The synthesizer circuit is based off a circuit (WSG by Ray Wilson) I found on the internet which uses three schmitt triggers. One for each of the oscillators and an extra for modulation. It also uses an op-amp to make a simple, yet effective, filter. In the tongue of the shoe, there is a piezo transducer which is set up in parallel to one of the oscillators causing the voltage to spike, giving the synthesizer some cool rhythmic sounds. There is a switch that allows you to choose one or both oscillating frequencies to be mixed in to the output. The knobs were placed where the shoe-lace holes are and the output jack was placed in the toe area. I built this circuit during my freshmen year of high school for a science fair.