THEREMIDI is the nickname of a more elaborate type of capacitive sensor, that in the past has been given name "theremin" or originally "termenvox" - by his inventor Lev Nikolajevic Termen. The invention goes hand in hand with the pioneering years of radio and electronic amplification - the 1920s. In fact, it is a cast-away of the development in the field of radio - namely: the heterodyning / frequency mixing principle and the notion of capacitive potential of human body.

THEREMIDI has the provision for voltage control of the volume - as does the right hand of the concert theremin do. But even more than before, it is meant as an interface or controller of some sort.

Description of the circuit

The demodulation is done in the similar way as before - but here in parallel the two frequencies are sent to a PLL frequency comparator. This is performed by another readily available CMOS chip 4046. At the output (pin 13) the comparator generates a proportional DC voltage that can be used to control a motor, an actuator (via current buffer) or be used as an input to some voltage to midi converter (Doepfer A-192 or Arduino) - and to the computer. 

On the board the output voltage from 4046 is used as controlling voltage in a simple voltage controlled amplifier  (VCA, 4007), where we can change the level of any audio signal (by moving closer to the antenna). The source can also be another theremin. But this may not be the best solution in view of the non-musical sound of theremin output.

Better solution would be to use controlling dc voltage and/or direct theremin frequency to drive an Arduino microcontroller board and connect it with USB to the computer. There is a patch for Pure Data available - see the arduino homepage.

The VCA (4007) uses one transistor pair as a very non-linear amplification cell that introduces quite a bit distortions and is also dependent of the level of controlled input signal itself. The solution is to keep this one low, so we have to amplify in the 4007 more to get the corresponding output level. The dynamics of VCA amplification is between 10 and 20 dB.