First of all, the threshold level of a valid pulse is set to be
1.75 V to eliminate noise or spike. As soon as the amplitude
of a pulse is identified, the microcontroller will ignore the signal
for 450 ms to prevent any false identification due to the
presence of premature pulse "overshoot" due to oscillation.
Hence, this algorithm can only detect pulse rate which is less
than 133 beats per minute.