Heart rate detection circuit

Cadmium sulfide photoresistor CD1 can detect the filter passing through the fingertip. CD1 forms the feedback network of the sensor amplifier. A part of the sensor amplifier, ICA, can generate weak signals, which can be further amplified by the ICB. These signals are often compared with a user-adjustable threshold, and the comparator ICD triggers the switch of the piezoelectric buzzer PZ1.