[sdiy] Vactrols subbed for dual-gang pot

[Harry Bissell]

That would be the same as the guitar effect Mutron III. you should
be able to see schematics on the web that detail how they did it. They
used a dual photocell vactrol with a single LED.

I'd make up a fixture to measure the cell resistance by having two current
sources or bias resistors to make a divider, and send the outputs to a scope
as an X-Y plot. Now you could choose cells to have the best 'linearity' over
the range instead of just the endpoints.

If you did get dual cells, you might use the technique in National Semiconductor
AN-20 "Analog Multiplier". I've used that to replace a resonance pot in a 
state-variable filter (to get VC resonance).  It works well, but use an LED rather
than the lamp. The lamp has hysteresis problems and a lot of thermal lag that makes it
unstable at really low illumination (high resistance values)

H^) harry

----- Original Message -----
From: Dave Kendall <davekendall at ntlworld.com>
To: sdiy DIY <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 09 Jul 2010 13:23:20 -0400 (EDT)
Subject: [sdiy] Vactrols subbed for dual-gang pot

Hi All.

Had an idea to replace the dual-gang FREQ pot in the CGS30 band-pass  
filter with a pair of home-rolled Vactrols driven from the output of an  
opamp buffer for CV control of frequency.  It doesn't need to be  
massively stable, as I'm expecting to use it with a modulation source,  
rather than to set a fixed frequency via CV. Details of the BPF circuit  
here ;  http://www.cgs.synth.net/modules/cgs30_bpf.html

To the best of my knowledge, the CGS30 circuit is very similar to the  
Roland SH5 BPF.

By hand-selecting LED/LDR pairs, I can match the off resistance of the  
home-rolled vactrols to within a few ohms, and limit the max resistance  
fairly easily, but between those points, I suspect the LDRs may "drift  
apart" due to different response curves to incoming CV. How would this  
affect the BPF circuit? Does it even matter?

FWIW, I used white or red superbright LEDS with these LDRs and got the  
"off" resistance down to around 130 ohms in the best cases.  (white  
LED)

http://www.rapidonline.com/productinfo.aspx? 
tier1=Electronic+Components&tier2=Optoelectronics&tier3=Photodetectors&t 
ier4=Miniature+light+dependent+resistor&moduleno=34796#techspec

Any and all thoughts welcome....

cheers,
Dave

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-- 
Harry Bissell & Nora Abdullah 4eva