Vactrol Breakout

last updated: Mar 14, 2023

14-mar-2023 > proto 0.1 boards arrived, and assembled! photos 2-mar-2023 > proto 0.1 boards ordered, stay tuned

This little PCB breakout is set up to be used an re-used, for prototyping, or modular designs. It's kinda nice because you can just care about the 5-pin header and the rest of the vactrol circuit is just kinda taken care of.

The values for the resistors are all blank because you have to adjust them a bit. I'll fill in some values as I find sweet spots for the vactrols on hand. (And some of those will for sure be home-made!)

This design uses an MCP6004 since it can go rail-to-rail, and given some designs it's useful if you've already got a 5V power rail from a microcontroller or something. Pin-wise you could totally drop in a TL074 and that'd probably be fine. You'd really have to figure out the R values because you'd be dealing with larger range of voltage.

Although not perfect, it seems like a decent option that will at least provide a bit of stability and consistency to most simple designs that use it.

Info

input signal from 0-5V
CV input from 0-5V
output signal from 0-5V
a buffer on the input signal, so it won't mess with anything upstream
a linear configuration for the vactrols' LED (see: article, simulation)
output buffer
signal output LED indicator

caveats

your cv will be limited to the power rails of your op amp (and also the limits of your op amp, so beware you can fry it)
negative voltages might do weird things or fry your op amp
there is NO protection in this circuit
you'll probably want pulldown resistors on the inputs

Les Instruments