Fig. 1 CD4047 Monostable Mode Multivibrator circuit.

CD4047 Monostable Multivibrator Circuit

by Lewis Loflin

A Monostable Multivibrator is defined as:

Monostable Multivibrators or "One-Shot Multivibrators" as they are also called, are used to generate a single output pulse of a specified width, either "HIGH" or "LOW" when a suitable external trigger signal or pulse T is applied.

Another definition:

Monostable multivibrator, in which one of the states is stable, but the other state is unstable (transient). A trigger pulse causes the circuit to enter the unstable state. After entering the unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot.

To simplify it's a circuit that when it receives an input pulse will produce an output waveform whose ON time is based on a capacitor-resistor combination then it will switch off.

These can be built with logic gates, transistors, or specialized integrated circuits. I'll explore some basic theory I'll focus on the CD4047 astable/monostable multivibrator.

The CD4047 is easy to configure for either mode, is low power, and has a voltage range of 3-15 volts, but works best at 5 volts. The circuit is strait forward just as shown.

Fig. 2 CD4047 Monostable Mode Multivibrator Waveform.

Fig. 2 illustrates the output from the CD4027 in the monostable mode. The input is a square wave but the differentiator formed by a 22pF capacitor created a positive going 'spike" that triggers a HIGH output whose ON time depends on the values of C1 and R1. The formula is Q = 2.48 * R1 * C1. In the example above Qon = 2.48 * 0.0000001 * 90,000 = 22.34 mSec.

Changing C1 to 2200pF and R1 to 10,000, Qon = 2.48 * 0.0000000022 * 10,000 =~ 55uSec.

A monostable can be useful for signal conditioning or time delay circuits. My test using polarized capacitors shows polarity doesn't matter.