The Shoe-thesizer is another addition to the crazy collection of “cases” for Ray Wilson’s WSG (Weird Sound Generator). The circuit is based on 3 simple schmitt trigger oscillators that modulate and gate each other + a low pass filter. Schmitt trigger oscillators work similarly to 555-based oscillators by snapping back and forth as a cap charges and discharges through a resistor. Just three components and you’re oscillating.
Another example (Kipp Wieland): 