Optical Pots, anyone?

Ok, here is what I want to do. I have a cheapo Dan electro Echo pedal and a Morley wah wah pedal. I want to pull the wah wah guts out and put the echo guts in, using the pedal to control the rate of the echo.

The catch is that Morley used an optical system instead of a regular pot. I want to build something that I can use to simulate a 10k pot with an LED light source and a photo cell. I've searched the web but I'm not really finding example circuits. I'd like to power this with just the same 9V that the echo pedal uses and have it be independent from the echo pedal itself so I can wire it into the place where the pot goes now.

Am I dreaming here? I only want to do this because I have some stuff here that I don't like as is and want to make them into something I think I might use.

Matt

EDIT: Here is the Morley Schem that uses the optical pot.

http://www.morleypedals.com/pwa-iies.pdf

EDIT #2: I should mention that there is a plate on the pedal that varies the amount of light that reaches the LDR. It has a slot that varies in width that lets the LEDs light get through at varying rates depending on pedal position.

here is an example of a pot replaced by a ldr.
http://geofex.com/Article_Folders/wahpedl/wahped.htm

Thanks for that. I think it should do pretty well if I can figure out how to set the max ohms. I'm guessing that just putting a trimmer in place of the wah pot would allow me to adjust the brightness of the LED and then I'd just have to adjust that to taste?

Cheers

Matt

I`m not sure if you can do that , the resistance range of a ldr is of some few ohms to mega ohms if you want to reduce that range to some fews ohms to 10k you could have tolerance problem(temperature, battery voltage, etc.

do you have a schematic of the dan electro pedal? if it use the pot just as voltage divider you can use the R.G.Keen solution. if it is a variable resistor is a bit more complex. Maybe you can try with a 10k resistor in parallel with the ldr.

the marshall jmp1 use two ldr to simulate one pot but I`m not sure how they do it, high speed pwm maybe.

To reduce the maximum value of a LDR you simply put a resistor in parallel with it...

The "pot" law you're needing/getting is another matter.

Samuel

A simple way to get any pot law you like is to use a center-tapped vactrol like the VTL5C2/2 and implement the lower half of figure 10 (page 66) of this Vactrol datasheet. By placing one half of the vactrol in the feedback loop of an op-amp, the other half's resistance curve should be reasonably controllable (within the min/max resistance of the vactrol, naturally). The schematic you've linked shows a ground-connected LDR, so this scheme should work.

JDB.
[but then, why not simply use a pot? It's not like you need the isolation]