Expression pedal jack (modification to replace internal pot in circuit)!

[quietchannel]

Hello all! This is my first post here!
I'm working on a modification to add an expression pedal jack on a commercially available fuzz pedal. It uses a 50k pot from the positive rail to vary a transistor's bias.

I assume I'll need a plastic neutrik jack to keep the expression pedal's pot isolated from ground? Also, most expression pedals use different pot values-- can anyone think of some sort of universal solution?

I'm open to using an op-amp, opto-isolator, or whatever it may take...

Is it somehow possible to use a pot with 0ohms from +9v connected to one side and 50kohms from +9v on the other, with the wiper running to the transistor bias?

Any ideas??!

Thanks!!
-Robert

[smallbearelec]

If you mean to replace the internal control with swell-pedal control, a TRS stereo jack

http://smallbear-electronics.mybigcommerce.com/1-4-in-stereo-12b/

mounted with shoulder washers

http://smallbear-electronics.mybigcommerce.com/fiber-shoulder-washer/

would give you fully isolated contacts. If you intend to bypass the internal control when the pedal is plugged in, things get Much more complicated.

What pedal are you working on? Is there room for another jack?

[quietchannel]

Thanks for the reply!
Yes, I've installed a TRS (stereo) jack. I plan to use a stereo cable and an external swell pedal (b 20k pot). I have a plastic neutrik stereo jack with NC switched terminals, allowing me the option to bypass the original (b 50k) pot when the expression pedal is plugged in. So, I think that should all work mechanically!

The issue now is scaling the 20k to have the range of the original 50k--  and making it more universal so I could use any expression/volume pedal. Also, what if I want to externally control the 1M rate pot on my phaser, for example? I'm seeking some sort of universal expression pedal retrofit.

Is it possible to add a daul op-amp or an LED/LDR circuit to effects pedals for scaling the signal coming from expression swell pot? Maybe with an internal trimmer to set the range? I guess the real crux of my question is the ability to scale the swell potentiometer.

[quietchannel]

Does anyone know of a more in depth article than this??

http://www.geofex.com/Article_Folders/xprnped.htm

He mentions the necessity of an LED/LDR for certain situations.. But no details.