Fig. C: Transmitter ready for testing with a radio.
Fig. D: Simplest receiver: just an earphone and
a solar cell.
Fig. E: The photocell (i.e. photoresistor) uses battery
power. Fig. F: The working transmitter is fed by
Photography by Ed Troxell (A, C and F) and Simon Quellen Field (D and E)
cell or photoresistor. You may hear some clicks or
pops. Now turn on the radio and adjust its volume
until you hear it across the room. If you don’t hear it,
try increasing the amp’s volume before you turn up
the radio. If you pull out the earphone plug, the radio
should be just audible.
Depending on your signal source, you also might
want to reverse the transformer. Some devices,
like iPods, don’t have enough power to drive 8Ω
speakers, so you should connect them across the
1kΩ side. This arrangement will dim the laser, but
won’t affect its range much.
When you can hear the radio, break the laser
SETUP AND TESTING beam with your hand, and notice that the music
We’ll test the system by transmitting a radio signal stops. Try chopping up the audio with your fingers.
and amplifying the receiver so we can hear it across The system is ready. To send secret voice
the room. First, replace the earphone of your receiver communications, move the amp from the receiver
with an audio plug (or just clip the audio plug to to the transmitter and plug in a mic. You’re ready for
the earphone wires), and plug it into an amplifier the field; just be careful with the volume, to protect
or stereo. the laser.
With the radio off, plug in the transmitter. Turn up
the volume on the amplifier until you hear a hiss,
then turn it down until it isn’t noticeable.
Aim the laser across the room so it hits the solar
soldering, which can be difficult on solar cells
For a sturdier and cheaper alternative, use a cadmium sulfide (CdS) photoresistor, which changes
its resistance proportional to the amount of light
hitting it. Paired with a battery, this acts like a solar
cell. To make the receiver, connect the earphone
and a 9V battery across the photoresistor, so that
battery, earphone, and photoresistor are all parallel.
Add a 220Ω resistor in series with the battery to
reduce power consumption and prevent heating of
the photoresistor (Figure E).
Simon Quellen Field (
firstname.lastname@example.org) is the president
and CEO of Kinetic MicroScience (
scitoys.com), and the
author of several books on science and computing.