When I first got into making Theremins I ordered the Theremin Vision kit. I had a hard time getting it to work at first, but finally got it running OK.
At the moment I only seem to get about 12″ of usuable range.
My reference oscillator runs at about 1.67MHz and the sensor oscillator is running at 1,54MHz.
A 4013 flip-flop does the necessary combining and filtering. A 14 bit ripple counter then gives a range of frequency responses.
Hooking up a piezo buzzer to any one of the “bit” output lines creates an audio tone – just like the original Theremin.