The DFROBOT Gravity optical Heart Rate Monitor has an excellent WIKI page

Last Updated on December 27, 2022 by pg@petergamma.org

https://wiki.dfrobot.com/Heart_Rate_Sensor_SKU__SEN0203

The Gravity is one of the best optical heart rate sensor we found to this date, which can be developed further, for instance to combine multiple sensors and to use HeartPy, to replace consumer grade heart rate monitors which do not have open standards.