I built this mechanical keyboard intending to create a low-cost, simple design for a Bluetooth keyboard. It is constructed from two laser-cut acrylic plates that house switches hand-wired to a microcontroller. For additional functionality, I added indicator LEDs for power, low battery, and Bluetooth connection. The microcontroller was flashed with a custom version of the QMK firmware. The firmware does not yet implement the low battery and connection LEDs, but the keyboard is otherwise fully functional and is my current daily driver.
Electronics
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Adafruit Feather 32u4 has ~20 GPIO pins and bluetooth
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Microcontroller scans matrix by setting column state and reading each row
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Demultiplexer connects to 8 different inputs based on the state of 3 pins
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3 UI LEDs: power, connection, low battery
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Enameled wire used to avoid shorts
Software
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Implementation of QMK firmware in C
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Custom matrix scanning code to use demultiplexer
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Custom layout
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Pin limitations mean LED pins are also matrix pins; this functionality is not added yet
Hardware
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2 laser cut plates
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Top plate contains mounting for keys, LEDs, and the power switch
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Bottom plate contains mounting for microcontroller
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Plates screwed together using standoffs
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Key switches were disassembled and lubricated
