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We have been working on a small project trying to build a fun game but are having some issues. We basically need an oscillator that can be controlled by varying the distance between our hands and a metal plate (capacitor type of sensor), which then the signal converts into a PWM signal and is fed to a H-bridge that controls the fans. Our issue is the sensor part, oscillator and the converting into a decent PWM signal that idealy would go from 0-100% based on the capacitance. We have tried several types, relaxtion oscillator (square), 7414 schmitt trigger (square) and recently I also built a Variable pitch oscillator taken from a theremin guide, which generated a nice sine wave but only varied from the intervals 120-142kHz. We also could't figure out how to utilize it.

Is there any kind soul that have a great idea of how to make this a reality? Basically what we need is an oscillator that can be affected by just one capacitor where the interval of the capacitance (the hand) is around 20pF, this oscillation then somehow needs to be converted into a PWM signal. Our only challenge here is that we are limited to only analog components but pretty much have anything at our disposal, but mostly standard components, and also the 7414 is something we managed to get our hands on. We have tried to come up with a solution for a long time at a daily basis and are getting pretty frustrated.

Thank you in advance,

Chris.

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  • \$\begingroup\$ google theremin \$\endgroup\$
    – jsotola
    Dec 12, 2017 at 22:17
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    \$\begingroup\$ "only varied from the intervals 120-142kHz" is that 120hz or 120kHz? Seems like a very small range if it is the latter. \$\endgroup\$
    – Trevor_G
    Dec 13, 2017 at 0:19

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Two hands about a foot apart is less than 1pF and with inverse distance vs C you need a fairly high f and low C LC resonator to modulate the frequency. Theramins used a thin insulation over the electrode surfaces of a Unijunction oscillator to create this sensitivity over many decades of f using the small [mm] modulation of hands near the surface using high voltage and high R, small C.

Back in 1970, my friend used a waving hand with just 60Hz to couple the stray E field thru buried plates to detect the E field into 10 MOhms and could modulate the light intensity using a logic cct to drive a staircase generated phase angle on the triac.

So it depends on your impedance and the stray E field noise nearby. You can use high f or low f but it depends on the interference levels.

But to increase f modulation or to detect the level of induced E field, C fixed must be as small as the smallest C variable of 1pF, so coax cable is out and ground capacitance must be minimized between electrode plates but each plate can be insulated with thin plastic coating.

It is best to use 10k series R with ESD protected CMOS for high impedance input stages to detect E fields. Thus use 74HC14 or similar not TTL 7414. And the series R is to limit ESD current.

It is possible shape the attenuation of unwanted noise with filters. But to make a relaxation oscillator sensitive to 1 to 10pF changes in frequency, one needs a very high R feedback into the Schmitt inverter oscillator. such as 50M * 10pf = T = 50us then scale this frequency down using a CD4060 binary counter to use the frequency of the output to control the on time of a fixed PWM driver such that the highest frequency resets the PWM one shot the quickest and the lowest frequency gives the widest pulse using reset. More clever ways can eliminate aliasing effects.

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