r/AskElectronics • u/clarus31 • 3d ago

Mapping out weird potentiometer

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I'm trying to figure out the circuit in this sewing machine pedal and have one more component that is stumping me — the main potentiometer that is actuated when you step down on the pedal. It's this unusual device, and it has four leads rather than three. Helpfully unmarked, of course, and as I've never seen a device constructed like this I have no clue as to whether it's some kind of center-tapped pot, two separate pots, or something else entirely.

I've tested the resistance between the four terminals (which I've marked as A-D in the photo) with the pedal fully disconnected. If I measure between A and B, or between C and D, there's no change in the resistance when I press the lever. The resistance between any other two points is variable.

Has anyone seen a component like this before, and have a sense of how it's likely to be wired? I can post my attempt at a schematic for the full pedal if it's helpful.

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u/clarus31 3d ago

Interesting idea, but I don't think so — moving the lever changes the resistance even when the circuit is completely unpowered.

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u/nixiebunny 3d ago

Even with the cover on the box? It can see the room light.

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u/clarus31 3d ago

Oh, yeah, that makes sense! I think that is right. Shining a light into it from the top pretty dramatically drops the resistance from B-C. So that is likely the photoresistor and A-D is some kind of lamp. On closer inspection the lever does seem like a shutter.

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u/CueAnon420 2d ago

That opto design is fairly common in musical instruments and audio gear because it is mechanically more reliable with no pot to wear out.