Sunday, September 20, 2009

The Wrong Vactrols


So I was all set to prototype the control voltage input circuits for the Super Psycho LFO rebuild. The basic idea is to feed the control voltage to the LED half of a Vactrol, and put the light-dependent resistor (LDR) half of the Vactrol in series with the rate pot of the specific oscillator (so there would be 6 Vactrols total). I have some VTL5C7's that I've had laying around from a group buy I participated in on Synth-DIY several years ago. I've never done anything with them.

So I went to the data sheet to find out how much current would be required to make the LDR vary between about 0 and 100K, which seems like a good operating range. (You don't want to get into the high resistances because the data sheet says the response is not specified above 1M or so.) And I saw these curves:


Note in particular the turn-off curve. If I want to bias it to operate between, say, 1K and 100K, then the turn-off time from full on is about half a second! Obviously, if you feed the input something like a 10 Hz square wave, the output is going to flatline. Never mind modulating at audio frequencies.

I know this is a fundamental characteristic of LDRs. However, Perkin-Elmer does make different types. On the same data sheet are the specs for the VTL5C6. Its turn-on and turn-off curves look like this:


Note the difference in the time scale. This one has a turn-off from 1K to 100K of 2 milliseconds. That's more like it! Trouble is, I don't have any of these. My usual go-to parts source, Mouser, doesn't carry Perkin-Elmer. Google showed me that Allied carries them, and I checked and they have hundreds in stock. So that's on order, and I expect them Wednesday or Thursday. Until then...



1 comment:

God Of Bacon said...

And this is related to the mating ritual of the wombat how?