Sound Driven Laser Show

This instructable will show you how to build a laser light show that moves and pulses with the music. Instead of making a circle or series of circles, this creates a chaotic pattern. Here’s the really short version:

1. Take apart a CD or DVD drive.
2. Remove laser focuser.
3. Glue small mirror to focuser.
4. Attach speaker wires to the focuser coils.
5. Bounce a laser beam off the mirror.
6. Pump up the volume.

The beating heart of this device is the laser focuser* out of a CD or DVD drive. When it’s doing its mundane job in a CD player, it moves up, down, left, or right to keep the laser focused on the surface of the disc. The lenses are mounted on a set of very thin flexible wires that require very little force to move. The two electromagnets are powered by a DC voltage and are attracted or repelled by two permanent magnets. For this device, we’re going to use the speaker output from a stereo to provide voltage to the drive the electromagnets. A speaker uses a coil and a magnet, so it’s the same sort of concept.

WARNING: If the impedance of the coil is too low, it could cause damage to your stereo amplifier. I’ve had this run for hours at a party with no ill effects to my equipment. I can’t guarantee the same will happen for you. I’ve measured a few of them and they ranged from 2.5 ohms to 6 ohms. Home speakers are usually 8 ohms, car speakers are usually 4 ohms. If you connect to both coils in parallel, the impedance will be cut in half.

This video shows the device in action.  Since it's dark, it's hard to see the scale, but it was projected about 25' across the shop.  The pattern was about 3' across for the first part, and about 10' across for the second part with higher volume.

*I don’t know if that is the correct name for that part, but that’s what I’m calling it. See Mitch Hedberg’s Appliance Naming Institute for more on that.

An old CD or DVD drive.  If you're making a burning laser from a DVD or BluRay drive, this makes good use of the rest of the parts. (From here out, I’m only going to refer to CD drives. CD and DVD drives are interchangeable for the purpose of this instructable.)

A laser.

Screw drivers, soldering iron, hand tools, etc.

Where to start:
On the CD drive, remove every screw you see. That will usually let the cover come off. There may still be some clips holding it on. A little prying will usually defeat them and allow access. Keep going until you can see the focuser. I would recommend leaving it in its housing and on the carriage. If you don’t, you’ll have to mount it in something.

A word of caution, the magnets are powerful, and will pick up every little metal chip or shaving, rendering it useless for our purposes. If this happens, all is not lost, you can still harvest the neodymium magnets for another project.

Next you need to locate the electrical connections. They are usually four solder points on the side of the focuser. You’ll know you have it if the lens bounces when a low DC voltage is applied. One or two batteries or an AC adapter should be enough. I wouldn’t go over 3 volts. One set of connections will make it bounce up and down, the other side to side. I used the set that makes it bounce up and down, but you could try the side to side ones as well. I doubt it would make much of a difference. I wouldn’t connect to both at the same time as it might reduce the impedance too much. Solder your wires to the two connection points. These wires will connect to the speaker connections on your stereo.

Any piece of mirror will work, but the CD player has some little mirrors built into it. They’re usually glued into place, but pop out with a little careful prying.

Glue the mirror onto the top of the focuser, being careful not to glue the focuser to itself.

The laser needs to be mounted so it shines on the mirror. I used a piece of steel with a hole bored in the center and welded that to a piece of TIG rod. If you don’t happen to have a lathe and welder laying around, you could use a hose clamp and piece of coat hanger or wire. The goal is to have a mount that is stiff enough to hold the laser in place, but can also be adjusted. I originally had a red laser that ran off a transformer, but for this one I’ve switched over to the green machine. You could also do this with multiple lasers bounced off the same mirror.

Take the wires from the focuser coil and connect them to the speaker connections on your stereo. I’ve used my old school big honkin’ stereo amplifier (maybe 100w) as well as a bookshelf stereo (maybe 10 watts). Whatever you use needs to have enough wattage and enough low end to drive the coil. I tried to run it  directly off  the headphone jack of the computer, but it wouldn't move it enough. 

Anything over 200hz or so doesn’t bounce it enough to make the pretty patterns. As you’d expect, the higher the volume, the more the mirror will move. The more it moves, the bigger the pattern.  Turning up the bass will help with the movement too.  I used mp3's played through a laptop.  Since some mp3's don't have as much bass, you might do better with FLAC files or better yet, a real CD or LP.

I used a tone generator to run a steady 100hz tone into the speakers.  It made a perfect oval that increased in size when the volume was increased.  See the end of the video for a demonstration of that.