This is a home laser show I built with a 35mW green lab laser. The laser bounces off a mirror attached to a speaker, then goes through a diffraction grating so the patterns will cover my entire ceiling. It's pretty sweet.
You'll need a green lab style laser module, a pair of old headphones, a box, a switch, an AC/DC adapter, and some hand tools.
Here's a link to more detailed instructions with a full parts list.
You can see some videos of it in action here and here.
Step 1: Power Supply and Control
The first thing we want to do is provide power to the laser itself, and provide any easy method to turn it on and off. The power is coming from the AC/DC adapter, but it ends in one of those plugs we're all so familiar with. What we need to do is wire a receptacle (which connects to our power adapter plug), to a switch (which allows us to turn power on and off), which then goes to the laser driver board itself.
Don't connect anything to the laser yet! Why? We need to mount the switch and the receptacle in the box, and you'll quickly find that you need to pass the wires through the little hole in order to mount the switch. The laser, needless to say, will have a bit of trouble fitting. Put everything into place, and apply glue to the inside to ensure everything stays put. You don't need to do this, but it doesn't really hurt.
Now we can solder those leads to the laser, and fix the laser driver board to a part of the box. If you look at the board there should be a big piece of metal attached to a component, that's the heatsink for the power transistor. Make sure it has lots of free space around it, it can get hot and needs to cool off.
Make sure the laser is pointing away from you at something non-reflective and cheap (a piece of wood is great), plug the AC/DC adapter in, and turn the switch on. You should see a whitish dot appear. Alright, now we have a working laser that we can easily turn on and off!
Step 2: Laser Modulation
Now we need to grab those headphones and take them apart. You should have one speaker attached to a cord that you can plug into an iPod or a stereo. Plug it in and start playing music to make sure it all works. We want to be able to glue the little circular mirror on the speaker itself.
Grab your glue gun, dab a bit of glue on the back of the mirror, and quickly and carefully place it right in the middle of the speaker, ensuring it remains flat.
Step 3: Alignment and Mounting
Now we want to place the headphone/mirror inside our project box, and align the laser correctly so it bounces off the mirror. I found it easiest to first place the speaker at a 45 degree angle, and fix it in place at one of the box with the glue gun.
Ensuring the laser remained parallel to the bottom of the box, I then I adjusted the height of the laser with a mounting made of popsicle sticks. The end result is a beam which bounces off the mirror at a 45 degree angle and then heads straight for the ceiling.
Now we need to place the diffraction grating so that it intercepts the laser being reflected off the speaker, and breaks it up into hundreds of different beams. We want to place the diffraction grating directly above the mirror if you've aligned your laser with a 45 degree reflection angle.
Use a Dremel tool or similar to cut a hole in the top of the project box that's slightly smaller than the diffraction grating. Then mount the diffraction grating in place with the glue gun.
Step 4: Final Testing
Don't worry about that tan circuit board with the LEDs attached the to the diffraction grating, they're some lights that came with the specific type of headphones I used. I kept them in because they looked kind of cool, not for any technical reason. You don't need them. Check to make sure that the laser turns on, and that it bounces off the mirror correctly at rest and when music is playing. Then box it all up, and you're ready to go!