I made these homebrew vactrols (photo-resistor opto-couplers) at a total cost of less than 40 cents each.. They are typically used in compressors, tremolo, phaser, flanging, envelope filters and other modulation effects. These were made using flat top and rectangle shaped LEDS super glued to Radio Shack LDRs (Light Dependent Resistors or also called photo resistors). The black one was made with Sculpty clay, and the other two I used epoxy puddy to encase the LED/CdS combo. That was done to insure no light can get in. I had tried just using shrink wrap, but the shrink wrap still let some light thru. Having to heat the black one in the oven to set the Sculptey clay didn't seem to effect it's function. From what I could test using only a multi-meter they spec out close to the CLM6000 and should work in most circuits calling for that vactrol with little or no adjustment of other components.