[sdiy] Vactrol control ideas wanted

[jhaible at debitel.net]

Depends on what you want to do.
If you want accurate V/Oct tracking, the feedback method
might not be good enough. OTOH, if approximate tracking
is all you need, why not just use the Vactrol LEDs 
as en expo converter? Connect them all in series, and
drive them with a _voltage_, not a current. 
Don't forget a current limiting resistor, and don't 
forget to include that resistor into the feedback
loop of your driving opamp (or you'll be bak to 
current control).

JH.


> Hiya all...
> 
> I'm planning a vactrol based phaser module (yeah, I know...
> how original :^)
> 
> I have a couple of ideas for controling the Vactrols.. like to have
> any feedback. Maybe you've been there, done that...
> 
> 1)  Control all the Vactrols with a series current (possibly including
> the
> LEDs in an opamp feedback.
> 
> pro: its easy
> con: it probably does not cause the interesting part of the sweep to
> occur
> over a wide range
> 
> 2) Control the Vactrols by running them all in series, with one
> Photocell fed
> back into the control circuit.  This is an adaptation of the National
> Semiconductor
> "Analog Multiplier" found in AN-20.  However, this version will not have
> one LED
> illuminating multiple photocells.
> 
> pro: (subject to matching) the photocell resistance can be accurately
> specified
> con: its more complex
> 
> Your thoughts ladies and gentlemen ???
> 
> H^) harry
> 
> 
> 




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