[sdiy] Endless pots?

Paul Perry pfperry at melbpc.org.au
Sun Dec 5 23:21:34 CET 2010

For an 'endless' rotation to voltage converter, you don't
need a polariser - just a cunningly shaped piece of card that partly
occludes the light to the LDR. Like optical wah pedals.

But, a magnet and a linear hall effect device might have
possibilities too.

paul perry Melbourne Australia

----- Original Message ----- 
From: "Mike Pepper"

I can see a couple of snags in the endless pot idea; when the slider hits
the deadband the output voltage is indeterminate, and as the slider crosses
the deadband, the output jumps from one end of the scale to the other,
depending upon the direction of rotation.

It might be worth mentioning an old idea I was thinking of using to provide
panning voltages for a surround controller, that avoids the problems.

Basically it's this: you have a knob with a disc of polariser film on it's
shaft. The disc endlessly rotates, when the knob is turned. Above the disc,
set at 90 degree angles, you have two Light Dependent Resistors, (LDRs),
each which has another piece of polariser film over it, the films aligning
to 90 degrees also. Under the disc are two LED's pointing at the LDRs. When
the disc rotates, the effect of the crossing and uncrossing of the polariser
films is to vary the light falling on the cells, in a roughly sine/cosine
fashion. This means the voltages derived from the cells go smoothly from min
to max and down again as the disc rotates, and there is no sudden jump at
the dead-band as there would be with an endless pot.

This is not an original idea - I got it from a suggestion by F C Judd in a
1960's article in 'Practical Electronics', where he suggested it for a
no-thump tremolo, and I adapated it, by using two cells, into a sort of
Leslie Simulator, which panned the sound nicely through a stereo PA system,
(quite radical then!), but didn't actually sound like a real Leslie, to to a
lack of the Doppler effect.

Another thought is to use a fine resolution Gray coded shaft encoder, and
decode that with a D->A into whatever voltage range was wanted. In fact, a
bit similar to a wind vane sensor.

Just a couple of ideas, anyway.


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