to 3.
Robot Control with Distance Detection · Page 277
' -----[ Variables ]---------------------------------------------------------freqSelect
irFrequency
irDetectLeft
irDetectRight
distanceLeft
distanceRight
pulseLeft
pulseRight
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
Nib
Word
Bit
Bit
Nib
Nib
Word
Word
' -----[ Initialization ]----------------------------------------------------FREQOUT 4, 2000, 3000
' -----[ Main Routine ]------------------------------------------------------DO
GOSUB Get_Ir_Distances
' Calculate proportional output.
pulseLeft = SetPoint - distanceLeft * Kpl + CenterPulse
pulseRight = SetPoint - distanceRight * Kpr + CenterPulse
GOSUB Send_Pulse
LOOP
' -----[ Subroutine - Get IR Distances ]-------------------------------------Get_Ir_Distances:
distanceLeft = 0
distanceRight = 0
FOR freqSelect = 0 TO 4
LOOKUP freqSelect,[37500,38250,39500,40500,41500], irFrequency
FREQOUT 8,1,irFrequency
irDetectLeft = IN9
distanceLeft = distanceLeft + irDetectLeft
FREQOUT 2,1,irFrequency
irDetectRight = IN0
distanceRight = distanceRight + irDetectRight
NEXT
RETURN
Page 278 · Robotics with the Boe-Bot
' -----[ Subroutine - Get Pulse ]--------------------------------------------Send_Pulse:
PULSOUT 13,pulseLeft
PULSOUT 12,pulseRight
PAUSE 5
RETURN
Your Turn – Stripe Following Contest
You can turn this into a contest with the lowest course time winning, provided the BoeBot faithfully waits at the “Start” and “Finish” stripes. You can make up other courses
too. For best performance, experiment with different SetPoint, Kpl, and Kpr values.
ACTIVITY #4: MORE BOE-BOT ACTIVITIES AND PROJECTS ONLINE
So, what do you want to do with your Boe-Bot next? Possible next steps include:




Projects with Boe-Bot accessories
Contests and Challenges
More activities with your Boe-Bot using the kit you’ve already got
IR Remote for the Boe-Bot text and kit
All of the resources discussed in this activity can be accessed through the
www.parallax.com/go/Boe-Bot page.
Projects with Boe-Bot Accessories
Parallax has additional sensors and accessory kits so you can add capabilities and keep
exploring with your Boe-Bot. Here are some examples:







Ping))) Ultrasonic Distance Sensor (#28015) provides longer range and more
accurate object distance measurements. An optional Mounting Bracket Kit
(#570-28015) allows the sensor to sweep an area.
Dual-axis accelerometer for tilt sensing (#28017)
Compass Module for navigation (#29123)
A Crawler Kit to make your Boe-Bot a 6-legged walker (#30055)
A mechanical Gripper Kit for picking up and moving items (#28202)
A Tank Tread Kit for all-terrain navigation (#28106)
XBee RF modules and adapters for wireless control and communication (see
www.parallax.com/go/XBee)
Robot Control with Distance Detection · Page 279
Figure 8-14: Ping))) Sensor and Bracket, Crawler Legs, and Gripper Add-Ons
Contests and Challenges
Interested in a contest? The www.parallax.com/go/Boe-Bot page also has links to rules
to contests ranging from simple to complex and very challenging.
Some ideas are also included here in Appendix C: Boe-Bot Navigation Contests which
begins on page 299.
Page 280 · Robotics with the Boe-Bot
IR Remote for the Boe-Bot
IR Remote for the Boe-Bot is available in print (#28139) and as a free PDF download.
This book uses the same circuit you currently have built on your Boe-Bot, and has
example programs that:


Make it possible for you to drive your Boe-Bot around by pressing and holding
certain buttons on the remote.
Make your Boe-Bot to listen for configuration commands from the remote that
tell it what to do next, like roam, follow objects, allow remote control, and
more…
The only additional piece of equipment required is a universal TV remote, which is a
common item in most households and can be obtained inexpensively through many stores
as well as through www.parallax.com (#020-00001).
More Activities with Your Boe-Bot Using the Kit You’ve Already Got
Here are some examples of activities you can find at www.parallax.com/go/Boe-Bot that
utilize the parts in your Boe-Bot kit. You won’t need to buy any extra parts to try these
activities:




Better distance detection by varying IR LED brightness instead of frequency
Navigate in a maze
Detect a candle flame
Climb uphill on a moving, tilting surface with an accelerometer
SUMMARY
Frequency sweep was introduced as a way of determining distance using the Boe-Bot’s
IR LED and detector. FREQOUT was used to send IR signals at frequencies ranging from
37.5 kHz (most sensitive) to 41.5 kHz (least sensitive). The distance was determined by
tracking which frequencies caused the IR detector to report that an object was detected
and which did not. Since not all of the frequencies were separated by the same value, the
LOOKUP command was introduced as simple way to use the counting sequence supplied
by a FOR…NEXT loop to index sequential lists of numbers.
Control systems were introduced along with closed loop control. Proportional control in
a closed-loop system is an algorithm where the error is multiplied by a proportionality
Robot Control with Distance Detection · Page 281
constant to determine the system’s output. The error is the measured system output
subtracted from the set point. For the Boe-Bot, both system output and set point were in
terms of distance. The BASIC stamp was programmed in PBASIC to operate control
loops for the both the left and right servos and distance detectors. By re-sampling
distance and adjusting the servo output before sending pulses to the servos, the control
loop made the Boe-Bot responsive to object motion. The Boe-Bot was able to use
proportional control to lock onto and follow objects, and it also used it to track and
follow a stripe of black electrical tape.
Last but not least, pointers to more activities, resources and contests were covered since
you’re about done here.
Watch the Boe-Bot in Action at www.parallax.com!
You can see the Boe-Bot and other robot video clips in the Robot Videos section of the
Video Gallery at www.parallax.com/go/videos.
Questions
1. What would the relative sensitivity of the IR detector be if you use FREQOUT to
send a 35 kHz signal? What is the relative sensitivity with a 36 kHz signal?
2. Consider the code snippet below. If the index variable is 4, which number will
be placed in the prime variable in this LOOKUP command? What values will
prime store when index is 0, 1, 2, and 7?
LOOKUP index, [2, 3, 5, 7, 11, 13, 17, 19], prime
3. In what order are PBASIC math expressions evaluated? How can you override
that order?
4. What PBASIC directive do you use to declare a constant? How would you give
the number 100 the name “BoilingPoint?”
Page 282 · Robotics with the Boe-Bot
Exercises
1. List the sensitivity of the IR detector for each kHz frequency shown in Figure
8-1.
2. Write a segment of code that does the frequency sweep for just four frequencies
instead of five.
3. Make a condensed checklist for the tests that should be performed to ensure
faithful stripe following.
Projects
1. Create different types of electrical tape intersections and program the Boe-Bot to
navigate through them. The intersections could be 90° left, 90° right, three-way,
and four-way. This will involve the Boe-Bot recognizing it is at an intersection.
When the Boe-Bot executes StripeFollowingBoeBot.bs2, the Boe-Bot will stay
still at intersections. The goal is to have the Boe-Bot realize it’s not doing
anything and break from its proportional control loop.
Hints: You can do this by creating two counters, one that increments by 1 each
time through the DO…LOOP, and the other that only increments when the Boe-Bot
delivers a forward pulse. When the counter that increments each time through the
DO…LOOP gets to 60, use IF…THEN to check how many forward pulses were
applied. If less than 30 forward pulses were applied, the Boe-Bot is probably
stuck. Remember to reset both counters to zero each time the loop counter gets
to 60. After the Boe-Bot recognizes that it is at an intersection, it needs to move
to the top edge of the intersection, then back up and figure out whether it sees
electrical tape or white background on the left and right, then make the correct
90° turn. Use a preprogrammed motion for turning 90°, without proportional
control. For three-way and four-way intersections, the Boe-Bot may turn either
right or left.
2. Advanced Optional Project - Design a maze-solving contest of your own, and
program the Boe-Bot to solve it!
Robot Control with Distance Detection · Page 283
Solutions
Q1. The relative sensitivity at 35 kHz is 30%. For 36 kHz, it's 50%.
Q2. When index = 4, prime = 11.
index = 0, prime = 2
index = 1, prime = 3
index = 2, prime = 5
index = 7, prime = 19
Q3. Expressions are evaluated left to right. To override, use parentheses to change
the order.
Q4. Use the CON directive.
BoilingPoint CON 100
E1. Frequency (kHz): 34 35 36 37 38 39 40 41 42
Sensitivity :
14% 30% 50% 76% 100% 80% 55% 35% 16%
E2. To solve this problem, put only four frequencies in the LOOKUP list, and decrease
the FOR…NEXT index by one.
FOR freqSelect = 0 TO 3
LOOKUP freqSelect, [37500, 38750, 39500, 40500], irFrequency
FREQOUT 8, 1, irFrequency
irDetect = IN9
… commands not shown
NEXT
Add a DEBUG command to the IF...THEN. Don't forget the ENDIF.
READ Dots + index, noteDot
IF noteDot = 1 THEN
noteDuration = noteDuration * 3 / 2
DEBUG "Dotted Note!", CR
ENDIF
E3. • Sniff for IR interference with IrInterferenceSniffer.bs2.
• Run Display BothDistances.bs2.
• White readings should be 0-1 in both sensors.
• Black readings should be 4-5 in both sensors.
• Straddle the line, both sensors should read 0-1.
• Move Boe-Bot back and forth over line, sensor over black line should read 4-5.
Page 284 · Robotics with the Boe-Bot
P1. In the solution below, the Check_For_Intersection subroutine implements
the algorithm outlined. The left servo was arbitrarily chosen for counting the
forward pulses. A bit-sized variable named isStuck is used as a flag to let the
Main Routine know whether an intersection has been reached. In the
Navigate_Intersection subroutine, the Boe-Bot goes forward past the
intersection and then backs up, checking the sensors, using DO…LOOP…UNTIL.
Then it makes a preprogrammed 90 degree turn in the correct direction. If the
intersection is a 3-way or 4-way intersection, the Boe-Bot will arbitrarily turn in
the direction that black is first detected. A constant, Turn90Degree, is provided
to tune the 90 degree turn. Some audible and visual indicators are included,
which aid in troubleshooting and understanding what the Boe-Bot is seeing and
deciding, as well as adding a bit of personality and fun.
'
'
'
'
-----[ Title ]------------------------------------------------------Robotics with the Boe-Bot - IntersectionsBoeBot.bs2
Navigate 90 degree left/right, 3-way, and 4-way intersections.
Based on StripeFollowingBoeBot.bs2
' {$STAMP BS2}
' {$PBASIC 2.5}
DEBUG "Program Running!"
' Stamp directive.
' PBASIC directive.
' -----[ Constants ]--------------------------------------------------Kpl
Kpr
SetPoint
CenterPulse
Turn90Degree
CON
CON
CON
CON
CON
35
-35
3
750
30
' Left proportional constant
' Right proportional constant
' 0-1 is White, 4-5 is Black
RightLED
LeftLED
PIN
PIN
1
10
' LED Indicators
' Pulses needed for 90 turn
' -----[ Variables ]--------------------------------------------------freqSelect
irFrequency
irDetectLeft
irDetectRight
distanceLeft
distanceRight
pulseLeft
pulseRight
numPulses
fwdPulses
counter
isStuck
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
VAR
Nib
Word
Bit
Bit
Nib
Nib
Word
Word
Byte
Byte
Byte
Bit
' Sweep through 5 frequencies
' Freq sent to IR emitter
' Store results from detectors
' Calculate distance zones
' Servo pulseWidths
' Count total pulses
' Count forward pulses
' Boolean variable,is bot stuck?
Robot Control with Distance Detection · Page 285
' -----[ Initialization ]---------------------------------------------FREQOUT 4, 2000, 3000
' -----[ Main Routine ]-----------------------------------------------DO
GOSUB Get_Ir_Distances
GOSUB Update_LEDs
' Read IR sensors
' Indicate white/black line
' Calculate proportional output and move accordingly.
pulseLeft = SetPoint - distanceLeft * Kpl + CenterPulse
pulseRight = SetPoint - distanceRight * Kpr + CenterPulse
GOSUB Send_Pulse
GOSUB Check_For_Intersection
IF (isStuck = 1) THEN
GOSUB Make_Noise
GOSUB Navigate_Intersection
ENDIF
' Are we stuck at intersection?
' Audible indication
' Navigate through it
LOOP
' -----[ Subroutines ]------------------------------------------------Navigate_Intersection:
' Go forward until both sensors read white, through the intersection.
DO
pulseLeft = 850: pulseRight = 650 ' Forward
GOSUB Send_Pulse
GOSUB Get_Ir_Distances
GOSUB Update_LEDs
LOOP UNTIL (distanceLeft <=2) AND (distanceRight <=2)
GOSUB Stop_Quickly
' Don't coast forward
' Now back up until one detector sees the black.L & R turn will see
' black on one detector.3- or 4-way will see both black, turn toward
' whichever the bot sees first (random).
DO
pulseLeft = 650: pulseRight = 850 ' Backward
GOSUB Send_Pulse
GOSUB Get_Ir_Distances
GOSUB Update_LEDs
LOOP UNTIL (distanceLeft >=4) OR (distanceRight >=4)
GOSUB Stop_Quickly
' Don't coast backward
' Make 90 degree turn in direction of the detector which sees black
IF (distanceLeft >=4) THEN
' Left detector reads black
FOR counter = 1 TO Turn90Degree
' Turn 90 degrees left
Page 286 · Robotics with the Boe-Bot
PULSOUT 13, 750
PULSOUT 12, 650
PAUSE 20
NEXT
ELSEIF (distanceRight >=4) THEN
FOR counter = 1 TO Turn90Degree
PULSOUT 13, 850
PULSOUT 12, 750
PAUSE 20
NEXT
ENDIF
' without proportional control
' so use PAUSE 20
' Right detector reads black
' Turn 90 degrees right
' That's it. At this point the Boe-Bot should have turned 90 degrees
' to follow the intersection. Continue following the black line.
RETURN
Check_For_Intersection:
' Keep track of no. of pulses vs the forward pulses. If there are less
' than 30 forward pulses per total of 60 pulses, robot is likely stuck
' at an intersection.
isStuck = 0
numPulses = numPulses + 1
' Initialize Boolean variable
' Count total pulses sent
SELECT numPulses
CASE < 60
IF (pulseLeft > CenterPulse) THEN
fwdPulses = fwdPulses + 1
' Count forward pulses
ENDIF
' (forward is any pulse > 750)
CASE = 60
IF (fwdPulses < 30) THEN
isStuck = 1
ENDIF
CASE > 60
numPulses = 0
fwdPulses = 0
ENDSELECT
RETURN
' If we have sent 60 pulses
' how many were forward?
' If < 30, robot is stuck
' Reset counters back to zero
' (Could reset in =60 case but
' it spoils cool Make_Noise)
Make_Noise:
' Makes an increasing tone, proportional to number of forward pulses
FOR counter = 1 TO fwdPulses STEP 3
FREQOUT 4, 100, 3800 + (counter * 10)
NEXT
RETURN
Robot Control with Distance Detection · Page 287
Update_LEDs:
' Use LEDs to indicate whether detectors are seeing black or white.
' White = Off, Black = On. Black is a distance reading > or = 4 .
IF (distanceLeft >= 4) THEN HIGH LeftLED ELSE LOW LeftLED
IF (distanceRight >= 4) THEN HIGH RightLED ELSE LOW RightLED
RETURN
Stop_Quickly:
' This stops the wheels so the Boe-Bot does not "coast" forward.
PULSOUT 13, 750
PULSOUT 12, 750
PAUSE 20
RETURN
Get_Ir_Distances:
' Read both IR object detectors and calculate the distance.
' Black line gives 4-5 reading. White surface give 0-1 reading.
distanceLeft = 0
distanceRight = 0
FOR freqSelect = 0 TO 4
LOOKUP freqSelect,[37500,38250,39500,40500,41500], irFrequency
FREQOUT 8,1,irFrequency
irDetectLeft = IN9
distanceLeft = distanceLeft + irDetectLeft
FREQOUT 2,1,irFrequency
irDetectRight = IN0
distanceRight = distanceRight + irDetectRight
NEXT
RETURN
Send_Pulse:
' Send a single pulse to the servos in between IR readings.
PULSOUT 13,pulseLeft
PULSOUT 12,pulseRight
PAUSE 5
' PAUSE reduced due to IR readings
RETURN
P2. If you create an interesting Boe-Bot maze project and you want to share it with
others, you may want to join the Stamps in Class or Projects forums at
http://forums.parallax.com. Or, you can email the Parallax Education Team
directly at education@parallax.com.
Page 288 · Robotics with the Boe-Bot
Parts List and Kit Options · Page 289
Appendix A: Parts List and Kit Options
To complete the activities in this text, you will need a complete Boe-Bot robot and the
electronic components necessary to build the example circuits. Kit options are described
in this appendix. All of the information in this appendix was current at the time of
printing. Parallax may make part substitutions at our discretion, out of necessity or to
upgrade the quality of our products. For the latest information, downloads, and
accessories, visit www.parallax.com/go/Boe-Bot.
Complete Boe-Bot Robot Kit Options
Aside from a PC with a serial or USB port and a few common household items, the
Boe-Bot Robot Kit options contain all the parts and documentation you’ll need to
complete the experiments in this text.
Boe-Bot Robot Kit - Serial with USB Adapter (#28132)
Parts and quantities subject to change without notice
Stock Code
Description
Quantity
BS2-IC
BASIC Stamp 2 microcontroller module
1
28124
Robotics with the Boe-Bot Parts Kit
1
28125
Robotics with the Boe-Bot Student Guide
1
28150
Board of Education - Serial
1
700-00064
Parallax Screwdriver
1
800-00003
Serial cable
1
28031
USB to Serial Adapter and USB A to Mini B Cable
1
Boe-Bot Robot Kit - USB Only (#28832)
Parts and quantities subject to change without notice
Stock Code
Description
Quantity
BS2-IC
BASIC Stamp 2 microcontroller module
1
28124
Robotics with the Boe-Bot Parts Kit
1
28125
Robotics with the Boe-Bot Student Guide
1
28850
Board of Education USB
1
700-00064
Parallax Screwdriver
1
805-00006
USB A to Mini B Cable
1
Page 290 · Robotics with the Boe-Bot
Robotics with the Boe-Bot Parts kit
If you already have a Board of Education and BASIC Stamp 2, you may purchase the
Robotics with the Boe-Bot Parts Kit, with or without this printed book:
Robotics with the Boe-Bot Parts & Text, #28154
Robotics with the Boe-Bot Parts only, #28124
Parts and quantities subject to change without notice
Stock Code
Description
Quantity
150-01020
1 kΩ resistor
2
150-01030
10 kΩ resistor
4
150-02020
2 kΩ resistor
2
150-02210
220 Ω resistor
8
150-04710
470 Ω resistor
4
150-04720
4.7 kΩ resistor
2
200-01031
0.01 µF capacitor
2
200-01040
0.1 µF capacitor
2
350-00003
Infrared LED
2
350-00006
Red LED
2
350-00029
Phototransistor
2
350-00014
Infrared receiver (Panasonic PNA4602M or equivalent)
2
350-90000
LED standoff for infrared LED
2
350-90001
LED light shield for infrared LED
2
400-00002
Pushbutton, normally open
2
451-00303
3-Pin Header
2
700-00056
Whisker wire
2
800-00016
Jumper wires (bag of 10)
2
Piezospeaker
1
Boe-Bot Hardware Pack
1
900-00001
Boe-Bot Hardware Pack Contents
Hardware replacement parts for the Boe-Bot can be purchased individually, as found in
our on-line Robot Component Shop. Please note that the Hardware Pack is not sold as a
unit separately from the Boe-Bot Robot (Full) Kits or the Boe-Bot Parts Kit.
Parts List and Kit Options · Page 291
Boe-Bot Hardware Pack Contents
Parts and quantities subject to change without notice
Parallax
Stock Code
Description
Quantity
700-00002
Machine screw, 3/8” 4-40 pan-head, Phillips
8
700-00003
Hex nut, 4-40 zinc plated
10
700-00009
Tail wheel ball
1
700-00015
Nylon washer, #4 screw-size
2
700-00016
Machine screw, 4-40 x 3/8” flathead
2
700-00022
Boe-Bot aluminum chassis
1
700-00023
Cotter pin, 1/16" x 1.5” long
1
700-00025
Rubber grommet, 13/32"
2
700-00028
Machine screw, 4-40 x 1/ 4” pan-head Phillips
8
700-00038
Battery holder with cable and barrel plug
1
700-00060
Standoff, threaded aluminum, round 4-40
4
710-00007
Machine screw, 7/8” 4-40 pan-head, Phillips
2
713-00007
1/2” Spacer, aluminum, #4 round
2
721-00001
Parallax plastic wheel
2
721-00002
Rubber band tire
4
900-00008
Parallax Continuous Rotation Servo
2
Building a Boe-Bot with a BASIC Stamp HomeWork Board
The HomeWork Board, which is included in the BASIC Stamp Activity Kit (#90005),
may be used with the Robotics with the Boe-Bot Parts kit and these additional items:
(2) 3-pin male/male headers, #451-00303
(1) Tinned-lead battery pack, #753-00001
A note to Educators: Quantity discounts are available for all of the kits listed above; see
each kit’s product page at www.parallax.com for details. In addition, the BASIC Stamp
HomeWork Board is available separately in packs of 10 as an economical solution for
classroom use, costing significantly less than the Board of Education + BASIC Stamp 2
module (#28158). Contact the Parallax Sales Team toll free at (888) 512-1024.
Page 292 · Robotics with the Boe-Bot
Resistor Color Codes and Breadboarding Rules · Page 293
Appendix B: Resistor Color Codes and
Breadboarding Rules
RESISTOR COLOR CODES
Resistors like the ones we are using in this student guide have colored stripes that tell you
what their resistance values are. There is a different color combination for each
resistance value.
There may be a fourth stripe that indicates the resistor’s tolerance. Tolerance is measured
in percent, and it tells how far off the part’s true resistance might be from the labeled
resistance. The fourth stripe could be gold (5%), silver (10%), or no stripe (20%). For
the activities in this book, a resistor’s tolerance does not matter, but its value does.
Each color bar that tells you the resistor’s value corresponds to a digit, and these
colors/digits are listed in the table below. Figure B-1 shows how to use each color bar
with the table to determine the value of a resistor.



Digit
Color
0
1
2
3
4
5
6
7
8
9
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Gray
White
Tolerance
Code
First Digit
Number of Zeros
Figure B-1
Resistor Color
Codes
Second Digit
First stripe is yellow, which means leftmost digit is a 4.
Second stripe is violet, which means next digit is a 7.
Third stripe is brown. Since brown is 1, it means add one zero to the right of the
first two digits.
The value of this resistor is 470 Ω.
Page 294 · Robotics with the Boe-Bot
BREADBOARDING RULES
Look at your Board of Education or HomeWork Board. The white square with lots of
holes, or sockets, in it is called a solderless breadboard. This breadboard, combined with
the black strips of sockets along two of its sides, is called the prototyping area (shown in
Figure B-2).
The example circuits in this text are built by plugging components such as resistors,
LEDs, speakers, and sensors into these small sockets. Components are connected to each
other with the breadboard sockets. You will supply your circuit with electricity from the
power terminals, which are the black sockets along the top labeled Vdd, Vin, and Vss.
The black sockets on the left are labeled P0, P1, up through P15. These sockets allow
you to connect your circuit to the BASIC Stamp’s input/output pins.
Vdd
Vin
Vss
X3
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure B-2
Prototyping Area
Power terminals (black sockets
along top), I/O pin access (black
sockets along the side), and
solderless breadboard (white
sockets).
The breadboard has 17 rows of sockets separated into two columns by a trough. The
trough splits each of the seventeen rows of sockets into two rows of five. Each row of
five sockets is electrically connected inside the breadboard. You can use these rows of
sockets to connect components together as dictated by a circuit schematic. If you insert
two wires into any two sockets in the same 5-socket row, they are electrically connected
to each other.
A circuit schematic is a roadmap that shows how to connect components together. It uses
unique symbols each representing a different component. These component symbols are
connected by lines to indicate an electrical connection. When two circuit symbols are
connected by lines on a schematic, the line indicates that an electrical connection is made.
Lines can also be used to connect components to voltage supplies. Vdd, Vin, and Vss all
Resistor Color Codes and Breadboarding Rules · Page 295
have symbols. Vss corresponds to the negative terminal of the battery supply for the
Board of Education or BASIC Stamp HomeWork Board. Vin is the battery’s positive
terminal, and Vdd is regulated to +5 volts.
Let’s take a look at an example that uses a schematic to connect the parts shown in Figure
B-3. For each of these parts, the part drawing is shown above the schematic symbol.
Gold
Silver
or
Blank
Yellow
Violet
Brown
Figure B-3
Part Drawings and Schematic
Symbols
LED(left) and
470 Ω resistor (right)
+
470 
LED
Figure B-4 shows an example of a circuit schematic on the left and a drawing of a circuit
that can be built using this schematic on the right. Notice how the schematic shows that
one end of the jagged line that denotes a resistor is connected to the symbol for Vdd. In
the drawing, one of the resistor’s two leads is plugged into one of the sockets labeled
Vdd. In the schematic, the other terminal of the resistor symbol is connected by a line to
the + terminal of the LED symbol. Remember, the line indicates the two parts are
electrically connected. In the drawing, this is accomplished by plugging the other resistor
lead into the same row of 5 sockets as the + lead on the LED. This electrically connects
the two leads. The other terminal of the LED is shown connected to the Vss symbol in
the schematic. In the drawing, the other lead of the LED is plugged into one of the
sockets labeled Vss.
Page 296 · Robotics with the Boe-Bot
Vdd
Vin
X3
Vdd
470 
LED
Vss
Vss
+
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Figure B-4
Example Schematic
and Wiring Diagram
Schematic (left) and
wiring diagram (right)
Figure B-5 shows a second example of a schematic and wiring diagram. Here, P14 is
connected to one end of a resistor, with the other end connected to the + terminal of an
LED, and the – terminal of the LED is connected to Vss. These two schematics differ by
only one connection. The resistor lead that used to be connected to Vdd is now
connected to BASIC Stamp I/O pin P14. The schematic might look more different than
that, because the resistor is shown drawn horizontally instead of vertically. But in terms
of connections, it only differs by one, P14 in place of Vdd.
Vdd
X3
P14
470 
LED
Vss
P15
P14
P13
P12
P11
P10
P9
P8
P7
P6
P5
P4
P3
P2
P1
P0
X2
Vin
Vss
+
Figure B-5
Example Schematic
and Wiring Diagram
Schematic (left) and
wiring diagram (right)
Resistor Color Codes and Breadboarding Rules · Page 297
Here is a more complex example that involves two additional parts, a 1 kΩ resistor, a
phototransistor, and a capacitor. The schematic symbols and part drawings for the
components you are not already familiar with are shown in Figure B-6. The
phototransistor’s terminals are labeled C, B, and E. The B terminal is optical, so it
doesn’t have any electrical connections. The C terminal is the longer pin, and the E
terminal is the shorter pin that comes out of the plastic enclosure closer to a flat spot on
its side.
Light
B
Collector
C
Base
Flat spot and shorter
pin indicate the
emitter (E) terminal
B
E
Current
Figure B-6
Part Drawings and
Schematic Symbols
Emitter
E
C
Phototransistor (top)
Non-polar capacitor
(bottom)
0.1 μF Capacitor Schematic
Symbol and Part Drawing
Since this schematic shown in Figure B-7 calls for a 1 kΩ resistor, which is 1000 Ω, the
first step is to consult Appendix C: Resistor Color Codes to determine the color code.
The color code is Brown, Black, Red. This resistor is connected to P6 in the schematic,
which corresponds to the resistor lead plugged into the socket labeled P6 in the
prototyping area (Figure B-8). In the schematic, the other lead of the resistor is
connected to not one, but two other component terminals: the phototransistor’s C
terminal and one of the capacitor’s terminals. On the breadboard, that resistor lead is
plugged into one of the of the breadboard’s 5-socket rows. This row also has the
phototransistor’s C lead and one of the capacitor’s leads plugged into it. In the
schematic, the phototransistor’s E terminal and the capacitor’s other lead are connected to
Vss. Here is a trick to keep in mind when building circuits on a breadboard. You can use
a wire to connect an entire row on the breadboard to another row, or even to I/O pins or
power terminals such as Vdd or Vss. In this case, a wire was used to connect Vss to a
row on the breadboard. Then, the phototransistor’s E lead and the capacitor’s other lead
are plugged into the same row, which connects them to Vss, completing the circuit.
Page 298 · Robotics with the Boe-Bot
Figure B-7
Resistor,
Phototransistor, and
Capacitor Schematic
Figure B-8
Resistor,
Phototransistor, and
Capacitor Wiring
Diagram
Flat spot and
shorter pin
Keep in mind that the wiring diagrams presented here as solutions to the schematics are
not the ONLY solutions to those schematics. For example, Figure B-9 shows another
solution to the schematic just discussed. Follow the connections and convince yourself
that it does satisfy the schematic.
Control
Horn
Flat Spot,
Shorter Pin
Figure B-9
Resistor,
Phototransistor, and
Capacitor Wiring
Diagram
Note the alternative
parts placement.
Boe-Bot Navigation Contests · Page 299
Appendix C: Boe-Bot Navigation Contests
If you're planning a competition for autonomous robots, these rules are provided courtesy
of Seattle Robotics Society.
CONTEST #1: ROBOT FLOOR EXERCISE
Purpose
The floor exercise competition is intended to give robot inventors an opportunity to show
off their robots or other technical contraptions.
Rules
The rules for this competition are quite simple. A 10-foot-by-10-foot flat area is
identified, preferably with some physical boundary. Each contestant will be given a
maximum of five minutes in this area to show off what their robot can do. The robot's
contestant can talk through the various capabilities and features of the robot. As always,
any robot that could damage the area or pose a danger to the public will not be allowed.
Robots need not be autonomous, but it is encouraged. Judging will be determined by the
audience, either indicated by clapping (the loudest determined by the judge), or some
other voting mechanism.
CONTEST #2: LINE FOLLOWING
Objective
To build an autonomous robot that begins in Area "A" (at position "S"), travels to Area
"B" (completely via the line), then travels to the Area "C" (completely via the line), then
returns to the Area "A" (at position "F"). The robot that does this in the least amount of
time (including bonuses) wins. The robot must enter areas "B" and "C" to qualify. The
exact layout of the course will not be known until contest day, but it will have the three
areas previously described.
Skills Tested
The ability to recognize a navigational aid (the line) and use it to reach the goal.
Page 300 · Robotics with the Boe-Bot
Maximum Time to Complete Course
Four minutes.
Example Course
All measurements in the example course are approximate. There is a solid line dividing
Area "A" from Area "T" at position "F.” This indicates where the course ends. The line is
black, approximately 2.25 inches wide and spaced approximately two feet from the walls.
All curves have a radius of at least one foot and at most three feet. The walls are 3 1/2
inches high and surround the course. The floor is white and made of either paper or
Dupont Tyvek®. Tyvek is a strong plastic used in mailing envelopes and house
construction.
Positions "S" and "F" are merely for illustration and are not precise locations. A
Competitor may place the robot anywhere in Area "A,” facing in any direction when
starting. The robot must be completely within Area "A.” Areas "A,” "B" and "C" are not
colored red on the actual course.
Figure D-1
Sample Contest Course
Boe-Bot Navigation Contests · Page 301
Scoring
Each contestant’s score is calculated by taking the time needed to complete the course (in
seconds) minus 10% for each "accomplishment." The contestant with the lowest score
wins.
Line Following Scoring
Accomplished
Percent Deducted
Stops in area A after reaching B and C
10%
Does not touch any walls
10%
Starts on command
10%
("Starts on command" means the robot starts with an external, non-tactile command. This
could, for example, be a sound or light command.)
CONTEST #3: MAZE FOLLOWING
Purpose
The grand maze is intended to present a test of navigational skills by an autonomous
robot. The scoring is done in such a way as to favor robots which are either brutally fast
or which can learn the maze after one pass. The object is for a robot, which is set down at
the entrance of the maze, to find its way through the maze and reach the exit in the least
amount of time.
Physical Characteristics
The maze is constructed of 3/4" shop-grade plywood. The walls are approximately 24
inches high, and are painted in primary colors with glossy paint. The walls are set on a
grid with 24-inch spacing. Due to the thickness of the plywood and limitations in
accuracy, the hallways may be as narrow as 22 inches. The maze can be up to 20-feet
square, but may be smaller, depending on the space available for the event.
The maze will be set up on either industrial-type carpet or hard floor (depending on
where the event is held). The maze will be under cover, so your robot does not have to be
rain proof; however, it may be exposed to various temperatures, wind, and lighting
conditions. The maze is a classical two-dimensional proper maze: there is a single path
from the start to the finish and there are no islands in the maze. Both the entrance and exit
are located on outside walls. Proper mazes can be solved by following either the left wall
Page 302 · Robotics with the Boe-Bot
or the right wall. The maze is carefully designed so that there is no advantage if you
follow the left wall or the right wall.
Robot Limitations
The main limit on the robot is that it be autonomous: once started by the owner or
handler, no interaction is allowed until the robot emerges from the exit, or it becomes
hopelessly stuck. Obviously the robot needs to be small enough to fit within the walls of
the maze. It may touch the walls, but may not move the walls to its advantage -no
bulldozers. The judges may disqualify a robot which appears to be moving the walls
excessively. The robot must not damage either the walls of the maze, nor the floor. Any
form of power is allowed as long as local laws do not require hearing protection in its
presence or place any other limitations on it.
Scoring
Each robot is to be run through the maze three times. The robot with the lowest single
time is the winner. The maximum time allowed per run is 10 minutes. If a robot cannot
finish in that amount of time, the run is stopped and the robot receives a time of 10
minutes. If no robot succeeds in finding the exit of the maze, the one that made it the
farthest will be declared the winner, as determined by the contest's judge.
Logistics
Each robot will make one run, proceeding until all robots have attempted the maze. Each
robot then does a second run through the maze, then the robots all do the third run. The
judge will allow some discretion if a contestant must delay their run due to technical
difficulties. A robot may remember what it found on a previous run to try to improve its
time (mapping the maze on the first run), and can use this information in subsequent
runs-as long as the robot does this itself. It is not allowed to manually "configure" the
robot through hardware or software as to the layout of the maze.
Index · Page 303
Index
<>, 134
=, 54
3-pin male-male headers, 45
3-position switch, 42
90° turns, 111
accelerometer, 278
aerospace projects, 58
alarm circuit, 88
ambient light, 172
amps, 31
analog sensor, 179
artificial intelligence, 160
ASCII, 131
Base
Phototransistor, 170
Basic Analog and Digital, 38
BASIC Stamp Editor, 12
Memory Map, 127
batteries, 42, 46
battery pack, 77
BIN1, 150
binary sensor, 179
BINx, 150
Bit, 53
Board of Education, 41
Board revisions, 41
Boe-Boost, 42
brownout, 86
Byte, 53
camera, to see infrared, 223
Capacitor
and RCTIME, 184
polar – identifying terminals, 212
Polar – identifying terminals, 227
schematic symbol and part drawing.
Capek, Karl, 5
carpeting, 112
charge transfer, 183
CLREOL, 202
CLS, 202
collector, phototransistor, 170
Compass Module, 278
CON, 200
connected in series, 176
constants, declaring, 200
Contests, 279, 299
control characters, DEBUG. See DEBUG
corners, escaping, 160
CR, 21
Crawler Kit, 278
CRSRX, 202
CRSRXY, 151
crystal, 87
current, 31, 177
cursor, 151
data collision, 126
DATA directive, 127
Word modifier, 132
DEBUG
? formatter, 55
BIN, 150
CLREOL, 202
CLS, 202
CR, 21
CRSRX, 202
CRSRXY, 151
Page 304 · Robotics with the Boe-Bot
HOME, 202
REP, 202
Debug Terminal, 92
DEBUGIN, 92
declaring constants, 200
desk lamp, 171
digitized measurement, 179
diode, 28
distance calculations, 112
DO WHILE...LOOP, 128
DO UNTIL...LOOP, 128
DO...LOOP, 26
DO...LOOP UNTIL, 128
driving direction, 104
Duration argument, PULSOUT, 37
Educators Courses, 8
EEPROM, 126
and navigation, 127
data collision, 126
Memory Map, 127
electric potential, 177
electrical tape, 243
ELSEIF, 155
Emitter
Phototransistor, 170
Encoder kits, 117
END, 235
ENDIF, 155
ENDSELECT, 127
EndValue, 56
escaping corners, 160
flashlight, 174
fluorescent lights, 222
and phototransistors, 171
infrared interference, 222
foot-candle, 171
FOR...NEXT, 56
to control servo run time, 64
formatters, DEBUG. See DEBUG
FREQOUT, 89, 229
frequency, 87
frequency generation, 229
frequency sweep, 256
GOSUB, 120
halogen, 170
hardware adjustment, 109
hertz, 90
HIGH, 32
HOME, 202
Hz, 90
IF…THEN, 155
illuminance, 171
incident light, 171
infrared interference, 233
initialization routine, 91
input register, 150
INx, 176
INx variables, 150
IR wavelengths, 223
iterative process, 111
kilohertz, 90
LED
part drawing and schematic symbol, 29
Light
ambient, 172
binary light sensor, 171
color spectrum, 171
desk lamp, 171
flashlight, 174
fluorescent, 222
fluorescent interference, 233
foot-candle, 171
Index · Page 305
halogen, 170
ramping, 117
illumninance, 171
rotating, 107
infrared, 221
stopping under bright light, 175
infrared interference, 222
subroutines, 120
LED part drawing and schematic symbol,
29
tactile, 143
luminance, 171
lux, 171
measure brightness with phototransistor,
179
LOOKUP, 257
LOW, 32
luminance, 171
lux, 171
maneuvers. See Navigation
Mars, 58
MAX, 205
Memory Map, 127, 131
microfarad, 181
milliamps, 31
millisecond, 25
MIN, 205
music, 90
nanofarad, 181
Navigation
with EEPROM, 126
with infrared object detection, 237
with whiskers, 155
Nib, 53
nodes, 184
not-equal operator "<>", 134
ohm, 28
Ohm’s Law, 177
operator block, 264
Operators
equals "=", 54
greater than >, 204
greater than or equal to >=, 204
less than <, 204
less than or equal to <=, 204
MAX, 205
MIN, 205
90-degree turns, 112
not-equal <>, 134
adjusting for straight travel, 110
with variables, 54
backward, 107
contests, 299
custom routines, 135
distance calculations, 112
errors on carpet, 112
escaping corners, 160
pivoting, 107
oscilloscope, 90, 188
PAUSE syntax, 24
PBASIC language
acronym definition, 11
BINx, 150
CLREOL formatter, 202
CLS, 202
Page 306 · Robotics with the Boe-Bot
CON, 200
PULSOUT syntax, 36
CR, 21
PWM, 190
CRSRX formatter, 202
PWM syntax, 189
CRSRXY, 151
RCTIME, 184
DATA, 127
READ, 127
DEBUG ? formatter, 55
REP formatter, 202
DEBUGIN, 92
RETURN, 120
DO WHILE...LOOP, 128
SDEC formatter, 55
DO ...LOOP UNTIL, 128
SELECT...CASE, 127
DO...LOOP, 26
STEP, 56
ELSE, 155
STOP, 235
ELSEIF, 155
VAR syntax, 53
END, 235
Word modifier for DATA, 132
ENDIF, 155
ENDSELECT, 127
FOR...NEXT syntax, 56
FREQOUT syntax, 89
GOSUB, 120
HIGH syntax, 32
HOME, 202
IF…THEN...ELSE syntax, 155
INx, 176
INx variables, 150
LOOKUP syntax, 257
LOW syntax, 32
MAX, 205
MIN, 205
Operators. See Operators
PAUSE, 24
phototransistor, 170
picofarad, 181
piezoelectric crystal, 87
piezoelectric element, 87
piezospeaker, schematic symbol, 87
Ping))) Ultrasonic Distance Sensor, 278
pliers, 73
potentiometer, 52
PowerPoint presentations, 8
Program Listings
AvoidTableEdge.bs2, 245
BoeBotForwardTenSeconds.bs2, 110
BoeBotForwardThreeSeconds.bs2, 105
BothServosThreeSeconds.bs2, 66
CenterServoP12.bs2, 51
CenterServoP13.bs2, 52
Ch01Prj01_Add1234.bs2, 21
Ch01Prj02_ FirstProgramYourTurn.bs2,
21
Index · Page 307
Ch02Prj01_DimlyLitLED.bs2, 70
OneSubroutine.bs2, 121
Ch02Prj02_4RotationCombinations.bs2,
71
P1LedHigh.bs2, 235
Ch03Prj01_TestCompleteTone.bs2, 100
Ch03Prj02_DebuginMotion.bs2, 101
Circle.bs2, 140
CirclingWithWhiskerInput.bs2, 168
ControlServoRunTimes.bs2, 65
CountToTen.bs2, 57
DisplayBothDistances.bs2, 261
EepromNavigation.bs2, 129
PulseBothLeds.bs2, 39
PulseP13Led.bs2, 37
RightServoTest.bs2, 83
RoamAndSniffBoeBot.bs2, 252
RoamingWithIr.bs2, 238
RoamingWithWhiskers.bs2, 156
ServoP12Clockwise.bs2, 59
ServoP12Counterclockwise.bs2, 60
ServoP13Clockwise.bs2, 59
EepromNavigationWithWordValues.bs2,
134
ServosP13CcwP12Cw.bs2, 62
EscapingCorners.bs2, 161
StartAndStopWithRamping.bs2, 118
FastIrRoaming.bs2, 240
StartResetIndicator.bs2, 90
FollowingBoeBot.bs2, 267
StripeFollowingBoeBot.bs2, 276
ForwardLeftRightBackward.bs2, 107
SumoBoeBot.bs2, 251
ForwardOneSecond.bs2, 114
TestBinaryPhototransistor.bs2, 175
HalfLightSensitivity.bs2, 191
TestBothIrAndIndicators.bs2, 232
HaltUnderBrightLight.bs2, 175
TestLeftFrequencySweep.bs2, 259
HelloOnceEverySecond.bs2, 27
TestLeftIr.bs2, 227
HighLowLed.bs2, 32
TestMaxDarkWithHighPause.bs2, 194
HighVsPwmInRctime.bs2, 192
TestMaxDarkWithPwm.bs2, 194
IntersectionsBoeBot.bs2, 284
TestServoSpeed.bs2, 94
IrInterferenceSniffer.bs2, 234
TestWhiskers.bs2, 150
LightSensorValues.bs2, 197
TestWhiskers_UpdateEaOnNewLine.bs2,
167
MotionActivatedBoeBot.bs2, 250
MovementsWithSubroutines.bs2, 123
MovementWithVariablesAndOneSubrouti
ne.bs2, 124
TimedMessages.bs2, 25
Triangle.bs2, 141
TwoSubroutines.bs2, 122
Page 308 · Robotics with the Boe-Bot
VariablesAndSimpleMath.bs2, 54
PropScope oscilloscope, 90, 188
pseudo code, 160
pulse train, 59
pulse width modulation, 61
PWM, 190
PULSOUT, 36
PULSOUT Duration argument
maximum value, 37
PWM, 61, 189, 190
QT circuit, 183
quantized measurement, 179
RAM Map, 210
ramping, 117
RCTIME syntax, 184
READ, 127
rechargeable AA batteries, 46
remote, 233
REP, 202
reset indicator, 86
resistance, 177
Resistor
and RCTIME, 185
color codes, 293
part drawing and schematic symbol, 28
RETURN, 120
Revolutions Per Minute, 59
Rossum's Universal Robots, 5
RPM, 59
screwdriver, 49, 73
SDEC formatter, 55
second, 25
SELECT...CASE, 127
Sensors
analog vs. binary, 179
series resistance, 236
Servos
and PWM, 61
avoiding damage, 42, 51
centering procedure, 49
control run time with FOR...NEXT, 64
horn styles, 24
mounting options, 76
removing servo horns, 75
servo circuits for HomeWork Board, 46
servo port power supply selector jumper,
43
servo signal monitor circuit, 44
standard vs. continuous rotation, 24
testing, 58
transfer curve, 96
troubleshooting, 85
wiring diagram for HomeWork Board, 47
sine waves, 229
software adjustment, 109
square waves, 229
StartValue, 56
StepValue, 57
STOP, 235
subroutines, 120
subsystem testing, 58
Tank Tread Kit, 278
TestP6LightSense.bs2, 186
timing diagram, 34, 38
tokens, 126
transfer curve for servos, 96
transistor, 169
Troubleshooting
electrical tape course, 274
Index · Page 309
infrared object detection, 228
light sensor navigation, 214
Programming connection. See BASIC
Stamp Editor Help
servos, 85
Understanding Signals with the
PropScope, 90, 188
USB drivers, 16
V (volts), 31
VAR, 53
variables
default value, 54
Variables
aliasing, 210
declaring, 53
INx, 150
math, with operators, 54
sizes, 53
Vbp, 46
Vdd, 34
vibration, 87
Virtual COM Port, 16
voltage, 31
voltage decay graph, 188
Vss, 34
What’s a Microcontroller?, 27
What’s a Microcontroller? Student Guide,
12
wheel direction, 104
wheels, 79
whiskers, 144
schematic, 146
Word, 53
Word modifier, 132
wrench, 73
XBee RF modules, 278
Ω, 177
Ω - ohm symbol, 28
Parts and quantities are subject to change without notice. Parts may differ from what is shown in this
picture. If you have any questions about your kit, please email stampsinclass@parallax.com.