TIMERS AND MULTIVIBRATORS

# Timers And Multivibrators

## Overview

Multivibrators are 2-state electronic circuits used to make simple waveforms, oscillators and timers.

There are three main types of multivibrator:

• Astable multivibrator: A circuit which is not stable (hence astable) in any state, in continuously switches from one state to the other.
• Monostable multivibrator: One state is stable, while the other is not. A trigger input puts this circuit into the unstable state, and at some time after this, the circuit will automatically switch back to the stable state. This is also called a one shot.
• Bistable multivibrator: The circuit is stable in both states. A trigger input is needed to switch from one state to the other. This circuit is much more commonly known as a flip-flop.

## History

The first recorded multivibrator was the Abraham-Bloch multivibrator oscillator in 1919. It functioned as a astable multivibrator oscillator. The circuit was called a multivibrator because the square wave output contained a large proportion of harmonics1. Instead of being unwanted, these harmonics were actually useful in the calibration of RF devices.

## Basic BJT Astable Multivibrator

A basic astable multivibrator can be made from two BJT NPN transistors and a handful of passive components, as shown in the schematic below:

As a throw back to the past, Dick Smith’s Fun Way Into Electronics (first printed in 1979) contains a “flasher” which is exactly this circuit. This book is arguably what sparked my interest in Electronics!

## The 555 Timer IC

### Modes Of Operation

#### Monostable Mode (Time Delay Mode)

Monostable mode is when the 555 timer is configured to output a single pulse after a fixed amount of time. It only outputs one pulse and then stops until it is externally reset. This mode is used for creating a time delay.

#### Astable Mode

Astable mode is when the 555 timer is configured to output a continuous waveform with a fixed frequency and duty cycle. It is similar to monostable mode, except that it continually resets itself after every pulse.

Astable mode is also called running the 555 timer as a multi-vibrator. The duty cycle of the output waveform cannot be reduced below 50%. If you want a duty cycle lower than that, you have to use an inverter on the output.

Equations:

\begin{align} t_H = 0.693 \cdot (R_1 + R_2) \cdot C \end{align}

\begin{align} t_L = 0.693 \cdot R_2 \cdot C \end{align}

\begin{align} T = 0.693 \cdot (R_1 + 2R_2) \cdot C \end{align}

\begin{align} f = \frac{1}{T} \end{align}

## References

1. Abraham, H.; E. Bloch (1919). “Mesure en valeur absolue des périodes des oscillations électriques de haute fréquence”. DOI: https://doi.org/10.1051/anphys/191909120237 ↩︎