To convert the analogue signal received by the photodetector into a digital signal that can be processed by the counter, a monostable multivibrator is used. The configuration used incorporates two NOR gates, a capacitor and a resistor.
The monostable multivibrator, as implemented in the heart rate monitor
Consider the initial condition where the input voltage from IC2 is LOW. This causes the left NOR gate to output a HIGH. The resistor, connected to the +5V supply, is also a HIGH. This results in HIGHs at both the inputs 8 and 9 on the right NOR gate and so it outputs a LOW. The two LOW inputs on the left NOR gate consequently induces a matching HIGH output to the capacitor. This is the stable state of the multivibrator.
As the photodetector relays the pulse and IC2 outputs a HIGH to indicate heartbeat, the monostable multivibrator is triggered into its transient state. The different inputs on the left NOR gate results in a LOW output to the capacitor while the resistor and supply maintains a HIGH leaving a potential difference across the capacitor. As the capacitor is charged, the right NOR gate receives the HIGH inputs and outputs a LOW. This reverts the circuit back to its stable state. The time spent in the transient state[1:1] is completely dependent on the resistor and capacitor and can be determined by
During this time when the capacitor is charging, any changes in the input from IC2 will not affect the pulse length. This multivibrator is therefore said to be non-retriggerable.