A friend have a Zoom 505 and asked me about to make a expression pedal. I told him that the easiest way is to use a LDR and control the effects he want using "shadows and light". He liked the idea, but the LDR have only two legs, and the pot in Zoom expression pedal use all the three legs, so I'm wondering how I can adapt the Zoom expression pedal to use a LDR. I need to do this with a stereo plug, like in the Zoom expression pedal. Maybe if I use two LDRs the job will be done easily.

Hi.  Sounds like the pot is used as a voltage divider.  So attach a fixed resistor to the LDR.  Try a 100k for starters.  That'll give you a range of resistance ratios from 10M/10.1M in the dark to about 10k/110k in the light.  cheers

But how I will do this? The Zoom expression pedal is like this (hope ascii works):

to tip
|
|
\
/<------ to ring
\
|
|
to sleeve

I'm thinking about this (not sure if will work):

to tip and ring-------\/\/\/\/--------to sleeve

Or this (this I think will work):

to tip----/\/\/\------to ring------/\/\/\-------to sleeve
Using both LDRs at the same time.

I'd use your diagram at the bottom, but replace the "tip to ring" LDR with a 100k resistor.

ie tip - 100k - ring - LDR - sleeve

That way, covering the LDR (increasing it's resistance) will be like turning a pot to maximum (the ring voltage will approach the tip voltage).  Uncovering the LDR, the ring voltage will approach the sleeve voltage.

hope that helped

Thanks, this helped a lot. I will give a try.

Take two LDR's and series them, use one end of this ckt as lug1 where the two LDR's tie together as wiper and the other LDR's lead as Lug3...
 Mount them far enough apart so that one or the other can be covered?

The two LDRs in series will work but you have to change the intensity of each LED in direct opposite proportion in order for it to simulate the behavour of a pot.

Check out the Rock 'N Control article at the site.

http://www.geocities.com/thetonegod/

Andrew

I once did an attenuator, put two 33k resistors in series and two leds, one from end of first resitor to ground, and other from second resistors end. Like

input - 33k series - 1st ldr other end - 33k series - 2st ldr other end - output, and connected ldrs other end to ground.

The resistors maybe were something other value, 100 k might be better, depens what the ldrs are. I saw this kind of circuit in some opto compressor, maybe adds little noise but gives more "range"