# Monostable Multivibrator

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.

![](/content/images/2015/09/circuit.png)

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[1].

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

$t=ln(2)RC$

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.