- How would you like to talk over a laser beam? In about 15
minutes you can set up your own laser communication system, using
cheap laser pen pointers and a few parts from Radio Shack. The
obvious uses for this is as an undetectable phone or room bug, as
there would be no betraying RF signal.
For the transmitter you will need:
For the receiver you will need:
- A laser pen pointer. You can get one for $10
- A battery holder that holds the same number of batteries as
the laser pointer (often 3 cells). The batteries can be any size,
but they must be the same voltage as the laser batteries. You may
need to get one that holds two cells, and another that holds one
cell, and wire them together in series. Radio Shack has a decent
- A transistor radio. Later we will use a microphone and an
amplifier (Radio Shack #33-1067 and #277-1008), but at first we
will send your favorite radio station over the laser beam.
- An earphone jack that will fit your transistor radio (Radio
- A transformer of the type known as an audio output
transformer. It consists of an 8 ohm coil and a 1000 ohm coil. The
one I used is the Radio Shack #273-1380.
- Some clip leads (wires with alligator clips on the ends) to
put it all together. At least one of the clip leads should be the
type with a long slender point (Radio Shack #270-334), to connect
to the inside of the laser pointer. You can substitute regular
wire and solder if you like, but the clip leads are fast and
simple. Radio Shack has a wide selection of clip leads (such as
- A small solar cell (such as Radio Shack #276-124). You may
have to solder your own wires to it if it doesn't come with wires
- A microphone jack that will fit the phono input of your stereo
(Radio Shack #42-2434 or ##42-2457). Instead of a stereo, you can
use the small amplifiers that Radio Shack sells (#277-1008).
It may be hard to find a battery holder that holds
three batteries. You can use two battery holders (one that holds two
batteries, and one that holds a single battery) and connect them in
Remove any batteries from the laser.
Connect a clip lead to the inside of the laser pointer
where the battery touched. Usually there is a small spring to which
you can attach the clip lead. The other end of the battery usually
connects to the case of the laser. Since there are many different
styles of laser pointer, you may have to experiment with clip lead
placement to get the laser to work with the new external battery
pack. You may also have to hold down the laser's push button switch
by wrapping a rubber band or some wire around it. Test the
connection before you attach the transformer, to make sure the laser
works with the new battery pack. If it doesn't light, try reversing
the battery. Battery reversal will not harm the laser.
Connect the 1,000 ohm side of the transformer between
the battery and the laser. The 1,000 ohm side of the transformer has
three wires coming from it. We only use the outside two wires. The
inside wire is called a center tap and we do not use it in
this circuit. Test the laser by attaching the battery. The laser
should operate normally at this point.
Connect the earphone jack to the 8 ohm side of the
transformer. The schematic of the transmitter looks like this:
The transformer modulates the power going to
the laser. The signal from the radio is added to and subtracted from
the battery power, and the laser gets brighter and dimmer along with
the volume of the music or voice in the signal.
The receiver is the simplest part. You simply connect
the solar cell to the microphone jack, and plug it into the
amplifier or stereo phono input. It does not matter which way the
wires are connected to the solar cell.
Here is the schematic of the receiver:
Setup and testing
Make sure the transistor radio is turned off, and the
laser is on. Plug the earphone jack of the laser into the earphone
socket of the radio.
Connect the solar cell to the amplifier or stereo, and
turn the volume up until you hear a hissing noise, then turn it down
slightly until the hiss isn't noticeable. The volume control should
be fairly high, corresponding to an ear splitting level if it was
Aim the laser across the room so it hits the solar
cell. You might hear clicks or pops coming from the stereo or
amplifier as the laser beam passes over the solar cell. This
indicates that everything is working fine at this point.
Now carefully turn on the radio and slowly adjust the
volume until you hear the radio station voices or music coming from
the amplifier across the room. The radio should be just audible if
the earphone jack is pulled out, not loud. If you can't hear the
sound from the amplifier across the room, make sure the laser is
shining on the solar cell, then try increasing the volume of the
amplifier before you increase the volume of the radio.
At this point you should be hearing the radio station
coming in loud and clear in the amplifier across the room. Put your
hand in front of the laser beam to break the connection, and notice
that the music stops. Wiggle your fingers in the beam and listen to
the music get chopped up by your fingers. Your laser communicator is
ready for the next step.
To send your voice over the laser beam, you simply
replace the transistor radio with a microphone and amplifier. Radio
Shack sells small amplifiers (about the same size as the transistor
radio) that have sockets for microphones and earphones. You can also
use another stereo system, but be very careful with the volume
control to prevent damage to the laser.
Using a disassembled laser pointer.
For this project we have removed the laser assembly
from a small $10.00 laser pointer. The power supply circuit is the
green board attached to the brass laser head.
The power supply circuit came conveniently marked with
a plus and a minus next to two holes in the board. We solder the
black negative lead from the battery clip to the hole marked minus.
We solder one of the 1000 ohm coil leads to the hole marked plus. We
solder the red positive lead of the battery clip to the other lead
from the 1000 ohm coil.
The battery clip is attached to a 4.5 volt battery
pack (not a 9 volt battery!). Since I didn't have a pack that
takes 3 cells, I used one that takes 4 AA batteries, and I replaced
one of the four batteries with a straight piece of bare wire.
That's it! We have a laser transmitter, in just a few
A new receiver
The solar cell receiver has some drawbacks. It is
expensive (solar cells are a few dollars each), and fragile.
A cheaper, sturdier alternative is to use a cadmium
sulphide photoresistor instead of the silicon photocell.
A cadmium sulphide photoresistor is shown below
(magnified many times). It does not produce electricity from light
the way the solar cell did. Instead, the light that falls on it
changes its resistance to electricity.
If we connect a battery and a photoresistor together,
they can act like the solar cell. As the intensity of the light
changes, the amount of electricity output changes in response.
The new receiver is very simple, and looks like
Super simple receivers
Using a super sensitive piezoelectric earphone you can
make a laser voice receiver that doesn't need any expensive
amplifiers or power source. Just connect it to a small solar cell,
as shown below:
If a solar cell is too expensive or fragile, a
cadmium-sulfide photoresistor can also be used. The earphone wires
are connected across the photoresistor, and the battery is also
connected across the same wires. The battery, the earphone, and the
photoresistor are in parallel. A 220 ohm resistor is placed in
series with the battery, to reduce power consumption, and prevent
heating of the photoresistor.
Either of these earphone approaches has the nice
feature of making the communication private. Only you can hear what
is coming over the secret laser link.
How does it work?
In all of the laser communicators on this page, the
laser light is amplitude modulated. This simply means that
the amount of light the laser emits varies over time.
To understand what is going on, it helps to consider
how a loudspeaker makes sound. A loudspeaker is a paper cone
attached to a coil of wire that sits in a magnetic field from a
strong permanent magnet.
When an electric current flows in the loudspeaker
coil, the coil becomes an electromagnet, and it moves toward or away
from the permanent magnet. As it moves, the paper cone pushes on the
air around it, compressing the air in front of it, and expanding the
air behind it. Waves of compressed and expanded air travel to your
ear, and cause your eardrum to move in time to the movements of the
The laser communicator adds two components to the
loudspeaker concept. We take the electrical signal that goes to the
loudspeaker, and connect it instead to the laser, so the laser gets
brighter and dimmer as the electric current varies. The second
component is the receiver, which converts the light back into an
electric current. This current varies in time with the first
current, because the amount of light that it receives is varying in
This second electric current is used to move the paper
cone of a loudspeaker, just as before. However, now the loudspeaker
can be quite a distance away from the original electric current,
without any wires connecting the two.