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Introducing Bonsai C

Bonsai C is an open-source drop-in-replacement for Proton C by customMK.

Github repo:

Brought to you by customMK, Bonsai C is an open source microcontroller board featuring an STM32F303CC microcontroller. Designed to be a drop-in-replacement for Proton C rev 2, Bonsai C is functionally equivalent to the Proton C, while incorporating several design and manufactring improvements, including being "JLCPCB-ready".

To facilitate building of your own Bonsai C, we have designed it to be “JLCPCB ready” using components commonly in stock at JLCPCB and with file formats that can be directly uploaded to JLCPCB. The only issue is the microcontroller… As you have likely observed, the Proton C has been out of stock for quite some time, now, and with very good reason: the microcontroller that powers it is far more expensive and harder to obtain these days – customMK.

As for what makes this particular microcontroller so special, it is one of the few that has a built-in USB bootloader, making it extremely convenient to flash QMK firmware onto it. Despite that the microcontroller is hard to get, it is still obtainable by either paying a premium or by waiting around a year for factory-produced components. Both options may be viable...

Btw, the photo above is a prototype built by JLCPCB (with the microcontroller added manually) and it is wider across the top (20 mm) than Proton C because JLCPCB requires a 20 mm minimum board width. If you build it elsewhere, that width constraint does not apply.

Published on Thu 7th Oct 2021. Featured in KBD #47 (source).


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