Make potentiometer more sensitive.

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Pommie

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• Dec 27, 2017

• #1

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.

 

dknguyen

Well-Known Member

Most Helpful Member

• Dec 27, 2017

• #2

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

Beau Schwabe

Active Member

• Dec 27, 2017

• #3

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

 

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Pommie

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• Dec 27, 2017

• #4

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

Thanks,

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

Mike.

 

MikeMl

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Most Helpful Member

• Dec 28, 2017

• #5

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

:

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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Last edited: Dec 28, 2017

dknguyen

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• Dec 28, 2017

• #6

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

 

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Pommie

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• Dec 28, 2017

• #7

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.

 

Beau Schwabe

Active Member

• Dec 28, 2017

• #8

MikeMl said:

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

Click to expand...

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

 

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Pommie

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• Dec 28, 2017

• #9

Sorry MikeMI, missed your post earlier. Thanks.

Mike.