|A Theremin is decribed in Wikipedia as...
"... originally known as the aetherphone / etherophone, Thereminophone or termenvox / thereminvox is an early electronic musical instrument controlled without contact from the player. It is named after its Russian inventor, Professor Léon Theremin, who patented the device in 1928."
This version I found on the internet many years ago but have only recently got round to building it. Normally it is both frequency and volume controlled moving your hands near the antennae of the Theremin but this version only seems to alter the frequency.
Anyway I decided to try it out, designed a PCB, and it worked straight away, very impressive. I have yet to house it properly with a decent ground plane and antennae but it is quite fun to play with, difficult to produce a tune worth listening to though.