Building and applying a Vactrol

Building and applying a Vactrol28.06.2010 20:21:37

Today at RuinWesen we bring you another technical blog. This time, we will be discussing the construction and applications of a Vactrol. A Vactrol can be thought of as a very simple way of getting voltage control into a circuit. The Vactrol couples an LED (Light Emitting Diode) with an LDR (Light Dependent Resistor) to provide a conversion from voltage to resistance, which allows easy integration into a circuit such as a guitar effect pedal.

The basic components you will need for the assembly of the Vactrol are:

• an LED: Classic LEDs are favored over Super Brights (3mm are best)
• an LDR: Usually any LDR will do, but some have larger ranges than others. If this is important, check the specs (smaller LDRs are preferred, to make for a smaller package)
• shrink tubing OR electrical tape
• 220 ohm resistor (or similar low value)

These are the basic components you will need. Other useful components may be a jack for the incoming signal, or a 50k potentiometer for a 'depth' control.

Now that these components are all wrangled up, sit them down on the bench (or suitable working surface- the floor or back of the nearest person will do fine) and get to it. Shrink tubing is preferable to electrical tape since it will have greater isolation from external light. It is also just easier and quicker to use, dammit! But I will understand if you can't get any in your town, if you are too poor to buy it, or if you lack the balls to steal it*. If you have some shrink tubing, please skip to chapter 9. This here is for the electrical tape losers. (Seriously, if you are going to be building more than one Vactrol, get yourself some shrink tubing.) Okay, grab your electrical tape, LED and LDR. Put the LED and LDR ass to ass (or face to face? I just wanted an excuse to say ass to ass). Once in the position, begin wrapping the tape around the pair several times so that they are snug in their sticky blanket. Okay, this is getting creepy. I probably should have never mentioned ass to ass! Make sure the legs of both are separated so they do not cause a short when wrapped. Now, begin wrapping some tape between the legs to seal off any further light from getting in. Make sure the enclosure is sealed well. Add some more tape if you want to.

Grab the shrink tubing, LED, and LDR, and put the LED and LDR face to face. Slip a suitable length of the shrink tubing on the two and apply some heat. Done. See, you electrical tape losers?! It's that easy. No nightmares about naughty electrical components.

Once your LED/LDR combo package is complete, you will need to solder on the resistor. This resistor is important since it prevents your LED from dying a horrible death. Would you like to be killed by electrocution? Neither does the LED. Identify the anode of the LED, which is the longer of the two leads. Clip the leads of the resistor and the anode to a suitable length and apply some solder. That's it! Now, some fun!

Do you have a favorite delay pedal but wish it could be adjusted from your modular? Well you are in luck, fine citizen. With your newly constructed vactrol, you can open up your delay pedal and apply the leads of the LDR end to the delay time potentiometer. Stick some wobbly voltages into the LED side and BOOM, chorus! This is where a depth knob voltage divider comes into play nicely.

Vactrols are also well suited for circuit bending. If you have a pitch mod, you can now add external control of the pitch so you can sequence your toy with your music without having to resort to sampling.

There are really many ways that a Vactrol can be used. Think of the vactrol as a replacement for potentiometers, or place them across currently existing potentiometers.

I hate to get you down, but Vactrols aren't perfect as you may have already noticed. They are a bit slow to changes in voltage. This is known as Slew. They are also a bit tricky to tune properly and are best left for quick and dirty methods of control voltage. Do not expect to use them as a 1V/Oct control of your circuit bent creation without a considerable amount of effort put into it. With that said, even the man himself, Don Buchla, used Vactrols to control some of his synthesizer modules. So I say, if they are good enough for him, they are good enough for me.

*RuinWesen does not support the theft of shrink tubing, or any object for that matter.