1
Laser Communication Link Assembly
By Eric J. Stanton, Victoria Rosborough and Takako Hirokawa
This activity leads students through the construction of a very simple circuit which will
transmit audio (from a smart phone, for example) using a laser pointer. The laser
signal is then detected and played back through headphones on a simple photodetector circuit.
This activity is based on Simon Field’s plans at sci-toys.com: Simple laser communicator
Materials:
-
8 wires with alligator clips
1 1/8th-inch audio jack
1 audio transformer
1 battery holder
-
Triple LR44-battery Holder
Post-it notes
1 photoresistor
Tape
(Find part numbers and links to purchase in the last section)
Step 1: Disassemble laser & LED package
- Unwrap laser pointer and set batteries aside
- Pull off the front piece (you may need to remove the sticker and pull hard!)
- Remove the laser/LED board
Step 2: Assemble battery pack
-
Stack the laser pointer batteries and place them in the
battery pack (you may need to use some folded post-it
notes to make everything snug)
-
Using an alligator clip, connect the positive (+) end of
the battery to the exposed LED wire (you may need to
adjust the red sleeve on the other wire and the alligator
clip sleeve to make sure the clip does not make any
unwanted contacts)
-
Now connect the negative (-) end of the battery to the
spring
-
While pointing the laser at the table, push the buttons
on the laser/LED board to check that they both light up
-
Disconnect the alligator clip from the negative side of
-
+
2
the laser (the spring)
Step 3: Connect the transmitter circuit
-
Connect one of the audio cable wires to pin 4 and the other to pin 6 on the secondary side of the audio
transformer (the primary side has a “P” printed on it)
P
-
Connect the negative side of the battery pack to pin 1 on the audio transformer
-
Connect the negative side of the laser to pin 3
-
Test your laser and LED to make sure they still turn on
Step 4: Connect the receiver circuit
-
Connect one of the photoresistor pins to the negative side of a AA battery (use the tape to secure the
clip to the battery)
+
-
-
Connect the other photoresistor pin to the negative contact of the earphone jack
-
Connect the positive side of the earphone jack to the positive end of the AA battery
Step 5: Initial test of the optical communication system!
-
Plug in the audio jack into your music player
-
Aim the laser or LED at the photoresistor
-
If you can hear the music, then you successfully assembled your communication link!
3
-
If you cannot hear the music, recheck all of your connections (it may be helpful to tape down some
components to the table or to secure and isolate some connections)
Try these experiments:
Laser vs. LED
-
What is the longest link you can make using the LED? What about the laser?
Which transmitter gives a clearer sound? Why? Try waving your hand through the beam path.
Which transmitter (laser or LED) has the best alignment tolerance with the receiver? i.e. how far
can you move the laser and LED to the side before you lose signal?
What happens when you place a polarizer between the laser or LED and receiver and slowly
rotate it through 360 degrees?
Does transmitting outdoors affect the signal from the laser or LED?
Fiber guiding
-
Try sending the signal through fiber #1 (200 µm glass core with plastic cladding). Is the signal
affected?
What happens when you bend fiber #1? (Don’t bend too sharply or the fiber will break!)
Repeat the above steps with fiber #2 (800 µm diameter plastic cable). How does it compare?
Can you see any light escaping the circumference of the fiber? Is it easier to couple light into
fiber #1 or #2?
Cut fiber #2 in half. Does the signal change when you use a shorter piece?
Use sandpaper to slightly roughen one of the fiber halves. What do you observe? How is the
signal affected?
Since this fiber is plastic, it will expand when heated. Try heating one or both ends (see
instructor). What happens to the signal and alignment tolerance?
Try adding a 90° bend to the fiber by heating it up
Design your own experiment!
This activity was funded by and held in classes run by the School of Scientific
Thought (SST), a program of the Center for Science and Engineering Partnerships
at the Univ. of California Santa Barbara. SST classes hosted high school students
at the University, in once-a-week classes designed and led by UCSB graduate
students. The IEEE Photonics Society student chapter at UCSB designed this lightbased activities for SST, with funding from AIM-Photonics and the IEEE Photonics
Society.
4
Parts Shopping List
•
•
•
•
•
•
•
•
•
•
•
Laser Pointers
o Scitoys.com "Four Hologram Laser Pointer", or
o Dollartree.com "2-in-1 Laser Pointer Key Chains". $1 ea.
8 wires with alligator clips:
o newegg.com “10 pcs 5-Color Double Ended Alligator Clips Test Lead Jumper Wire 48cm 1.6
Ft”. $2 ea., or
o Radio Shack #278-016, #270-372, or #270-334
1 1/8th-inch audio jack
o Radio Shack part #42-2434 or #42-2457, or
o Firgellirobots.com “Through-hole 3F07 Audio Jack Socket- Hole Dia.: 3.5mm”. $0.30 ea.
1/8” audio cable
o mycablemart.com “1.5FT 3.5MM MONO TS (2 CONDUCTOR) MALE TO MALE AUDIO
CABLE”. $1 ea.
1 audio transformer (8Ω to 1000Ω conversion)
o Radio Shack #273-1380, or
o scitoys.com “Audio Transformer”, or
o mouser.com “Xicon Part# 42TL013-RC”, $2 ea.
1 battery holder for three LR44 button batteries
o firgellirobots.com “Single Compartment AG13 LR44 Button Battery Holder with Switch”,
$1.25 ea.
1 photoresistor (CdS, approx. 10kΩ to 200kΩ range)
o mouser.com “Adafruit 161”, Part# 485-161
Optical Fiber – (Multimode)
o Thorlabs.com “FP200URT - 0.50 NA, Ø200 µm Core Multimode Fiber, High OH”
Post-it notes or any small pieces of paper
1 AA battery
Tape
Download PDF