Reading control voltage

[jim68000]

Hi

I'm thinking about adapting a Fuzz Factory to work with modular synths. Specifically I'd like to control the Stability pot using a CV input, as the Stab control gives the widest range of oscillation once the pedal is oscillating. The puzzle I have is how to convert a 5V CV input into resistance. If I had control over the input in terms of current I'd use an LED/LDR, but as I understand there's no guaranteed current associated with a CV signal, just voltage. So what's the simplest way of using CV input? Add the CV to a transistor amplifier and raise the current that way? Use a FET? Any ideas gratefully appreciated

Jim

[GibsonGM]

I'm going to watch this one, as I'm interested in this, too.   From my (1 hour) of research on it, it appears to be a little more complex than we thought    
 
Depends on if you need to have that resistance be JUST a variable resistance, in which case your LED/LDR might work great - or if you need a pot, which is much harder to come up with.   I found that the LM13700 has been used to 'make a pot', but it involves more circuitry than most would want to put together (see the data sheet...).     

I'm hoping that someone out there has found a hack for using TWO LED/LDR's, out of phase, to simulate a pot in an ANALOG way rather than digital  (as digital pots require clocks, more lost real estate, etc).   Don't know if any of this applies to your issue tho, Jim.

[jim68000]

The Fuzz Factory Stab control is just a rheostat so a single LDR will do fine. This all started because I bought a Digitech RDS 3.6 from eBay and it turns out to have a CV control for the delay time. Making a CV pedal turns out to be easy: it's just a pot in series with a resistor to clamp the voltage and a 9V battery; meanwhile I have longstanding request from a friend for a Fuzz Factory with an expression pedal input. The problem being I don't know what kind of expression pedal he's going to put in there so it needs to be generalisable, which is where the idea of using CV came from. The LM13700 looks like it might be useful. I've spent the morning trying to understand synth module schematics attempting to spot the CV conversion stages nut without a lot of luck but now I have something to look for.

[Kipper4]

Maybe something like this.

http://www.diystompboxes.com/smfforum/index.php?topic=107812.0

A quick search for out of phase LEDs

[anotherjim]

Yeh, something like this...

LED controlled in op-amp loop.
The amp here is a single supply type that can sense and drive to ground. Aka LM358,LM324...
I think + supply of amp needs to be at least 1.5v higher than max CV to follow it all way.