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InfluxDB
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Not specifically what you're looking for, but https://github.com/nilokr/cantor-remix is a 36 key diode-less open-source board that you might be able to get some concepts from. Keep in mind, if you don't wire a switch, then nothing breaks. You just need enough pins on the controller to handle the number of switches, so you could go a controller with less pins on a smaller board.
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