ARP Axxe: Voltage-Controlled LFO Speed

Photocell Method

For this attempt at voltage-controlled LFO speed, I made a home-brew 'Vactrol.' Also known as an "opto-coupler", this is a combination of a cadmium sulfide (CdS) photoresistor and an LED. It functions as a voltage-controlled resistor. The photocell and LED are shrink-wrapped together so that the LED is shining right into the photocell, but no ambient light can get in. As the LED gets brighter, the photocell's resistance goes down.



(This schematic doesn't show the Fast LFO Mod, but the mp3 was recorded with it in place, and set in the super-fast mode. I figured it would be more useful to post a separate schematic showing each mod by itself.)

The LED is a generic yellow LED, and the CdS cell came from a kit from Radio Shack. I don't have exact part numbers on either one, sorry! I did this by trial and error.

The Continuum's "Y" voltage, which varies from 0-10 volts, is sent through an 18K resistor to the LED. The photocell is inserted into the LFO's charging circuit between the speed control pot (R18) and the fixed resistor R16. So as the "Y" voltage increases, the frequency of the LFO increases. This circuit made for some cool effects:

Hear it: (107K mp3)

But its response was slower than I wanted. Photocells take a while to get back up to a high resistance after you have removed the light. So after trying this I tried a much faster method using an analog multiplier circuit.