This hi-hat module is largely based on Moritz Klein’s hi-hat design, see below for my customization. The design uses a series of harmonically related Schmitt Trigger Inverters to make a series of related square waves that have enough harmonic content that they sound like they’re related (as opposed to simply being noise). From there, the sound is shaped by a band-pass filter, some distortion, and a high pass filter to get that hi-hat sound.

I found Mortiz’s design to be a bit odd so I made one change. In Moritz’s original design the hi-hat is ‘open’ by default and could only be ‘closed’ by applying control voltage to the module. The decay knob only applied to the ‘closed’ hi-hat and didn’t do anything unless there was control voltage present. In my design I added a switch across the transistor so that the hi-hat could be ‘opened’ or ‘closed’ with a flick of a switch and no control voltage is needed. This feels more intuitive to me and allows me to have one hi-hat module set up to be open, and another one closed.