Changing Resistance Mechanically

[David]

First off, I apologize if I come off sounding like that annoyingly android-ish individual who haunted this forum for awhile a couple of years back.  I just could not come up with a better subject line to save my life.  Anyway, to get to the point...

Anyone who has made or considered making some type of volume, wah, expression, CV or other type of pedal-actuated mechanism knows from experience or research that the construction of the electronics and the pedal itself aren't too hard.  The devil is in the details of the control mechanism.  Sometimes it's a rotary pot, sometimes a slide pot, maybe a Hall effect sensor (I hope that one works...)...

Here's something I haven't seen come up here before -- I don't think.  We change the resistance of an LDR all the time by varying the brightness of an LED or bulb.  OK, this would usually involve some kind of consumable control part, right?  Well, what about this:  how about changing the resistance of the LDR by physically moving the LED and LDR close or further away?  I would think it would be simple to get 10K of resistance in a short distance, depending on the brightness of the LED, whether it's in a dark or reflective tube, yada, yada, yada...  It might take a little more doing to get 100K (wah) or 250K (volume) out of it, but maybe it could be done.

Questions:

1)  Does anyone know if this would or would not work -- or have any advice on how the concept might be tweaked so that it would?  Specifically, I'm looking to limit the separation of the LED and LDR to 1.5 or maybe 2 inches.

2)  Is there some kind of mathematical relationship to describe this?  I think the inverse square law might be involved, but I haven't done anything with physics since the days of polyester leisure suits and the Bee Gees...   

[R.G.]

Good idea.

Yes, light intensity is inverse-square.

But it's easier to move some sort of shutter between the light source and LDR. That's what Moreley does in their wahs. They use a loop of black cloth. Other shutters, such as a triangular shaped window have been used too.

Don't get discouraged at it having already been done. Keep thinking!

[David]

Thanks for the encouragement, R.G..  At least I know I'm not about to wander down a primrose path.  I knew about Morley and the shade thing.  I just couldn't figure out how to implement that mechanically.  Your comment got me thinking, though.  What you must mean is that resistance would also change depending on the portion of the LDR (or the light source, I guess it wouldn't make any difference) that was covered or uncovered.  Geez, you sure wouldn't need much pedal travel to cover up an LDR...  What, half an inch tops?  Shouldn't be too hard to implement a shade of that size.  Hmmm...

[petemoore]

Does anyone know if this would or would not work
  Yes: Yes.
  As long as you get the varied resistances and everything works the way you want.
  See "Morley", some of these pedals put a light blocking sheet on the treadle, then when treadle moves, the light blocker also moves...between the LDR/LED, varying the brightness the LDR sees.

[~arph]

What about making a wah without moving parts? with a solid metal (or some other material, less affected by heat) plate and some strain gages?
This will require some additional circuitry for converting the gages output though.

[David]

What about making a wah without moving parts? with a solid metal (or some other material, less affected by heat) plate and some strain gages?
This will require some additional circuitry for converting the gages output though.

Yow!  Strain gauges... 

That would be interesting, but probably not easily duplicated.  I'm trying to come up with a design "template" and feeling a lot like how Don
Quixote must have felt... 

[foxfire]

i've been thinking about mounting both the led and ldr in the base pointing towards a mirror mounted to the underside of the rocker. as you rock the pedal the angle of the mirror would change there by changing the amount of light hitting the ldr. you would probably need a divider between them and i'm not sure how much range you'd get with the ldr exposed like that?

[theonlyrobkeyser]

What about two sheets of polarizing film between the LED and LDR?  That could give you varying light transmition just by rotating one of the sheets.   

[David]

What about two sheets of polarizing film between the LED and LDR?  That could give you varying light transmition just by rotating one of the sheets.   

That's a cool idea, Rob!  Trouble is, are they universally available?  Cheap?  Easy to use?

[theonlyrobkeyser]

Yeah, you can get it at camera shops and on the web.  It's not exactly cheap (~$8 for a small sheet), but if you only need a couple square cm, a sheet would probably last you a long time.  I haven't tried it out myself, I just thought of it when I read your post about varying the brightness of the LED. 

[David]

Yeah, you can get it at camera shops and on the web.  It's not exactly cheap (~$8 for a small sheet), but if you only need a couple square cm, a sheet would probably last you a long time.  I haven't tried it out myself, I just thought of it when I read your post about varying the brightness of the LED. 

Oh, crud!  I just realized you wrote "polarized"...  Isn't that going to work like a prism?

[Pushtone]

Oh, crud!  I just realized you wrote "polarized"...  Isn't that going to work like a prism?

I don't think so.

If you hold up two lenses  from polarized sunglasses so you are looking thru both and you rotate one of the lenses the amount of light that passes thru is reduced.

[theonlyrobkeyser]

Pushtone's right.  They sell that stuff in thin plastic sheets, like projector overlays in school. 

[David]

Pushtone's right.  They sell that stuff in thin plastic sheets, like projector overlays in school. 

OOOOooooooooo...  If one were held stationary and one rotated by a pedal-actuated mechanism, this would create a rotary shutter -- which would regulate the amount of light received by an LDR!

Very interesting...  Also seems like it might be practical.     

[Paul Perry (Frostwave)]

Polaroid is pretty cheap if you cut up old polaroid clip-ons from thrifts.
A posssible problem: even when rotated exactly 90 deg, it doesn't go TOTALLY dark.
An alternative is geometric patterns (parallel lines, positive and negative dot fields etc) copied onto overhead transparency plastic.
Warning: you might accidentally re-invent 1960s Op Art.

And note that the "shutter" mechanisms usually work with a 'wedge' pattern, so it can work over as long a throw as you want.

[darron]

i thought that i had seen some optical wahs that look like this? you use infrared LEDS to both transmit and receive (like using a regular led as a photoresistortor). then when you tilt the pedal back not only is the  distance increased, but the angle changes and points away a bit.

maybe i was dreaming...

[petemoore]

  Somewhere, maybe in the distant archives is a thread I started.."The Spinner"..soon thereafter claimed as an invention [I believe only in discoveries though].
  Discs with various hole shapes in them, place 1 disc with the light-holes of taper/shape you like between the LED/LDR, fix to the spinners axis shaft, the disc rotates allowing light to pass through the holes [between the light and light dependant element] or not / or some.
  Yupp, I built a prototype, but it could use a second drafting for major mechanical size reduction and cleanup, utilizing a flywheel or scaled down leslie motor affair to make the disc spin at less 'accellerative/de-accelerative' speeds.
  A bright flashlight and a bearing/shaft, [can be whipped up big and ugly in no time] I wrangled up a rubber belt/DC motor drive system for it, a 1 speed tremolo / science project, met design goals.
  The LDR element[s could be assigned to control various effect parameters.

[Darren N]

Getting back to the original post, the digitech xp100 whammy-wah uses this arrangement. The LED is fixed to the rocker so as it moves away from the LDR it also points away slightly. It looks really simple to implement, perhaps even more so than the shutter systems I've seen. If there is someone that can post a guts shot of the xp100 (or possibly other whammies?), it would probably help a lot.

[David]

Getting back to the original post, the digitech xp100 whammy-wah uses this arrangement. The LED is fixed to the rocker so as it moves away from the LDR it also points away slightly. It looks really simple to implement, perhaps even more so than the shutter systems I've seen. If there is someone that can post a guts shot of the xp100 (or possibly other whammies?), it would probably help a lot.

Cool!  The original concept has some merit, then.  Here's something else:  I don't have the construction details worked out yet, but I made a real narrow triangle out of foam that was about 2 inches long and tried that.  My test LDR had a dark resistance of 10K -- perfect for my target application.  Moving the triangle over the LDR from point to short side, I saw the resistance increase from about 1K to 10K.  I'm sure it wasn't perfectly linear, but I'll bet an ADC could handle it.  Just for grins, next up is the moving LED idea.

Now, guys, before you get too excited, this is going to drive a PIC.  You know -- digital?  If you're still interested, when I get started I'll post a new thread in the DSP section.