[sdiy] Vactrols subbed for dual-gang pot

[lanterma at ece.gatech.edu]

On Jul 10, 2010, at 12:39 PM, cheater cheater wrote:

> Gotcha. So the LDR works as an LPF. I wonder what the transfer
> function of that is?

Hmmm... "transfer function" analysis, as you are using the words, presumes linearity, and I think it would be pretty tricky to try to characterize the LED/LDR drive combination that way. It's probably better thought of a "slew," as in op amp slew (which can lead to frequency response sort of calculations), but even then there's probably all sorts of weird nonlinear and asymmetric behaviors going on. 

I did have a student try to build a weird nonlinear filtery kind of waveshaper by exploiting the limited vactrol slew:

http://users.ece.gatech.edu/~lanterma/studentprojects/andrew_SSM01_manual.pdf

>From the student's writeup:

"I don’t blame you if you still don’t exactly understand what the circuit is supposed to do. I think it best described as a low pass-ish, waveshapey thinga-mabob..."

Come to think of it, this is probably a similar notion to the Polyvox filter:

http://m.bareille.free.fr/modular1/vcf_polivoks/vcf_polivoks.htm

- Aaron