You might also look at the Art Harrison Minimum Theremin (http://home.att.net/~theremin1/100/minimum.html).
From the website:
"I designed the "Minimum Theremin" with the objectives of simplicity and economy, and in response to frequent requests for an instrument that can be built with readily available parts. This is a pitch-only instrument that demonstrates the basic theremin concept, perfect for science fair projects or as a coffee table conversation piece. Although it doesn't have the qualities of more expensive and complex theremins, it is very inexpensive, relatively easy to make, and sufficiently sensitive and stable for playing a melody. A suggested lay-out is included for perforated-board construction, however, any number of alternative methods may be used to build the Minimum Theremin."