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optical pot

Optical Potentiometer never wears out
  (+6, -1)
(+6, -1)
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Joysticks typically use cheap potentiometers (aka pots, variable resistors, rheostats) to translate an angular position into a voltage. Remote Control transmitters also use pots similarly, and audio equipment uses pots to variably attenuate signals (or provide a variable voltage to a VCA). In either case, the problem is that the electrical output depends on a mechanical "slider" in contact with a resistive material. The slider and the resistor always suffer from mechanical wear. This is especially a concern in a joystick or RC transmitter where the slider spends most if the time moving in the same area relatively close to the middle of their travel.

As the pot wears out, you start to get spurious noise and false signals as the contact degrades between the slider and the resistor. Dirt, contaminants and vibration make this problem much worse. I would like to see a pot that works by shining a light (from an LED) through a disk on the input shaft. This disk will have a groove cut into it that light shines

through onto a photo-detector. The width of this groove varies as the disk is rotated. See what I'm getting at? Send an appropriate constant current through the LED, and use the photo-detector as the output.

(You could in theory use this setup to directly attenuate audio signals, although you would need some biasing on the LED and AC coupling on the photdetector. unless you used two different diodes wired in different polarities to allow positive AND negative signals??)

I think this would be perfectly suited apps like the RC transmitter and joysticks where a constant voltage is applied and the output of the sldier is fed to PC or microprocessor, and can probably be made as "drop in replacements".

The main advantage of course is that there is absolutely nothing to wear out that would affect performance. It would be cheap, small and very robust, unafected by vibration. Instead of a disk with a varying width groove, it could instead have a clear plastic disk that the light shines through. By printing a fine mesh of varying darkness onto the disk, it could easily be made to have more or less opacity as the disk is turned. The printing of this mesh or whatever could easily be made to give you linear output, or logarythmic for audio apps.

Unlike using an optical encoder type setup (like the slotted wheels in a mouse) the output of this pot would have infinite resolution (since its output is analog).

It is also "absolute" and doesnt need a homing sequence to know the exact shaft position immediately after power up. It would be unaffected by magnetic or radio wave interference (unlike magneto-restrictive and hall sensors).

ServoMan314, Feb 01 2006

Quite a lot of patents on this... http://patft.uspto....Query=ref/4,284,885
[ldischler, Feb 03 2006]

[link]






       You can use < br > (except without the spaces) to induce line breaks where you want them. Or just hit Enter twice for new paragraphs.   

       What about a rotary polarized setup? A small polarizing filter on an little rotary dial thingy that could pass LED light through at varying amounts?
RayfordSteele, Feb 02 2006
  

       So, is this, like, really, really green?   

       Contactless Hall effect rotary pots like those made by Penny and Giles are immune to dirt and vibration and can output both analog and digital signals. Yeah, they're not optical but when a company such as Penny and Giles builds something intended to be robust they generally succeed.
bristolz, Feb 02 2006
  

       This is such a great and simple idea that it feels like it must be baked somewhere...   

       If not, then, congratualtions[+]   

       P.S. Welcome to the bakery!
Minimal, Feb 03 2006
  

       //This is such a great and simple idea that it feels like it must be baked somewhere// Down the edge of virtually every movie film since the 1930s?
coprocephalous, Feb 03 2006
  

       The idea is not movie film, it is joysticks.....   

       Why do people always say things are baked in other circumstances? If I suggest an electric dunce-cap, I don't get anno's saying that electricity is already baked!!!!   

       And the idea is Optical Pot, we know hall pots exist!!!
Minimal, Feb 03 2006
  

       The soundtrack pickup in a movie projector is an "optical pot" which varies a voltage based on the width of a "groove", ergo baked.
Besides - it isn't that simple - you'd need a reference detector to null out differences in light output of the source.
Years ago, linear optical pots using triangular slits and pairs of detectors were used for positional feedback in floppy disk drives, before stepper motors became common.
coprocephalous, Feb 03 2006
  

       I didn't know Hall pots existed until I went and looked it up. I'm familiar with Penny and Giles, though.   

       Although I agree that claiming an alternative exists as a reason that the proposed invention is a bad idea is irksome but when an idea is prevented as non-trivial, I tend to search for a differentiating benefit. That Hall effect devices are not immune to RF--as an optical device may be--is a good value-add but the vibration and dirt immunity are not distinguishing yet are the points of emphasis in this idea.
bristolz, Feb 03 2006
  

       This would be easy to make by using the components out of an optical mouse. The old ones with balls used pots which when they got dirty made the mouse pointer jump around on the screen, the optical ones work so much better. Great Idea.. Bun for you
dlapham, Feb 03 2006
  

       Older mice use digital relative direction sensors, not absolute analog sensors. Besides, they still work better at tracking. If I try to make a quick movement with an optical mouse, the cursor jerks everywhere.
Aq_Bi, Feb 04 2006
  

       I'm with Rayford - two polarisers, one fixed and the other moving, would achieve this very well. I would also use two photodetectors, one receiving unfiltered light and the other filtered, so that the system compensated automatically for the degradation of the LED over time.   

       However, as it would put out an analogue signal, the output voltage would be susceptible to RF interference.
david_scothern, Feb 04 2006
  

       When I was about nine or ten I decided to repair an old electric organ that was in the school hall - seemed like an interesting project. I don't remember how far I got, but I do remember that the swell/volume pedal had a curved piece of plastic protuding from the underside with steadily increasing width. This descended between a small filament lamp and a photoresistor in such a way that all light was cut off when the pedal was fully depressed. The first thing I did was replace that lamp which enabled it to make some (admittedly dreadful) sounds.
wagster, Feb 04 2006
  

       <obligatory drugs reference>
dbmag9, Feb 04 2006
  

       This system would be sensitive to light interference. It would need to be sealed, or changed to infra-red.
Ling, Feb 04 2006
  
      
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