With the help of a clever combination of
brightness sensors, varikabo can pick up the
tiniest contrasts in his surroundings and
react to them in various ways. varikabo‘s
brain cells are two transistors, his muscles
two motors.
The manual consists of three parts:
By using a selector switch and adjusting and
exchanging the sensors, you get to explore
ten different functions and astounding
1) Assembly
After a short introduction to soldering, all
necessary steps are described in detail and
are documented with numerous illustrations
in the construction manual.
This is a fun way to learn the basics of
The experiment manual explains the
configuration of the various functions,
showing how these can be set using the
selector switch and the three exchangeable
varikabo can do lots of things:
follow and avoid lines
circle in and follow light
circle in, follow, and avoid shadows
push, follow, and avoid objects
How varikabo works
In this manual, you're shown how the
control circuits are set up and how the
various components function. You can learn
to understand and comprehend varikabo's
multi-functional behaviors and what causes
Introduction to soldering
If you have little experience with soldering,
be sure to read this part of the manual very
carefully and begin by practicing, for
example with some old wire.
Soldering iron: 20 to 30 W / 300 to 350°C
Solder: 0.5 to 0.7 mm diameter
Small wire cutter
Damp, heat-resistant sponge
Possibly desoldering wick or desoldering
bulb for corrections
1. Install the components at the indicated
locations. Soldering is done on the other
side of the circuit board.
2. Clean the soldering tip lengthwise with a
damp sponge.
3. Press the soldering tip for approx. one
second simultaneously
to the soldering pad
and the connecting
wire of the
component so that
both are well heated.
4. Now feed the solder into the gap
between the soldering pad, the
component wire, and the soldering tip
while holding the soldering
tip in place. Use just
enough solder, to
cover the whole
soldering pad.
5. Keep the soldering tip in place on
the solder joint for about one
second until the applied solder is
spread evenly and a silvery metallic
cone forms around the wire.
6. Then shorten protruding wires
with a small side cutter.
• Heat up sufficiently and do not
“dab” with the soldering iron
• Do not heat up too long – the
solder will turn “sticky”
• Do not use too much solder,
otherwise there will be thick
In case it did not work:
Remove solder with desoldering bulb or desoldering
wick and start over. The
second time it will work
1) Assembly
You will need
A soldering iron and solder
Plyers and a wire cutter
9 V battery or 9 V rechargeable battery
Approx. two hours time
The components of the kit
2) Functions
Since varikabo generally follows or avoids either light or darkness, his numerous abilities can
be divided up into Shadow-Follower and Light-Follower functions.
The five Shadow-Follower Functions
Following lines
Following objects
Moves along a line
on the floor
Looks for a dark
line, speeds up on
straight stretch, and
brakes for sharp
Pushing objects
Following shadows
Circling in shadows
Follows dark objects Pushes small
in front of him
Avoids light and
looks for shadows
Circles when in a
Moves toward
objects next to him
yet keeps a
distance, stopping
in front of them
Moves toward
shadows, following
them if they move
and standing still
when in them
Speeds up to get to
a shadow and then
circles slowly in the
Stands still if there’s
no object in sight moves if there’s an
object in front of
him and follows it
The five Light-Follower Functions
Following light
Dodges objects
Dodges lines
Follows the light of
a flashlight
Masters an obstacle
Rides between dark Flees from the
areas on the ground shadow (of a bird)
Circles when under
a light
Moves toward a
light or spot of light
on the floor and
stands still in front
of it
Slows down at dark
obstacles, navigates
around them, and
then speeds up
The narrower the
path, the faster he
goes, and he stops
at the end of dead
Moves fast toward
the light of a lamp
from above and
then calmly circles
in the light
varikabo stands still
Avoiding shadows
Dodges a shadow,
speeds up, and
then continues
moving slowly
towards light
varikabo moves slowly
Circling in light
varikabo moves fast
Basic Settings
You can take off the left and right sensors from the LEDs and exchange them. They determine
whether varikabo will seek darkness or light.
 Sensor cables parallel:
varikabo moves toward darkness (Shadow-Follower)
 Sensor cables crossed:
varikabo moves toward light (Light-Follower)
With the selector switch, you decide whether the sensor signals are applied directly (=) or
interchanged (×) to the transistors (T1/T2) or to the motors (M1/M2). This determines
whether the range of speed will be fast or slow.
 Switch on „FAST (=)“:
varikabo accelerates if there‘s a shadow on the middle sensor
 Switch on „STOP (×)“:
varikabo brakes if there‘s a shadow on the middle sensor
In acceleration mode, varikabo‘s eyes shine red and in brake mode blue.
Positioning the sensors
varikabo has three rotatable brightness sensors that cause him to respond to lines, objects,
light, and shadows. By adjusting the sensors downward, upward, and forward, you decide
whether varikabo will respond to these when they're on the ground, in front of him, or over
Function Selections
The diagrams on the following four pages show you how to set the function you wish for. Each
of the ten function symbols appear two times. Follow these steps:
Put the left and right sensors parallel (Shadow-Follower) or crossed (Light-Follower)
Following the diagram, position the left and right sensors symmetrically
Set the selector to „FAST“ (acceleration mode) or „STOP“ (brake mode)
Position the middle sensor as shown on the diagram
The left and right sensors always need to be positioned as a mirror image of one another.
For the function, „Circling in shadows“, the left and right sensors need to be plugged onto
the LEDs backward, so that they are positioned 45° to the back.
If varikabo doesn‘t respond as you expected, simply readjust the sensors.
The brightness ratio between the left and right sensors determines varikabo‘s direction.
The brightness ratio between the middle sensor and the sensors on the sides determines
varikabo‘s speed.
sensor cables
sensor cables
Acceleration Functions
To choose an acceleration function,
flip the front switch to „=“ or „FAST“.
Brake Functions
To choose a brake function, flip the
front switch to „ד or „STOP“.
3) How varikabo works
The circuit diagram
The Sensors
varikabo‘s brightness sensors are so-called phototransistors. Let‘s simply
imagine the phototransistor (FT) as a fluctuating resistor whose resistance
decreases when brightness increases. However, you need to be sure the
phototransistor has the right polarity: The collector (C) should be on the
positive side, while the emitter (E) should be on the negative side.
In the circuit diagram, you see that all three phototransistors - FT1, FT3 und
FT2, are connected. One says they are connected in a series. This series
connection produces a so-called voltage divider, which distributes the voltage
from the 9 V battery among the sensors depending on their lighting.
When the lighting has the exact same intensity for all three sensors, the
potential difference at each sensor is 3 volts: U1 = U2 = U3 = 3 V
If the middle sensor FT3 is lit up four times brighter than FT1 and FT2, then
the potential difference at FT3 is four times less and the voltage supply is
divided up like this: U1 = 4 V, U3 = 1 V, U2 = 4 V (U1 + U2 + U3 = 9 V)
Both of the fluctuating voltages between the three sensors control the
speed of the two motors.
The Transistors
Since the motors need a much higher electric current than the
sensors can provide, varikabo has an amplifier. A transistor is a
simple electronic amplifier with three connectors:
base (B), emitter (E), und collector (C)
If the voltage between the base and emitter is high enough, namely
approximately UBE = 0.7 V (V = volt), the transistor reduces the
resistance between the collector and emitter, and – as one says, it
switches through. The collector current IC can be about 100 to 800
times larger than the base current IB with normal transistors.
So-called Darlington transistors, which have a very high current ratio of
30,000, are used for varikabo. To supply the motors and the LEDs with a
current of 0.03 A (ampere) = 30 mA (milliampere), a base current of only
1 µA (microampere) is necessary:
30 mA / 30,000 = 0.001 mA = 1 µA
A Darlington transistor consists of two cascaded transistors and needs
approximately UBE = 1.4 V instead of 0.7 V to switch through. So that the
motors will react obversely to the sensor signals, varikabo has a
complementary pair of transistors: a PNP transistor for T1 (BC516) and a
NPN transistor for T2 (BC517).
Transistor Functions
The figure at the top right shows a simple circuit with a
battery, motor, and NPN transistor. Under that is the
corresponding circuit with a PNP transistor. The black arrows
show the direction of the current: from positive to negative.
There are three basic circuits with a transistor. varikabo uses
the so-called common collector circuit. It‘s called a common
collector circuit, because the collector (C) is connected to a
constant voltage – the battery.
The red arrows show the voltages. As you can see, the voltage
on the motor UE is by UBE smaller than the base voltage UB.
UE = UB – UBE = UB – 1.4 V
Since the voltage on the emitter (E) responds to the
voltage on the base (B) - except for the difference of UBE,
this circuit is also called the emitter-follower.
So the voltage UE, and with that the speed of the motor, can
be controlled by UB and a very small current IB.
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