In this project I embed what I call a hackduino which is an Arduino-compatible circuit, in this case built out by hand on perfboard. I have made an instructable specifically about building one of these yourself, which you can read here.
Step 1: Build out the microcontroller circuit
1. the 7805 voltage regulator. This so we can take 12v DC output from the theremin, and lower it 5v which the ATMega chip needs. The code attached will work for both ATMega168 and ATMega328 chips.
2. the ATMega168/328 circuit. This consists of the chip itself, a 16MHz crystal oscillator, and several capacitors. I have not made an instructable for creating this circuit on a perfboard, however several tutorials exist for example here. Alternatively, you could use any Arduino compatible device - there is a list here.
I will be attaching the Arduino code to upload to your chip
I have built mine out on perfboard from radioshack. Reasons why I do not just use an Arduino board: This is cheap, and completely permanent. This entire circuit cost about $7 and is much smaller than an entire Arduino. I can permanently add components as I need to. I feel a sense of hacker legitimacy when I don't spend $35 on the bulky blue board.
3. LEDs.... [next step]
Step 2: LEDs!
I chose to use 2 RGB LEDs, which afford me cool colors, and perhaps some sort of synesthetic connection can be made. When dealing with installing LEDs outside of a breadboard, I am a huge proponent of wirewrapping. It's easy, clean, lightweight and with a bit of solder on top it can be completely permanent.
For one LED, I soldered directly on the breadboard. That's because the board will be attached on one side of the theremin, so I connected it on one corner of the board. The other, I wirewrapped 30AWG wire to each leg, and then to header pins on the other side (see image).
Step 3: Theremin hack!
To get solder to this pin, remove the board from the theremin and flip it over. Attached is a picture of exactly what pin to solder to. Run the wire under the board toward the left side.
The next step in the hack is to attach female header pins to the row of breakouts located near the top left corner of the board when you are looking at it face on, right-side up (see images). The female headers will let us prototype easily.
Step 4: Get that board and LEDs in there!
If you made your board like I did, you will have one RGB lined up nicely near the middle of the far left side now!
I used a *dab* of hot glue to hold the other LED in place, on the far right side of the theremin.
The other nice trick I like to do is use styrofoam spheres to diffuse the LEDs. They can get quite bright, and the balls create a nice large object.
I used needle nose pliers to create a hole in each ball. Then just plopped the balls on top of each LED! So simple, cheap, and effective! yay efficiency!
Step 5: Acrylic top time!
You will need approximately 2' x 1' piece of plexi, and using the Adobe Illustrator file I have attached, you shouldn't have any problem at all. Laser the attached file (we have an Epilog 13 watt, I believe), and you'll have a sweet top ready to go.
I decided I wanted both sides to be translucent, not clear (where the LEDs are). I drew a line at both sides, and with an exacto cut the paper along this line. I then take the paper off, and with a fine grit sandpaper (emory paper) go at the inside of the acrylic.