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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@Gordon
Great! If you had another spare pin (UARX?) you could put the piezo between the two which effectively doubles the voltage swing on it: https://www.espruino.com/NRF52LL#create-a-square-wave-on-pin-d0-with-the-inverse-of-the-square-wave-on-d1-
Just a thought, but if you're getting rid of the HRM I'd maybe think about drilling out the HRM hole and then gluing the piezo to the back of the case. That way the case would be sealed but you'd have a way for the sound to escape.
As for extra IO, if you're willing to dig around on the board you might be able to find something. For instance I don't know if
GPS BOOTis actually the GPS power signal or some other pin that I haven't noticed so far.Or maybe if you're disconnecting the HRM then the pins for that might be on testpoints?
https://www.espruino.com/Bangle.js2+Technical is a good start but I imagine you found that already!