I've been wanting to blog this for a few months, but life got in the way of preparing enough material for release. I've updated the static site at g1inf4u with enough information for an experimenter to try their luck with one. The full link is here, and do please comment if you'd like more information.
The theremin is a 'mature' project; it's been around for a couple of years. The original still works, and I find it a great asset when I'm teaching the art of the Boatswain's Call. The two instruments are very close in character, and the way they are played. They share the distinction of being the only instruments whose pitch is controlled without the player's fingers touching it; only the distance from the tone-determining antenna or hole is relevant.
Theremins are relatively rare. I've just had to add the word to my netbook's dictionary, and few people recognise the word, either. The sound is unmistakeable, however, and while similar to a bowed saw, the range of frequencies is quite large. My theremin can make any tone from a low ten-hertz growl to a ten-kilohertz shrill.
I've seen pocket-sized theremins cropping up here and there, but none offer level control. I've used an ORP12 photocell with a white LED, and this gives a useful dynamic range. It would need a window amplifier to cut off the signal entirely, but then the circuit begins to get complex. Currently, it's simple and elegant.