I was watching the Kristian Blåsol “Synthesizer in a Week” series and realized I had a couple of unspoken for photoresistors that could possibly be turned into vactrols for a passive VCA.
I would rather not waste them on a project that wont work out - is there any reason I couldn’t make a kosmo module out of it? Or maybe there is a better use for them?
Strange schematic. It shows only three terminals for the vactrol. The LED cathode should connect to ground. As for using a series LDR to attenuate a signal, its effect is going to depend greatly on the impedances of the input and output devices. If you do build this pay attention to the LED current. As shown, with the pot all the way up, there’s 330R in series with the LED and depending on how large your CV is that might draw more current than your LED is happy with. It also might try to draw more current than your input module is able to deliver — impedances again, this input impedance is very small.
Altogether it might work fine in a particular application but it’s very simplistic for a general VCA module.
There is a better version of the schematic at this timecode:
That’s an entirely different circuit. It’s not a VCA. It’s a super simple oscillator with the vactrol used for pitch CV. The LDR is in parallel with the pitch potentiometer rather than in series between a module input and output. The LED lacks a current limiting resistor entirely, which isn’t great. Essentially it’s this oscillator with a vactrol attached:
(from https://www.lookmumnocomputer.com/simplest-oscillator).
Apart from the remarks made by @analogoutput only if R14 has a low value the LED will light up (if it is connected to GND), but then it will draw say 5 to 10 mA and that current is too high for the potentiometer. It will probably heat up too much and burn. Potentiometers are not made to withstand that kind of current.