Mechanical Stop for Potentiometer

Discussion in 'The Projects Forum' started by nathomas, Mar 4, 2011.

  1. nathomas

    Thread Starter Member

    Mar 3, 2011
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    Hey I have this project where I need the potentiometer to rotate only a certain amount. i heard about mechanical stops but the potentiometer company says that it would cost atleast $10K to make changes to that. any ideas. i cannot change the box holding the potentiometer as it has already been designed and made.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Do you really need a limited range of physical rotation (tough), or do you need a limited range of resistance, like 0-3K with a 5K pot (easy)?
     
  3. nathomas

    Thread Starter Member

    Mar 3, 2011
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    I need a limited range in physical rotation. i did see that soldering a piece of wire might work but I was looking for a permanent solution.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Use a fixed resistor or trimpot in series with the end you want limited. That way you get full pot rotation for maximum sensitivity, and only requires adding one cheap part.
     
  5. nathomas

    Thread Starter Member

    Mar 3, 2011
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    well the resistance part of it is fine. but the pot cannot rotate 300 deg due to design limitations. it can rotate maybe 90 deg. i want to limit that on the pot with a stopper of some kind.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I suppose one way to do that would be to have a custom knob made that had a semicircular track cut in the underside that rode over a pin/stud/rivet on the panel; that would make it not visible. Getting custom knobs like that made could be expensive.

    Rather than a pot, you might consider a rotary Hall-effect sensor like those used in automotive applications; there are a number of them designed to rotate only 90 degrees for throttle position sensors. You feed them +5v and ground, and they output a voltage between 0v and 5v.
     
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  7. someonesdad

    Senior Member

    Jul 7, 2009
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    I concur with SgtWookie's pin/cutout method, as it's the first thing that popped into my mind. If you're modestly mechanically able, it shouldn't be too hard to do with an appropriate knob. I'd look into drilling a hole into the knob, then pressing a pin into the hole. I'd fill things with epoxy if I could to help hold the pin in place. If I was doing it, I'd just make my own knob from aluminum or brass, as it's simple lathe and drill press work. Made from metal, it would be trivial to just drive a spring pin into a drilled hole and construction is finished.

    For the arc-cutout in the panel, just scribe what you want and drill some holes and file to shape. This is easy and is done all the time by hobbyists. You may have to buy a suitable round needle file; look for one that's reasonably coarse -- most sold are too fine; look for almost a woodworking rasp if your panel is aluminum (if it's steel, you'll need the finer files).

    If you have a small router or Dremel tool, it's not hard to make a simple fixture that indexes on the hole for the pot's shaft and cuts the arc in a few passes.

    The hard part of all this is to do the measurement and layout so that things work the first time when you cut and assemble. It gets easier after a few mistakes though. :p

    If your pot doesn't have a shaft with a flat on it, then things are easier, as you can position both the pot and the knob at final assembly (unless you've got a small hole to keep the pot from rotating). If the shaft does have a flat, then measuring and layout are going to be important.
     
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  8. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    What does the pot look like? Photo? Is it the type of back cover that's held on with bent-over fingers? It might be possible to remove the back, and use epoxy to build up a stop-block inside the back cover.

    Ken
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Sort of a combination of the ideas so far; drill a single hole in the underside of a stock pot knob and insert the roll pin like someonesdad mentioned, and make a plate with either a couple pins sticking up, or some bend-up tabs to go under the pot nut.
     
  10. nathomas

    Thread Starter Member

    Mar 3, 2011
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    Thanks everybody for the suggestions. I am looking at the rotary effect hall sensor idea. The problem with making small mechanical changes would be that I need to make more than one of these devices and I need a way that would not need me to make changes on all of them individually. I ll look into these ideas and thanks once again.
     
  11. nathomas

    Thread Starter Member

    Mar 3, 2011
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