SPY TECH
MATERIALS
Laser pointer You can get one for $10 from my
Scitoys Catalog,
scitoyscatalog.com.
Batteries, 1.5V, any size Get the same number
of batteries and voltage that the laser takes,
typically 3x 1.5V.
Battery holder for the 1.5V batteries If you can’t
find a 3-battery holder, wire a 1-cell and a 2-cell
holder in series, or use a 4-cell holder and bridge
1 compartment with wire.
Stiff wire or rubber band
Audio output transformer, 1kΩ primary coil,
8Ω secondary RadioShack part #273-1380
or Scitoys Catalog #XFORMR
Alligator clip leads ( 2–10) with points fine enough
to connect to the inside of the laser pointer. You
can substitute wire and solder, but the clip leads
are easier. RadioShack #278-1156 will do nicely.
2-lead bicolor LED to protect the laser from voltage
spikes if it doesn’t have built-in protection.
Hookup wire
Mini portable amplifier such as RadioShack #277-
1008. Alternately, you can use a stereo system.
Microphone with cable and plug that fits amplifier
or stereo input
A
B
1kΩ
8Ω
4.5V
RECEIVER OPTION #1: SOLAR CELL WITH
EARPHONE (SIMPLEST)
Small solar cell Scitoys #3SOLARCELLS or
RadioShack #276-124
Piezoelectric earphone Scitoys #EARPHONE
Transparent tape
Fig. A: The laser pointer with its button taped down
to the always-on position.
Fig. B: The transmitter wiring diagram.
RECEIVER OPTION #2: PHOTOCELL WITH
EARPHONE (CHEAPER AND STURDIER)
Piezoelectric earphone Scitoys #EARPHONE
9V battery and battery clip
Photoresistor, cadmium sulfide (CdS)
RadioShack #276-1657
220Ω resistor
contacts conveniently marked with a plus and a
minus. If it doesn’t light, try reversing the power;
this won’t harm the laser.
Figure out how to hold the laser’s button down
with a rubber band, wire, or tape (Figure A).
TOOLS Remove the batteries. Following the schematic in
Figure B, connect the 1,000-ohm (1kΩ) side of the
transformer in-line between the battery pack and
the laser, using the outside 2 wires of the transformer.
We don’t need the center tap wire.
Connect the bicolor LED between the same
2 wires of the transformer. This protects the laser
from high voltage spikes, since so many cheap
lasers nowadays have no onboard protection circuit.
If you see the LED flash, that indicates a spike.
ASSEMBLE THE TRANSMITTER Now connect the earphone plug to the 8Ω side
NOTE: I recommend soldering this project, but initially of the transformer with alligator clips. That’s it! We
it’s easier to make it and test it using alligator clip leads. have a laser transmitter, in just a few minutes! We’ll
Remove the batteries from the laser. Connect the plug it into a transistor radio for testing (Figure C).
external battery pack to the laser’s power contacts
with 2 alligator clips; usually you’ll connect one ASSEMBLE A RECEIVER
lead to the battery case and the other to the spring For the simplest receiver, connect the 2 leads from
inside. Some laser pointers are easy to disassemble; a piezoelectric earphone to a small solar cell. You
you can remove the circuit board and see the power can attach them using transparent tape instead of
Illustration by Julian Honoré/ p4rse.com
Soldering equipment
Transistor radio
Earphone plug to fit your radio, such as
RadioShack #42-2434
Phono/mic plug to fit the phono/mic input jack
of your mini amplifier (or stereo), such as
RadioShack #42-2434 or #42-2457
68 Make: Volume 16
