THEREMICRO/ THEREMINI is the nickname of the simplest type of capacitive sensor, that in the past has been given the name "theremin" (originally "termenvox" - by his inventor Lev Nikolajevic Termen). The invention goes hand in hand with the pioneering years of radio - the electronic amplification - the 1920s. In fact, it is a cast-away of the development in the field of radio - namely: the heterodyne (frequency mixing) principle and the observation of the capacitive properties of human body. In termenvox this mostly undesirable effects were put to good use.

The original "termenvox" was made from vacuum tube amplification elements, but here we use the simplest, readily available CMOS logic gates, that were introduced in the beginning of 1970s. However, they do not provide the soft sound as termenvox used as an instrument, but do perform the proximity sensing function. The schematics was found in one of the journals for electronicians that was published in ex-Yugoslavia (originally published in some Russian journal) and then modified.

Termenvox is basically a capacitive sensor, that functions by comparing the frequencies of two oscillators. One oscillator is fixed, and the other is coupled with the antenna to it's surroundings. A human moving the hand acts as a capacitor to ground and adds him/herself to the oscillating system. The closer the hand (or any body) - the higher the total capacitance and thus the lower the oscillating frequency. The free-running frequency (no body close by...) of this variable oscillator should ideally be the same as that of the fixed frequency oscillator - then the difference of these two frequencies would be zero.

So:     F_fix - F_var = F_audio

The frequencies are compared by mixing them together (intermodulation) and filtering out all the higher intermodulation results - leaving the basic difference of the two frequencies - which falls mainly into the audio spectrum (about 20Hz to 20KHz). In the ideal case the resulting tone with nobody close-by would be 0 Hz.

Theremicro is the smallest and simplest "termenvox" - a proximity detection electronic module. It was first used in an object (and project) called "The Round Table" - made for a concert at ÖRF 1 program Kunstradio, next time in a more elaborate form at tproject Singer by Marija Mojca Pungerčar, where they were attached to three sewing machines and produced their sounds while also mixing the sound from three sources. 

I built a couple of these simple ones myself and others have built some more at the workshop of Cirkulacija 2 initiative at the Sajeta new music festival 2008. With the values shown they all performed well - so this is a kind of "production situation". A couple of similar circuits can be found around the net - they are all basically the same - built around CMOS NAND gates

Different metal objects acted as antenna and for this a wide range of tuning is necessary. The basic drawback that lowered the effective sensitivity of "hand", was always the closeness of wall or floor. During the testing with different wire forms the best one came out to be a "snake" design which i believe has something to do with "yagi" antenna design. Anyone can try to check out the necessary distances (the wavelength)  between the "snake's" ondulations and the length (the amplitudes) of it...

The output is the audio frequency which is very sharp and "dirty" because of odd harmonics that make up a "square" wave. And this is the main problem in using digital circuits. One can try to filter the higher harmonics out - but because of the wide frequency range - a couple of octaves - this would be quite impossible.

Still it was used for some noise music concerts by some people and myself in the last couple of years. I also used it a couple of times as controlling input to computer (and to software like Ableton Live or Pure data) and this I see as the right way to go - to use theremin as interface...