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Make potentiometer more sensitive.

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Pommie, Dec 27, 2017.

  1. Pommie

    Pommie Well-Known Member Most Helpful Member

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    I want to make the middle of a pot more sensitive. The pot will be connected between 0 and 5V and when the pot is centered the output will be 2.5V. I want to make the bit either side of center to be extra sensitive such that 3V on the pot gives 5V out and 2V on the pot gives 0V out. So, 2-3V in maps to 0-5V out.

    Suggestions.

    Mike.
     
  2. dknguyen

    dknguyen Well-Known Member

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    Maybe two comparators that each drive a transistor to short the wiper to the rail/pot end? Unless you can find a transistor output comparator like the LM311.
     
    Last edited: Dec 27, 2017
  3. Beau Schwabe

    Beau Schwabe Member

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    You can do it with a dual Op-Amp ...
     

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  4. dave

    Dave New Member

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  5. Pommie

    Pommie Well-Known Member Most Helpful Member

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    Perfect. Just emulated in LTspice and it does exactly as asked.

    Thanks,

    Think I've got a few LM358 that should surfice.

    Mike.
    LTspice.png
     
  6. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Try a single opamp with a non-inverting gain of 5.

    : t3.png

    The independent variable of the simulation is pot position (x-axis). The dependent variables plotted are V(wiper) and V(out).

    Gain is 1+200K/(100K//100K) = 1+(200K/50K) = 1+4 = 5 centered on 2.5V , so 3Vin = 5Vout and 2Vin = 0Vout.

    Try it for any reasonable value of pot resistance...

    It does depend on the opamp being rail-to-rail out, which an LM358 is not. If all you have is LM358s, then you will need more than a single 5V supply.
     

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  7. dknguyen

    dknguyen Well-Known Member

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    whoops. For some reason I read this as leaving 2-3V unchanged but snapping to the nearest rail beyond that.
     
  8. Pommie

    Pommie Well-Known Member Most Helpful Member

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    I assume the LM358 isn't suitable due to the max output voltage being Vsupply-1.5 which would only allow me to measure inputs from 2-2.7V. This will probably do for my situation but just in case I can get hold of an LMC6482 locally which should manage the full range.

    Mike.
     
  9. Beau Schwabe

    Beau Schwabe Member

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    ...Of course !... I was focused on level shifting/dividing the input first and then amplifying. Use the single Op-Amp solution.
     
  10. Pommie

    Pommie Well-Known Member Most Helpful Member

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    Sorry MikeMI, missed your post earlier. Thanks.

    Mike.
     

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