Vactrol and LFO Arrangements

by Michael Una.

This is the first of an occasional series explaining particular solutions for circuit bending and synthesis.  Here’s a little series of tricks I commonly use.  First, consider the vactrol. You can buy vactrols in a variety of manufactured forms, but you can also make your own quite easily.  It’s basically an LED coupled to an photoresistor.  As the brightness of the LED increases, resistance decreases in an inverse relationship.vactrol

Next, try pulsing the LED on and off using a 555 timer– the resistance rises and falls rhythmically. If you use that photoresistor to control the pitch clock resistor of a circuit-bent toy, I think you’ll open the door to an interesting palette of sounds.

vactrol pulse

At the Experimental Garage Sale I sold a toy guitar with this type of modification:[youtube][/youtube]

Next step:  try adding a large-value capacitor in parallel with the LED, which will cause the LED to fade out over time instead of quickly switching off.  This shapes the output into more of a ramp/sawtooth type waveform, which will cause the resistance to slide slowly downwards as the wave slopes off.

cap sawtooth

Anyone have their own variations for this type of circuit?  Drop some knowledge in the comments.  Also- big ups to Alex Inglizian for telling me about the capacitor fade a little while back.

8 thoughts on “Vactrol and LFO Arrangements”

  1. Now that’s what I’m talking about! You could use such an arrangement to create a feedback cycle between the audio output and the clock resistance.

  2. this (using vactrols) is also one of the easiest way to cross modulate two separate instruments.

    got a toy with a blinking led? now you have a modulator (toy a). extend the led (via wires, mono jack, fiber-optic line, whatever) to meet up with the photo resistor in another toy (toy b) and the first will modulate the sound of the second.

    keep in mind that many toys flash leds in interesting patterns (not just on/off square pattern of the 555). sometimes they have multiple patterns depending on what mode the toy is in… maybe the rate of the flashing changes with the clock rate…

    maybe it makes sounds itself. if so hook two together so the modulation happens both ways. now were talkin’ feedback loop.

    oh, sometimes the photo-resistor will replace a momentary button (solder one leg to each side of the switch – if it doesnt work try another photo-resistor, experiment). that means it can act as a trigger as well. the above example with the 555 can be used for “looping” by means of a trigger….

    now we’re talkin’ feedback web of any number of toys…. crazy

  3. Thanks for little article! It got me to head up to Radio Shack after work to pick up a few photoresistors and stuck them in a stupid toy I wanted to finish but didn’t want to get pots for. Its obnoxious and fun. Perfect for leaving in the living room for guests!

  4. Depending on what you’re using the CDS cell for, you can get differentiation and integration by putting the capacitor on the photoresistor side of the circuit as well, and you can probably get a greater range of variation. In a situation where you needed a pure resistance (like adjusting the gain of an opamp or something) the cap might be trouble there, but it would depend on the application.

    Also, you can get a series of sharp spikes from your square wave (differentiation) instead of a sawtooth (integration) by putting the cap IN SERIES with the LED instead of in parallel. Depending on your application this probably would require a pretty large value capacitor.

  5. im not smart enough to understand Vactrol and LFO Arrangements, but that toy guitar in the video was awesome! i want one/or the cheat sheet for the mod.

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