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theremin thoughts
willmatthews Posted: 17 May 2022, 12:30 AM
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average bovril enjoyer

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I just watched your tiny theremin video and had some thoughts as lots of the problems/questions were ground-related.

I'll scrawl down my ideas inline with points you mentioned:

There was a periodic interference present.
When a wrist strap was worn - you noticed the range improved and the signal to noise ratio also improved.

I suspect some mains coupling is going on. I have encountered this as a problem with the human body in close proximity to sensitive detection circuits. I might do some analysis on the audio from your video to check this, but to my untrained ear it sounds approximately 50 Hz.

When your hand is closer to antenna the measure of charge increases, opposite to what is expected from capacitance increasing.
This might play into the point above with mains hum - as your hand gets closer a larger amplitude is coupled into the high impedance line.

With the PWM in differential mode and by grounding touching the metal on your laptop the pitch range reverses.
While I'm not exactly sure of the implementation, this might reject common mode interference from mains hum, and you might be only measuring the capacitance in this case, a differential measurement(?).

I'll stew on what else could be happening and might have a crack at making one myself.

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mit Posted: 17 May 2022, 08:19 AM
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yeah whatever

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Yes, there's definitely mains hum in the video, but it was picked up from how I connected it to the speakers. They were pretty poor quality speakers and later I took them apart and replaced the power supply, which eliminated the hum entirely. It would be interesting to repeat the theremin experiment now.

It would also be interesting to try the theremin with battery power and outdoors, away from any power cables.

I did come up with some more theories about how it might be working, but at this point I have forgotten a lot of the details.

It should be very easy to recreate the experiment, all you need is an ATtiny85 and some wire.

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willmatthews Posted: 17 May 2022, 08:36 AM
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average bovril enjoyer

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Ah thank you, I'll let you know if I get around to building it and having a prod. I'm beginning to suspect some RF wizardry is afoot.

I just discovered your blog/work and there's a lot to get deeply interested in here, (if we compare blogs, yours is definitely something I aspire to be like someday) so I hope to stick around. Thank you for lots of interesting reading!

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