oem expression pedal with common pot

[beatnik]

Hi everyone,

I am working on a little project that requires a foot operated speed controller. The potentiometer value should be 1M.

Initially I thought I could use an old Crybaby shell I had laying around, but the potentiometer in that is 100K and it has a long d shaft with a groove in order to fit the pinion gear. Unfortunately I haven't had any luck finding a 1M potentiometer with a long d shaft.

The next thought was to find a cheap expression pedal on ebay, but from what I understand the typical resistance values are 10K and 50K. Again I will have to change the pot and from the photos online most of them seem to use special potentiometers like on the Crybaby.

Was wondering if someone can recommend an expression pedal that uses a standard potentiometer so I could easily replace it with a 1M value

[PRR]

> The potentiometer value should be 1M.

Why?

Is this an SCR/Triac dimmer?

[ElectricDruid]

> The potentiometer value should be 1M.

Why?

I think PRR's onto something. Changing the pot is only one way to fix the problem. Changing the circuit so it can use the pot the pedal's already got is the other way. So what's the circuit?

[beatnik]

yes I think is based on a triac. the circuit is extremely simple, only a handful of parts. I'll trace the circuit board and post the schematic asap

[PRR]

> the circuit is extremely simple

They all are; although there are multiple complications to improve response.


https://learnabout-electronics.org/Semiconductors/thyristors_64.php

As that text says, the values of R2 R1 and C1 depend on each other. To some degree you can change them all in the same direction (of impedance) and it will work the same.

However I am VERY concerned about bringing 120V into a stomp/wah pedal. It takes very sturdy work and wiring to avoid shocking the user. And there ARE pre-engineered and tested solutions at very reasonable cost.
https://www.sciplus.com/foot-pedal-speed-control-120-vac-16a-max-43485-p  surplus 200W job
https://www.amazon.com/Control-Variable-Speed-Pedal-Plastic/dp/B00WGVP6Q4  work-grade 400W

[ElectricDruid]

When the OP said "speed controller" I thought we were going to be looking at an LFO circuit. I was wrong!