# Potentiometer for PWM motor controller: how to deal with a limited input range?

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?
• What range does your PWM controller expect? 0-100k? then buy a 0-200k and use only half of it. – PlasmaHH Dec 6 '16 at 10:28
• 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. – Andy aka Dec 6 '16 at 10:33
• @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. – PlasmaHH Dec 6 '16 at 10:35
• @PlasmaHH: correct. – Hobbes Dec 6 '16 at 10:36
• 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. – crowie Dec 6 '16 at 14:53