The circuit outlined below provides an excellent theramin which is both inexpensive and easy to construct.
Only the pitch is proximity controlled, but if volume is controlled using a pedal, the theramin can be used whilst playing keyboards.
The circuit consists of two Hartley oscillators. One of these is fixed, while the other varies in pitch with the proximity of the player's hand to the pitch plate.
The oscillators are at around 470kHz and the beats are audible. The resulting signal is picked up by the second half of the circuit – a simple medium-wave receiver. You could omit this element and use an ordinary MW radio instead, but more often than not this results in a noticeable reduction in sound quality.
Oscillator coils L1/C1 and L2/C2 are Toko YMCS 17104 IF transformers, with their screening cans removed.
The sound is very much dependent on adjusting the slugs to obtain the desired effect.