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Replying to @bitomaxsp
It’s an old board I designed to play with pulse and pulse oximetry; more or less just a red/IR led and a phototransistor. It never worked well, just because the signal was so faint and I didn’t (yet!) know enough to know how to amplify it. OSO-MISC-20-004: https://github.com/joeycastillo/Sensor-Cap/tree/main/PCB/Sensor%20Boards
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