theremin project and capacitance to voltage IC question

Other Hardware General Electronics
1

Hello, this project may not require an arduino but this forum is the best place that I know to get electronics advice. I've cracked open my korg monotron and I'd like to build a mini theremin out of it. If you feed a voltage between 1.4 and 5 volts into the gate terminal, the monotron produces a pitch. Below 1.4 volts is silence. There are ground and regulated 5v terminals available on the monotron as well.

I would like to know how to build a theremin antenna circuit that can feed 1.4 - 5 volts into the monotron. I'm not looking for a traditional theremin RCL circuit that oscillates a current. After researching a bit I've discovered some I2C and SPI IC capacitance sensors. I think a capacitance to voltage IC such as this one (warning PDF) would be the perfect solution. Could anyone suggest a capacitance to voltage IC that is well documented, non SMD, and easy to use? Also what's a typical capacitance range for a theremin antenna?

Thanks a bunch!
Mike

2

I'd try something else.... I'd try a high-impedance (1M or more) high-gain (40-60dB) amplifier, which you can build with an op-amp. Connected to an antenna, it will pick-up AC signal from your body. (You know the "buzz" you hear when you touch the input to an amp?) Connecting a low-voltage AC signal to your body (maybe from a 6V or 12 V transformer) will inject a stronger signal from your body, and make the system less dependent on the electromagnetic energy in the environment.

If you have a musical instrument amp, try connecting an antenna to the input to see how close you have to get before you pick-up a buzz from your body.

Then you can use a precision rectifier (diode & op-amp) to convert the amplified AC to DC.

There may be a couple of problems with this... The antenna will pick-up some AC from the wiring in the room. And, the signal will be non-linear in relation to the distance from the antenna (inverse square law). If background AC pick-up is too high, you can connect a higher-frequency oscillator to your body and filter-out the low-frequency 50/60Hz AC.

Also, you may have to insulate the antenna, because if you make direct contact, you'll suddenly get a very strong signal, which might mess-up the "naturalness" of the sound.

Also what's a typical capacitance range for a theremin antenna?

Very-very-very small... I'm thinking a couple of picofarads or less.