I've got a 3V DC toy motor that I'd like to convert to variable speed using a PWM circuit. I found an $5 PWM controller that would work. This expects a 100 K\$\Omega\$ potentiometer as its speed input.

I want to set the speed using a mechanical trigger (like on a variable-speed drill), but that means I have an input that travels only 5-10 mm. The potentiometers I've found are rotary (270° travel) or linear/slider (20 mm travel). If I were to use these, I'd only be able to use part of the 0-100 k\$\Omega\$ range.

How should I approach this?

  • create a mechanical system to convert the trigger travel into the full range of the pot,
  • is there a way to get the full rpm range out of the PWM circuit using only part of the potmeter range?
  • or should I build a custom PWM circuit instead?
  • 2
    \$\begingroup\$ What range does your PWM controller expect? 0-100k? then buy a 0-200k and use only half of it. \$\endgroup\$
    – PlasmaHH
    Dec 6, 2016 at 10:28
  • \$\begingroup\$ If you want "variable speed" then you need a "once per rev" sensor. "I have an input that travels only 5-10 mm" - I don't understand the relevance given you want "variable speed". Your question is confusing. \$\endgroup\$
    – Andy aka
    Dec 6, 2016 at 10:33
  • 1
    \$\begingroup\$ @Andyaka: I think he means that he wants a mechanical trigger with 5-10mm travel that he uses to control the PWM with his fingers. \$\endgroup\$
    – PlasmaHH
    Dec 6, 2016 at 10:35
  • \$\begingroup\$ @PlasmaHH: correct. \$\endgroup\$
    – Hobbes
    Dec 6, 2016 at 10:36
  • 1
    \$\begingroup\$ Actually you could do what they put on Wha-Wha pedals, its a rotary pot on the inside with a cog, then a rack with teeth that turns the cog. Think of it like a rack and pinion style. Actually just google rack and pinion potentiometer and you'll see what I mean. \$\endgroup\$
    – crowie
    Dec 6, 2016 at 14:53

1 Answer 1


I would use a slider something like this; Slider potentiometer. I think it would be easier to make a linear mechanism than a rotary one.


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