WARNING: this project involves the use and modification of laser devices. While the lasers I suggest using (store-bought red pointers) are relatively safe to handle, NEVER LOOK DIRECTLY INTO A LASER BEAM, BEWARE OF REFLECTIONS, and be EXTREMELY CAREFUL when MODIFYING a laser product. Also, I am not liable for anything stupid you do.

Here is yet another thing to do with those promotional laser pointers: send music (or data) from point A to point B over the laser beam using amplitude modulation. All it takes is pointing the moded laser at a detector, and music can be heard from an attached amplifier. The range and quality (or data speed) can vary, but I have gotten a HALF MILE of range with excellent audio quality and around 300bps of throughput.

The image shown here is the transmitter and receiver working across my desk during a test.

TO SEE HOW TO USE TWO LASERS TO TRANSMIT TWO MUSIC CHANNELS AND MIX THEM WITH A PAIR OF SUNGLASSES, check out the blog post here.

A video of the system working can be found here: http://video.google.com/videoplay?docid=6895048767032879458&hl=en

Much inspiration for this project came from http://sci-toys.com/scitoys/scitoys/light/light.html#laser_communicator

Step 1: Gather Materials

To send music over a laser beam you will need the following parts, most of which can be gotten for less than 5 dollars at radioshack total (besides the pointer, which probably costs $15). If you are on a tight budget, try replacing the laser with a red LED and a 100ohm resistor attached in series.

for the transmitter:
a laser pointer
batteries (D-cell work best)
potentiometer (variable resistor) 50k ohm or less
audio source (iPod, cd player, mic preamp, PC line-out, etc.)
some wire (cat5 aka ethernet cabling works best)
toggle switch (a turbo switch from an old PC works well)
audio transformer (can be pulled from audio equipment)
1/8" audio jack (can be gotten from the end of a headphones cable)

for the reciever:
phototransitor (photodiodes or IR detectors also work)
1/8" audio jack
some more wire
high-gain amplifier (laptop with mic input, or mic preamp plus amplifier)
magnifying glass (helps at great distances)

tools:
wire cutter/stripper
soldering iron and electronics solder
tape (clear and/or electrical)
digital multimeter (can be useful... not realy required)
tripod (helps for aiming laser at a distance)
empty pizza boxes with white backs (for finding beam and for adjustments)
some assistants

Step 2: Hack the Laser

First the laser pointer needs to be modified. Remove all bateries and total up the voltage of the batteries to find the voltage required by the laser. For instance, mine takes two AAA batteries, so that is 2 x 1.5 or 3 volts. Now solder wires on to the positive and negative terminals inside the laser. This might require cutting open the case a bit (a dremmel is sometimes nessisary).

Next, figure out how to hold down the button on the pointer that makes it light. A shaved-down pencil eraser and a ruberband works for me.

Now test the modified laser by connecting batteries of appropriate voltage to the newly attached wires. If it does't work, try connecting them in the opposite direction. Laser pointers use laser diodes which only take current in one direction.

This mod will allow us to control the brightness of the laser by varying the voltage and current supplied to it.

In the photo below you can see my pointer, with two D-cell batteries attached.

Step 3: Create Transmitter Circuitry

Use the schematic below as a guide in soldering togeather the transmitter circuit. Everything to the left of the laser is the transmitter circuit.

Tape or glue the compents down to a piece of cardboard, or use a breadboard.
Check out the photo of my finished board. I used an 1/8th inch female jack to make connections easier.

To test the circuit, turn up the iPod volume to MAX, play some music with alot of bass, and turn the potentiometer all the way down. The laser dot should appear to pulsate with the music, since this is an amplitude modulated (AM) circuit.

Step 4: Setup the Receiver

Solder long leads onto the phototransistor (or photodiode). Attach these to an 1/8th inch audio jack (a headphones cable is perfect). Plug this into the MIC port on a laptop or PC or other MIC preamp/amp and turn up the gain and volume to a moderate level. Try to mount the whole setup (with room for a magnifying glass) on a sturdy but portable material (like a wooden board).

For this project the GAIN of the amplifier is critical. It must be very high to pick-up slight variations in input signal (light) to pull out the music. Therefore I build a breadboard preamp from RadioShack's 50-in-1 Sensor Lab breadboard kit, which I highly recommend. Check out the photos and schematics from the included book.

Step 5: Give It a Try

Point the laser beam at the photodiode, hit play on the iPod, and listen to any and all noises comming from the amp. Play with the iPod volume and potentiometer positions until music can be heard clearly and without distortion on the recieving side. Then turn up the reciever gain as nessisary.

Try mounting the laser on a tripod and sending music a longer distance. I recently was able to hear Starway to Heaven clearly at a beam distance of a half mile. This was done over a small lake, with an assistant in a canue with a pizza box to help with the aiming.

Step 6: How Does That Work? and Where Do I Go From Here?

This circuit works using amplitude modulation, exactly like AM radio, except using a visible light wavelength instead of a radio frequency. The audio signal leaves the iPod as a varying voltage which forces a varying current through the laser. Then the laser's varying brightness conveys the musical information. Finaly, the phototransistor varies in resistance as the brightness on it changes. The mic amp applies a small voltage to the phototransistor and amplifies the resulting current.

A problem with this system is that at each step there is a non-linear transfer function, that is, there is distortion that occurs because the brightness changes are not always proportional to the change in voltage applied. See the screenshot below for an example, and listen to the attached audio sample.

The next step in this project would be to use pulses (like fast, computer operated morse code) to convey digital information like text, crystal-clear audio, or even video. One could even network computers with laser beams in a manner similar to fiberoptics but in open air. I will post C code for my transmitting and recieving programs.