Music is stored on my mp3 player as magnetic patterns that are converted into vibrations of electrical current. In my headphone coils, these vibrations, in turn, create a vibrating magnetic field. Since there is a permanent magnet inside the coil, the magnetic interaction causes a physical movement creating sound waves (vibrating air).

I thought I'd add another step to this beautiful process by sending the electrical output of my mp3 player to a laser pointer and having the headphones pick up the signal via a light dependent resistor. "Why?" you may ask. "What is the practical implication?" "Why not?" I say, and "none as far as I can see." On the other hand, it is very cool to see (and hear) it work.

Step 1: Supplies

Here's what you'll need to recreate what I did:
* 1 cheap laser pointer
* 2 AA batteries
* 1 9V battery
* 1 battery holder for the AA batteries
* 1 battery holder for the 9V battery
* a few pieces of stiff insulated wire, each a few inches long
* 1 pair of cheap headphones
* 1 source of music, like an mp3 player
* 1 standard light dependent resistor (the small kind)
* 1 cheap electrical switch
* 1 soldering iron
* a bit of solder and aluminum foil
* some tape.

Step 2: Hack the Laser Pointer

The first thing to do is to remove the batteries from the laser pointer. You could probably use the original little button cell batteries, but I found them annoying to work with. Besides, it seems like replacing them costs more than buying a new laser pointer.

Once the batteries are out, you should see a spring. That spring is connected to the positive lead of the laser diode. The laser diode's negative lead is wired to the metal outer casing of the pointer itself. This is important, because like an LED, a laser diode has a polarity; if you wire the power up the wrong way, nothing will happen.

I power the laser with 2 AA batteries in a battery holder. Power will go from the positive wire of the battery holder, through the headphone cable, to the metal casing of the pointer. I'll describe that part of the circuit more in the next step.

The negative wire from the battery holder must be connected to the spring inside the laser pointer. Without cutting the laser pointer open somehow or stretching out the spring, it would be tricky to solder anything in there. In order to accomplish this contact, I used a piece of very stiff insulated copper wire. I bent a piece into the shape of a hook which I could just squeeze into the center of the spring coil (see image below). That way, there is pressure maintaining a good contact there.

With the other end of that wire sticking out the back of the laser pointer, just bend it around the side and back toward the front. Then put a final bend in the end to make it stick out a bit (The broken red line in the photo indicates the path of the wire.). That way, it's easy to solder it to the battery holder wire, which you can do now.

Finally, use a piece of tape to hold down the laser's on button so that it's permanently on. You should be able to test your circuit by temporarily making the contact between the positive wire from the battery holder and the outer casing of the laser pointer. The laser should light up.

As we all know, lasers are not so good for your retinas. Please be careful, and don't aim the beam into anyone's eyes.

Step 3: Hack the Headphones

The 3 Volts from the AA batteries should be enough to light up the laser. Now, the signal from the mp3 player is going to ride on top of that battery power and modulate the brightness of the laser.

Get a pair of super-cheap headphones and cut the cord at about the midway point; we'll be using both halves. Put the end with the earbuds aside for the next step. Right now, we will work with the other end, where the signal from the mp3 player will be coming in. The cable has two strands, one for each earbud. We have the option to work with one of the strands or both. This time, for simplicity, I chose to just deal with one of them. So, peel the other one back and cut it short to keep it out of the way.

Then strip the black rubber insulation off the strand you're working with. There should be 2 very thin wires and probably some fluffy stuff. You can cut back the fluffy stuff if it's in the way. The two wires are probably either red and copper or blue and copper. The copper one is the negative or ground wire, and the red or blue one is probably the positive wire. These usually have an invisible layer of resin insulation on them that you can burn off with a lighter. There should be a quick puff of flame that then goes out once the resin is gone. Once you do this, the colors of the wires may be hard to discern, so try to leave a little unburned as a reference.

The negative (copper) wire will get soldered to the positive wire from the battery holder, and the positive (red or blue) wire will get attached to the outer casing of the pointer. Unfortunately, my laser pointer is cased in aluminum, and I don't know how to solder to aluminum. So, instead of solder, I used aluminum foil. Wrap a small piece of foil around the wire and press it tight. Then lay that foil flat against the laser body and tape it on.

The laser should not light up as long as your audio cable is not plugged into anything. This is because the circuit is open; the little metallic rings at the plug end are separated by insulating rings. Plug into a powered-up mp3 player and the laser should turn on. Play a song and you should notice fluctuations in the brightness. Isn't this cool!?!?!

Step 4: Buld the Receiver Circuit

In order to turn that laser light back into sound, we will use the light-dependent resistor (LDR) as a sensor. If we apply a voltage across the LDR, a current will flow through it. Since changing the illumination of an LDR changes its resistance value, it should also change the current. If this current is fed to a pair of earbuds, we should be able to hear the result.

The circuit is a simple loop:
* First, current flows from the + battery holder wire (red) to the LDR.
* It then passes through the LDR, and out the other side to headphones ground.
* Next, the current flows through the headphones, to headphones +, and to switch terminal 1.
(I actually wired the headphones backwards, but it doesn't matter, since they are not polarized.)
* Finally, the current will (if the switch is on) flow to the - battery holder wire (black).

Since we leave switch terminal 3 empty, the circuit is broken when the switch is down as shown in the image below. When the switch is up, terminals 1 and 2 are connected. Without some sort of switch, there would always be current flowing and the battery would steadily die out.

The only really tricky bit is the headphone wires. As I described in Arduino Optical Theremin, you can use both earbuds if you pair the wires correctly (see photo below). Again, all these wires need the lighter treatment to burn off the resin insulation.

Again, my soldering is primitive, but it gets the job done. As you can see, I packed everything up into a little box so that it would be easy to carry around. The beam is small and so it's good for the receiver to be portable so you can catch it. The little plastic cover helps keep the battery from spilling out.

Step 5: Enjoy

Time to test out this wonderful madness!

Start up your favorite song on the mp3 player, stick the earbuds in your ears, and hold the receiver so the LDR catches the laser beam. You should be able to recognize the song.

Check out the video below for a demo (I taped the earbuds to the camera mic.). Unfortunately, there's a lot of background noise as there is construction on my street.

Pros:
* Sheer awesomeness: your music is playing through a laser beam!
* Range: coherent light travels far. I have tested my device at a range of about 15 meters, but I bet it could work at longer ranges, especially with a more powerful laser and/or with more/better LDRs,
* Cost: I think the whole deal was 10 bucks or under.

Cons:
* Unidirectionality: It's not very practical to have to hold your receiver in a particular spot to hear the music
* Signal to noise ratio: The signal is weak, even with the mp3 player at full volume. Also, there is a lot of distortion. Probably there are many improvements that can be made to help with these issues, but I think LDRs are inherently noisy devices.

Please feel free to suggest improvements or modifications. I have crazy dreams of building one with a fat green beam-visible laser and hacking into a keyboard so that the sound could travel, visibly, to an amp some distance away.