Heart rate monitor

  
Light filtering through the finger tip is detected by the cadmium sulphide photoresistor CD1 which forms the feedback network for transducer amplifier section ICA producing a weak signal which is further amplified by ICB. This signal is now compared against a user adjusted threshold, comparator ICD triggers gating on the piezoelectric buzzer PZ1. On each falligg edge of the comparator's output signal one-shot multivibrator ICD produces a 2 ^s pulse which is inverted by Ql and averaged by the RC network consisting of Ml, C6 and C7.
Heart rate monitor - schematic

The 10 trimpot R20 in Ql's collector circuit sets the scale factor for Mi where full scale is 150 beats per minute.