…and a fancy new theremin

(Actually I built this a few months ago but I’ve just recently made some modifications which kinda make it new again.)

As soon as I saw Moldover’s fancy CD case instrument I knew I had to make one. But not in a CD case – it had to be bigger, stronger, and more like a flying saucer from an Ed Wood movie.

And so:

Hear it in action in the last 30 seconds of this video.

What does it do? If you don’t know what a Theremin is, read about them here or watch theremin videos here. If you do know about Theremins then you’ll know that this isn’t really one but it’s a convenient shorthand for an instrument that you play by waving your hand through the air.

A true Theremin is controlled by playing the electro-magnetic field around an antenna – the amount of capacitance created by the proximity of your hand to the antenna determines the pitch – closer to the antenna for higher notes, further away for lower ones.

Lev Termen (AKA Leon Theremin), inventor of the Theremin and international man of mystery.

In an optical theremin, on the other hand, pitch is determined by the amount of light reaching a light-dependent resistor (also called an LDR or photocell). That’s the little round thing at the left end of the device in the first photo. More light = higher notes so moving your hand closer to the LDR lowers the pitch and pulling away raises it.

I’m pretty sure this is the circuit I started with, taken from here, with my slight modifications described below.

• Q1 = 2N3904 Transistor.
• Q2 = 2N3906 Transistor.
• C1 = 0.1µF ceramic capacitor but for some reason I seem to have used an electrolytic cap. Not sure if this is a good idea or not.
• PC1 is the Light Dependent Resistor.
• The battery is 1.5 volts
• The speaker is 8 ohms and is not optional. The circuit won’t work without it.

The first thing I added to this circuit was the momentary switch, just because it makes it more fun to play. I put it on the + battery lead. This switch is probably the most expensive component at about $10. I started with a much cheaper plastic one but it does get a bit of a hammering and could tell it wasn’t going to last long. YMMV.

Next, I added a fairly big capacitor to lower the range and a switch to bring it into the circuit when it’s needed. The reason I did this is so I can use it outdoors in the sunlight and other brightly lit environments. As it was, the pitch was just way too high in the daytime. The extra capacitor(s) to alter the range can go almost anywhere. I think I put it across the LDR. As with any of these things, try different values of capacitor until you find the range you want. I used 680nf.

The other way to deal with the sunlight problem is to put some really dark sunglasses over the LDR but I didn’t think of that until just after I’d mounted the switch Now that I think about it, a variable capacitor would be even better.

Anyone else using or making optical-Theremin-type instruments that I should know about?