A “Monostable Multivibrator” is so named because its output is only stable in one state. When triggered, this device will switch the output to the so-called 'unstable state' for a set period of time and then return to the stable state. Note that "stable" in this context refers to an output state that will remain the same without an external input and "unstable" refers to a state which will change without external input. Since one input trigger event results in one output pulse, this device is also known as a “one-shot.” The SN74LVC1G123 will be used herein as an example because it is one of our most popular devices, but TI offers many other monostable multivibrators in different logic families to fit a wide variety of applications.

These devices can be used as debouncers, pulse extenders, delays, and edge detectors. They are very useful for cleaning up input signals to logic circuits of all types.

• Stable State – The natural, resting, or ‘off’ state of a monostable multivibrator’s output; usually ‘low’
• Unstable State – The temporary, active, or ‘on’ state of a monostable multivibrator’s output; usually ‘high’
• Pulse Width – The amount of time for which the output is switched. Often referred to as ‘t_w’
• Trigger – The input signal edge which causes an output pulse to be generated. The trigger can be on a rising or falling edge depending on which input is used.
• Retrigger – The act of applying a trigger signal while the output of the system is already in the unstable state.
• Retriggerable - A monostable multivibrator that has the capability to retrigger.

Any monostable multivibrator that has an external RC circuit for timing will operate on the same basic principle. The capacitor in an RC circuit will take a set amount of time, referred to as the ‘time constant,’ to charge up to 63.2% of its full charge from a fully discharged state. It takes 5 time constants to reach 99.3% of its full charge, which is generally accepted as a fully charged state. These two values are derived from the RC circuit step response characteristic equation, Equation 1.

Equation 1.

A monostable multivibrator takes advantage of the consistency of RC circuit charge times to produce output pulses of set widths. Because external components are used, this consistency is reliant upon the thermal and manufacturing tolerances of those components in addition to the chosen semiconductor device.