The theremin is an electronic musical instrument controlled without physical contact by the thereminist (performer). It is named after the Westernized name of its Soviet inventor, Léon Theremin (Термéн), who patented the device in 1928.
Construction of this device is very simple - we have two frequency generators - one with stable output frequency, and another with floating frequency controlled with thereminist hand. Difference of two frequencies is a note frequency, and it is sent to volume regulator (which is controlled by another hand).
This is my frequency generator:
I could create pcb with modern components with smaller size, but wanted to have more serious look :)
Volume control board: