Theremin (Termenvox) was the first electronic instrument, invented by the Russian Leon Theremin (Lev Termen) around 1920. Having no moving parts, the pitch and the volume of the nearly pure sine wave sound are controlled by the proximity of hands from antennae. The pitch may span the entire audible range. It is probably most famous for its use as an effect generator for early B Sci-Fi movies such as Plan 9 from Outer Space :-), but it can be used for serious music, too. Details about Theremin, including building instructions, can be found here.
was built in July 1999, based on A. Zeyliger's schematics. I, too, have been unable to build a proximity controlled volume adjustment, so I had to resort to mechanical control. The system has been enhanced with a simple but versatile fuzz effect.
First of all, the Theremin is fairly hard to 'tune', that is, to set the frequency range that is accessible for playing. And it has to be tuned every now and then, sometimes during a song. Rather than a drawback, this is a 'feature' that makes the Theremin almost like an acoustic instrument with its unpredictabilities.
It is also difficult to play. But even more than, say, a fretless bass guitar. My prototype spans four octaves by a one-foot sweep of hand, and the response is far from linear! It has been said that the Theremin is harder to play than the violin.
A fairly technical point to those of you planning to build your own. Since the ear is the most sensitive in the mid-range of frequencies, the lower frequencies sound very quiet - after all, they have the same amplitude as the middle and high frequencies. A well-designed low-pass filter might solve the problem; however, my quick & dirty hack was simply to use a phono preamplifier to straighten the frequency curve. The output level then got too high for my sound card, so I had to reduce the input level to the preamp.
Phew. About a week's evenings of planning and hardhacking for Theremin 1.1: