The hardest part here is obvious on sight: a drum sequencer with only two buttons and no microcontroller, while still keeping it playable. The maker set that challenge deliberately, and the result shows a rhythm stored as nothing more than binary high and low states, yet you can tap beats in and wipe them out in real time. That is a tight brief handled well.
The circuit lives on an Erica Synths Labor prototyping board and is built from discrete logic and switching parts. At the core sits a CD4015 dual 4-stage CMOS shift register wired as an 8-bit register, with the data looping through a feedback path isolated by diodes and a 100k-ohm pull-down resistor. A CD4520 dual binary up-counter takes the sixteenth-note clock and puts out quarter-note pulses, and an op-amp buffer with a diode and a 100k-ohm resistor works as a clock gate.
Two details stand out to me. A PNP transistor paired with a 1M-ohm resistor interrupts the loop so bits can be dumped off the register, and the second pushbutton sits right in that feedback path to clear or edit stored steps. The two mechanical keyboard switches give it a satisfying, deliberate input feel, and there is even a small pink rectangular film capacitor for a spot of colour on the board.
The one trade-off: 60 additional LEDs were left out to keep the project feasible and costs down, so step feedback is sparser than on a typical sequencer. As a Eurorack kit it comes with a downloadable build guide and an illustrated manual, which seems a friendly entry point for anyone wanting to try discrete logic sequencing.
Project by Moritz Klein. Watch it on their YouTube channel: