Qprox(tm) Capacitance Sensor as a Simple Theremin CV Controller

This uses a QProx QT301 Capacitance to Analog Converter (CAC) chip to generate a control voltage dependant on hand position near an antenna.

Toby Paddock - Feb 2004

OK, it's not a real Theremin, except in the generic sense of something with an antenna that you wave your hand at. But it's cheap and real stinkin' easy.

WARNING - PRELIMINARY - some of this is untested speculation.

QT301 features:
8-bit PWM output, filter it to get a DC voltage.
Single supply 3-5VDC.
EEPROM remembers setup.
Not many other components needed. Really. Not much. Hardly any. No inductors, trim pots, fancy tuning, or critical values.
Optional sync input to help ignore power line noise.
Spread spectrum measurement burst.
Cheap, about US$4 at DigiKey
And my favorite...
2 switches for gain and range calibration.

Hold hand far, press CAL LOW.
Hold hand near, press CAL HIGH.
Full output range is now between those 2 points.

Check here for datasheets.
There is also a QT300 with a serial output.

A minimal demo circuit:
Weeeeoooooeeeoeoeoeoooooo . cable ties - state of the art in meter mounting . wobble the C, wobbles the V
Driving an analog meter movement right from the chip. No buffer amp since the meter load is so light. Pretty much right out of the datasheet. Capacitor Cs is 100nF (0.1uF, I have to convert to uF before my head can relate to it). Chassis parts are out of the scrap bin. 9V battery with 5V regulator. Power and the 2 cal switches at the front. Banana jack and binding post for antenna connections. The white rectangle on the left behind the meter is a cheesy backward acting 555 VCO on a musical greeting card piezo speaker left over from another project.

First impressions:
Fun. Good fun/effort ratio. I like.
Sounds a bit jittery/zippery/steppy, especially at the far end of the range. Larger filter cap may help some. But not too large or it might get sluggish.
It really likes the circuit ground tied to earth ground or something big and groundish to work against.
Increasing the capacitor Cs increases the sensitivity, but slows down the sample rate. With 400nF, you can easily hear the jumps.
Kids like it.

Filtered output voltage is high with hand near the antenna and low with it far.
Some possible ways to flip that over:
Use a 'digital' inverter to flip the raw PWM over before filtering.
(Maybe a 555 with the PWM either into pin 4 [no invert] or pins 2 and 6 [yes invert]. Then filter it.)
Or use a unity gain opamp inverter to flip the analog voltage.

Minimoog controller:
2 circuits. One for pitch plugged into the VCO jack (or VCO and VCF). One for volume plugged into the VCA jack. Shorting plug in the S-trig jack or hold a key down with something.

Odyssey pedal input:
1 circuit plugged into the footpedal jack. FM modulating VCO2 and filter. VCO2 sync switch is ON. That Odyssey sync sound. Will probably need a buffer opamp to override the pullup resistor inside the Ody. And flip the voltage because the filter or VCO frequency will go up as the voltage drops toward zero.

Odyssey acting more like a real theremin beating 2 oscillators.
1 circuit plugged into the footpedal jack. VCO1 set to a very high fixed frequency. FM modulating VCO2 at a very high variable frequency. VCO2 sync switch is OFF. VCA slider up. Ring modulator gives the beat frequency. See Halloween Patch for more patch info. No volume control because a stock Odyssey does not have a VCA control input.

Odyssey and Minimoog as a Theremin:
2 circuits. One for pitch plugged into the Odyssey as above. One for volume plugged into the Minimoog VCA jack. Connect the audio from the Odyssey to the AUDIO IN of the Minimoog. So the Ody does the pitch and the mini does the volume.

Check out the Listen Reader that uses an earlier QProx circuit to detect hand position. I had the opportunity to sit in the comfy chair and experience it. Very very cool.

Suggestions, corrections, comments, and general abuse are more than welcome.

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© 2004 Toby Paddock