John Keefe made this DIY pulse oximeters, that is available on Github:
The heart (ha) of the project is the Pulse Oximeter and Heart Rate Sensor, which includes the sensor itself, a tiny brain to make the measurement calculations, and two “Qwiic” connections. Qwiic is Sparkfun’s system of making it easier to wire up sensors. So I also bought a little box of Qwiic wires.
I’d also need a microcontroller, a credit card-sized hobby computer, to run the show. Sparkfun makes its own version of an Arduino called a RedBoard that is Qwiic-friendly, and that’s probably the easiest option for anyone starting from scratch.
See project details at johnkeefe.net.