[sdiy] Endless pots?

[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.

||\/||ike


----- Original Message ----- 
From: "karl dalen" <dalenkarl at yahoo.se>
To: <synth-diy at dropmix.xs4all.nl>; "Mike Pepper" <profpep at hotmail.com>
Sent: Sunday, December 05, 2010 1:57 AM
Subject: Re: [sdiy] Endless pots?


Ah!

Thanks for the link, had no idea Rapid had them! :)
and thanks for the generous offer but i may order
some 10kohm from Rapid later on if decide to use these.

Regards
KD


--- Den lör 2010-12-04 skrev Mike Pepper <profpep at hotmail.com>:

> Från: Mike Pepper <profpep at hotmail.com>
> Ämne: Re: [sdiy] Endless pots?
> Till: synth-diy at dropmix.xs4all.nl
> Datum: lördag 4 december 2010 19:42
> > Regarding past pots and decoder
> discussions, i wonder why there are
> > are no pots that turns endlessly with a small dead
> zone around 360 deg?
> > Could be neat!
> >
> There are: they are mainly made for servo applications.
> sadly because of
> this they are quite expensive. There were some exotic
> sine/cosine ones made
> to use as panpots in Ambisonic systems too.
>
> I have a few Vishay Spectrol 1K ones:
>
> 'Dead' zone is about 20 degrees
>
>
http://www.rapidonline.com/Electronic-Components/Resistors-Potentiometer/Control-Potentiometers/1W-Single-turn-precision-potentiometer/65229
>
> http://www.rapidonline.com/netalogue/specs/68-1300e.pdf
>
> If you want some, you can have up to four, for the postage
> plus a
> 'donation'.
>
> ||\/||ike
>
>
>
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