Thanks to Drew for sharing this project from Orlando Hoilett on hackaday.io
All the DIY pulse sensors I have used previously have some sort of threshold that has to be adjusted to each new user. The threshold is tuned for each person or is generalized so that it fits any user. I haven’t played with one (at least in the DIY space) that has an auto-adjusting threshold built in the hardware. So I figured I would give this project a try. I don’t know how well it will work, or even if it will, I was just curious.
The circuit incorporates the basic pieces of a typical pulse sensor. It contains an LED to shine light through someone’s finger, a photodiode to detect the light that passes through the finger, a transimpedance amplifier to amplifier the current from the photodiode, and an active bandpass filter to amplify the signal and filter out the noise. My main addition is a peak detector circuit at the output of the band-pass filter and a comparator.