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I'm a beginner with electronics. I'm trying to replace a softpot on a cheap synthesizer with a regular potentiometer. As far as I can tell this component (pictured below) behaves just like a 10K pot; leads 1 and 2 are like the outer leads in a pot and always measure 10K, between 1 and 3 the resistance varies linearly depending on where you touch along the strip:

enter image description here

I soldered a ribbon cable to a small breakout board (pictured below), and it is at this point that things start to go wrong.

enter image description here

  1. If I connect a 10K pot to the corresponding leads (before powering the synth on) The synth plays only the highest and sometimes second highest note as I sweep through the range
  2. if I disconnect everything from the breakout board the synth doesn't play, but then strangely if I touch my finger or an alligator clip to only terminal 3 ( with no other part of the circuit connected) it plays while the alligator clip is touching….

I have double checked for shorts from the breakout board to the terminals on the synth circuit board using the continuity checker on my multimeter and it all looks fine.

So I'm looking for tips to help debug this, but also I'm very curious how (2) above could be happening

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  • \$\begingroup\$ What's a "softpot"? \$\endgroup\$
    – Hearth
    Commented May 30, 2023 at 2:08
  • \$\begingroup\$ @Hearth maybe the proper term is "ribbon sensor"? It's something like this, though the component used here isn't made by them afaict: spectrasymbol.com/linear-position-sensors/… I don't see any markings on the one I'm dealing with \$\endgroup\$
    – jberryman
    Commented May 30, 2023 at 17:55
  • \$\begingroup\$ You need to provide more info. Where exactly do you measure 10K? Where do you put your finger (for example) and what do you measure (& where)? Do you easure with a multimeter? What setting of the multimeter? etc. etc. \$\endgroup\$ Commented May 30, 2023 at 18:46
  • \$\begingroup\$ @StefanWyss I'm using a multimeter to measure resistance, yes (mine has one setting for that), and using the mode that beeps to check continuity where I say "I have double checked for shorts". The other stuff seems clear from the question to me, but maybe I'm misunderstanding. Lead 3 is connected to wiper, you move your finger along the strip and resistance varies between 1 and 3, and between 2 and 3, just like a regular pot \$\endgroup\$
    – jberryman
    Commented May 31, 2023 at 12:27
  • \$\begingroup\$ One thing that puzzles me about your description of the soft pot functionality: Is it true that it works if you bridge terminals 1 and 3 with your finger alone? The resistance of skin is multiple kilo Ohms, so this bridge is very weak!? \$\endgroup\$ Commented Jun 1, 2023 at 18:40

1 Answer 1

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So uhhhh... it turned out it was actually one of the brand new alligator clips connecting the pot to my breakout board that was totally broken (I had even checked the range of the pot I was testing with using two of the three clips...). So, working as expected now.

Stefan Wyss explains what's probably happening in my second question:

It seems that terminal 3 is an input with very high impedance. In this case your body acts as an antenna and picks-up noise, mostly from the AC power outlets nearby. You can see the same effect if you touch an oscilloscope probe tip.

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