the blue wires hanging out of the rear end of my homemade crackle box clone are connected to one of the six touch points in the circuit. here's a recording and a patch diagram:
last weekend, i put the gray box up at the top together. it houses two 4049 preamps, a 2x1 active mixer (because i'm mono like that), and two voltage-controlled attenuators.
they are extremely simple - an led coupled with a photoresistor (before you say anything, i do have some vactrols on my bench, but i'm saving them for other, potentially more exciting things, and i've had a bunch of photoresistors lying around for the better part of two years, so whatever, synth-snobs), a 2n3904, and a pot and three resistors. the circuit craps out with high frequency control signals, but it has some neat behavior - as you decrease the amount of attenuation, the output signal has a longer decay time. if you increase the attenuation, the output is more percussive and momentary.
here's a demo recording using the setup pictured above:
and here's the schematic for y'all: