Strange Proximity Sensing Circuit

Posted: 6/20/2016 7:43:39 PM

From: Minnesota

Joined: 3/9/2013

This video demonstrates a very strange circuit that can detect hand proximity up to a meter from the antenna.  It shows this by varying the intensity of an LED as the person moves their hand around an antenna.  So obviously this made me think about how this could be used in the construction of a theremin. 

Here's the video...

Of course, I know nothing about how any of this works and was hoping to gather some comments from our experts on its practicality.

Posted: 6/20/2016 10:57:49 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Johnthom I really like this circuit as a fun project. The LED could be the one inside a VACTROL for volume control but it would be great if an engineer chimed in how to best reverse the direction to dimming the LED upon approaching the antenna.

This is an area dewster shines! 

Now a question arises, has the theremin and TW gone the way of the dinosaur?

PS: Yes it was 118 degrees F here today, but who goes outside, more Canadian Whiskey on the rocks please?



Posted: 6/21/2016 1:55:06 PM

From: Northern NJ, USA

Joined: 2/17/2012

The flashing, picked up by the camera and likely aliased down, indicates to me it's operating by picking up 50/60Hz mains field from the nearby hand via the tiny mutual capacitance.  Kind of like when you touch a microphone input and hear a loud buzz, but operating at a distance due to the very high impedances.

[EDIT] What's really creepy is making a DIY electroscope by putting a JFET in one leg of a resistor bridge, with the opposite resistor variable for null, and a small meter across the bridge, powering the whole thing with 9V or so.  Let the JFET gate float and connect it to a small antenna.  Re-positioning it vertically will show higher potentials at higher heights.  You can directly see the Earth's voltage gradient over a couple of inches with this crude setup.

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