[sdiy] An LDR predicament...

[Paul Higgins]

Hi,

IIRC, those dual Vactrols _share_ an LED, so they track (hopefully) 
identically.  That's your problem.  You have to do what Mesa did and 
run two series connected LDRs (voltage divider), driven with opposing 
CVs.  I think the deal with those dual Vactrols was to have two matched 
LDRs that tracked the same CV, so that you could use them independently 
in two different parts of the circuit.

HTH,

-PRH

On Sunday, January 25, 2004, at 02:47 PM, Paul D Brooks wrote:

> all this talk of LDR's brings up a question that i have been working 
> on for a while now... i am making a ring mod / wah pedal for a friend 
> of mine (was supposed to be a christmas present).  the dual 
> opamp-based vco was easy enough to get working with an LDR being 
> controlled from a voltage produced by the pot connected to the > pedal...
>  
> the wah side of the pedal was quite different.  the dual vactec 
> vactrols with three pins on the LDR side do not operate like a "three 
> wire pot"; the two resistors both increase in resistance and decrease 
> in resistance as the voltage going into the LED side is varied.  ok... 
> so i started working with two vactrols for the wah circuit instead... 
> and changed the pot to produce a CV from -9V to +9v off of the wiper.  
> well, for one thing, the wiper does not produce even remotely the 
> entire range... more like +4 -> -3V... what am i missing here?
>  
> also, the two led's seem to want to be either on or off... very little 
> effect is found in between.  do i need to bias them with some fraction 
> of the high or low rails?
>  
> basically, i'd like to produce a small circuit that acts as a 250k 
> pot, based on the CV produced by the actual pot in the pedal.  the 
> "on" resistance of around 400 ohms does not really bother me, but... 
> any ideas?  even messing around with spice programs does little to 
> provide solutions for me, it just clarifies that i'm missing > something!
>  
> thanks for any help!
>  
> (and yes, Mesa uses lots of vactrols... i used to be one of thier 
> authorized service centers, and i can tell you that they love them...)


Paul R. Higgins
email: higg0008 at tc.umn.edu