Passive Vactrol LPF

In this module I used a DIY Vactrol to build a voltage-controlled low pass filter.


Vactrols are made by coupling an LED and a photoresistor (LDR) together using either heatshrink tubing, clay, or electrical tape. In my case, I used black Sculpey polymer clay. I’ve also had good results with black heat shrink tubing.

A control voltage input can be used to turn on the LED, the brightness of the LED would then control the resistance of the LDR. In a lot of cases, this can be used a drop-in replacement for a resistor or pot in a synth circuit.

 Picture of a DIY vactrol and it’s components


CV_AMOUNT controls the brightness of the LED, which in turn controls the high frequencies that get through. CUTOFF controls the darkness of the sound when no CV is received though the CV input.

 Schematics for the Passive LPF

Let me know if you’ve built it! Tag me on IG @benjiaomodular.


Here’s a demo of my Vactrol LPF along with the Oscillator Bank I wrote about about in my previous post.


Check out Kristian BlĂ„sol’s video in his Modular in a Week series where he showed how to create a DIY vactrol at home: