[sdiy] Endless pots?

[Mike Pepper]

> 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.

Tried the shutter effect for touch sensitive keying: I'd been given a pile
of those slotted LED/Photodiode units,  it needed a fairly steep angle on
the shutter and alignment was not easy. The thing that made me drop it at
the time was thta I couldn't design a suitable multiplexer to scan all the
keys, though these days I might have a better grasp of programming. As an
aside, I did try the liquid keying idea used in some console organs, though
that is definitely not portable!

As a thought, if we explore that route, given a laser printer that will
print on acetate, how about a low-fi, (or should that be even lower fi!)
'Optigan'

One idea I had for a magnet and Hall sensors was to fit the sensors to the
base of one of those chaotic pendulum magnetic toys you see in novelty
shops. I think you could get mad patterns by adjusting the the base magnets
and swinging the pendulum. Some brushless DC motors, which can be harvested
from old VCR's or Laser printers, can contain up to 4 sensors, look for the
4 pin ones: the 3 pin ones, as used in keyboards, are switches, and not much
use for field sensing. I suspect Ian Fritz has already done this with some
of his legendary ingenuity and a couple of chips, though.

||\/||ike



> 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
>
>
>
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