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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
You can measure it very simply by looking at the change in light that passes through your finger. For example this was a quick hack I did a while ago that uses your webcam to measure your pulse. It's not great, but in the right conditions (light behind your finger) it does work!
https://gfwilliams.github.io/HeartRate/
So just a light on one side, and a light sensor on the other.
However you can buy more professional units which (I believe) have two lights and sensors at different wavelengths. One is a wavelength that is affected by blood, one isn't - and you can then just compare the two sensor readings over time to get a pulse.