Frank Pacquiao

user Test Engineer

city ISI

  • Heart Rate Monitor

  • Created: Sep 18, 2014

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Light filtering through the fingertip is detected by the cadmium sulphide photo resistor RCD1 which forms the feedback network for transducer amplifier section U1A producing a weak signal which is further amplified by U1B. This signal is now compared against a user adjusted threshold, comparator U1D triggers gating on the piezoelectric buzzer PZ1. On each falling edge of the comparator’s output signal one-shot multivibrator U1D produces a 2µs pulse which is inverted by Q1 and averaged by the RC network consisting of M1, C6 and C7. The 10kΩ trimpot R20 in Q1’s collector circuit sets the scale factor for M1 where full scale is 150 beats per minute.



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