Today's #fuzzfriday is an Tim Escobedo design: the Ugly Face. Here's what he had to say about it:
The Uglyface is a completely all-out fuzz unit for fat, noisy buzzsaw sounds. P1 sets the threshold for triggering the fuzz. This characteristic also means that notes can fade out abruptly when they decay below the set level. P2 is the frequency control. It functions as an unorthodox tone control, at one extreme producing synthesized octave down effects, at the other extreme, thin and nasal tones. When the threshold is set below the trigger point, the Uglyface becomes a free running oscillator, with P2 controlling the frequency. P3 is a Sensitivity control, regulating how much of the guitar signal will modulate the Frequency control.
Make sure the second IC is a CMOS 555 otherwise the effect won't work. Here's the schematic for reference.