Step 1: The Parts
You will need:
-A CdS Photocell
-Some battery holders with battery clips if needed (one for the audio-to-light modulator [transmitter], one for the receiver, and one for a microphone (optional))
-An Audio Output Transformer (RadioShack Catalog No. 273-1380)
-Some form of amplifier (I used a battery-powered one from RadioShack (Cat. No. 277-1008))
-Some MONO (NOT stereo) audio jacks (Three will do)
-Some form of LED, Preferably a bright one
-A container to hold all the doo-dads in (for the transmitter). I just used what I had: a pill container. You want to keep all the circuitry concealed in some form of container, because if you touch the circuitry, I found out it gives you a small unpleasant shock (because of the transformer?), and it disrupts the signal to the LED, therefore giving you a bad signal.
-Some hook-up wire
The Pictures below are what I had to buy (except for the mics). I salvaged the rest.
Step 2: The Schematics
These schematics were taken straight from the website I cited.
Step 3: Assembling the Transmitter
The assembled circuit should look something like this. Be sure not to bother with the black lead from the transformer; it is not used in this circuit. The cap of the pill bottle (to the right) has two holes drilled in it for the LED, which I just got from a solar patio light. Make all the connections with alligator clips FIRST to test it, then solder it all together when you know it works properly.
Step 4: The Receiver
The receiver is so simple, a monkey could do it. All you have to do is solder the CdS photocell in series with the battery pack (3 volts), and then solder the two resulting leads to another mono jack. Use the biggest photocell you can find. Also note that it does not matter which way you connect the photocell; it has not plus (anode) or minus (cathode) sides.
Step 5: The Microphone
The Microphone is also extremely easy to hook up. Connect it the same as you did with the receiver. You can find these microphones in cordless phones, answering machines, etc. The one I used came from a broken cordless phone. Once again, the batteries are just to give the microphone some power so that there will be some signal to the amplifier.
Step 6: Using It
To use it, just plug the transmitter into any audio source (CD player, radio, etc.), and turn it on! It will modulate the sound signals into light, and the receiver (plugged into an amp) will turn it back into sound! If you want to modulate your voice, plug the microphone into the input on an amp, and the transmitter to the output (to external speaker) jack. The final product may look a little like the below picture.