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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.
tom.gidden
@Gordon
Yes, I reckon you could get something in there... There's a plastic 'chassis' that comes out, and it takes up quite a bit of space, It'd be trivial to grind a bit out or 3D print a new one without the padding. Or worst case you could replace the battery with a smaller one.
In terms of IO, there are no 'free' pins brought out, but the piezo speaker isn't that great, and it's connected right to a GPIO pin via a wire that's easily accessible on the back of the watch. Something like a 433MHz transmitter that only needs one IO pin would be super easy to add to that.
Switched power is also available (less easily, but not that hard) on the heart rate monitor PCB, so you'd be laughing.