Simple Laser Communicator

• Remove the batteries. Following the schematic, connect the bicolor LED across the 1,000-ohm (1kΩ) side of the transformer. With the transformer shown here, this means connecting it to the green and blue wires. We don’t need the tap wire (the black wire in between).
• The LED 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.
• Since we're using a bicolor LED, we don't have to worry about polarity and can connect it either way.
• Connect the same two transformer leads in-line between the battery pack and the laser.

• Connect two leads to the earphone plug. Unscrew the cover, solder leads to the plug terminals, and replace the cover.
• Connect the earphone plug to the 8Ω side of the transformer using alligator clips.
• That’s it! We have a laser transmitter, in just a few minutes!
• Plug the headphone plug into a transistor radio, which will be our signal source for testing.

• For the receiver, connect a 9V battery across a cadmium sulfide (CdS) photoresistor and an earphone, so that battery, earphone, and photoresistor are all parallel.
• The photocell changes its resistance proportional to the amount of light hitting it. Paired with a battery, this acts like a solar cell.
• Add a 220Ω resistor in series with the battery to reduce power consumption and prevent heating of the photoresistor.

• We’ll test the system by transmitting a radio signal and amplifying the receiver so we can hear it across the room. First, replace the earphone of your receiver with an audio plug (or just clip the audio plug to the earphone wires), and plug it into an amp or stereo. Here we use a mini portable amplifier.
• 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.
• With the lights low, aim the laser across the room so it hits the solar 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 beam with your hand, and notice that the music stops. Try chopping up the audio with your fingers.

To send secret voice communications, move the amp from the receiver to the transmitter and plug in a mic. You’re ready for the field; just be careful with the volume, to protect the laser.