# Astable Multivibrator Circuit Design with Oscillations and Transistors

December 16, 2019

Growing up, I enjoyed taking piano lessons, but the same can’t be said of the teacher who patiently tolerates my inability to getting the beat right. Thankfully, the metronome came to the rescue, with each swing of the pendulum rod guiding me with ease. Eventually, my sense of rhythm improved, and piano lessons are no longer episodes of frustrations.

In electronics, rhythmic pulses are key to many applications. However, there are no swinging pendulums on the PCB. Instead, an astable multivibrator circuit is a decent alternative in providing consistent beats of electrical pulses.

## What Is An Astable Multivibrator Circuit

An astable multivibrator circuit also termed as a regenerative switching circuit, is an electrical circuit that switches between HIGH and LOW states continuously. The output of the astable multivibrator circuit is a square wave that goes on indefinitely.

As simple as it is, the astable multivibrator is often used as a pulse generator for delay circuits. It is designed in a way that the circuit will continue to oscillate without external triggers. The indefinite oscillation and simplicity of the circuit make it a popular choice for basic pulse generation applications such as switches, modulators, or signal generators.

## How Does Astable Multivibrator Circuit Work

It’s fascinating how a simple circuit consisting of transistors, resistors, and capacitors could produce continuous pulses of square waves without external triggers.

Below, you’ll see the circuit matching these observations. Here’s how it works:

1. Consider that transistor T1 is switched off while T2 turns on when the power supply is first applied. At this point, capacitor R1 is in a state of discharge, while capacitor C2 charges towards Vcc as current flows from R3.

2. The voltage that falls on C2 will continue to rise. As it breaches the base-emitter voltage of T1, which is typically 0.6V, transistor T1 goes into conduction or saturation mode.

3. When T1 goes into conduction, the voltage at the Collector of T1 immediately falls to 0.6V, causing a reverse charge to the base of T2. This sudden change of voltage at C1 puts T2 into off state immediately.

4. As T1 continues to conduct current, C1 now charges towards VCC. At the moment of the voltage on C1 hitting 0.6V, T2 will start conducting, thus completing a cycle of unstable states.

A typical astable multivibrator circuit.

The process will repeat itself and produces square waves of inverse amplitude at the collector of both transistors. The voltage on the HIGH state of the square wave rises close to the value of Vcc.

Both C1, R2 and C2, R3 determine the frequency of the pulse. The period of the high state is controlled by one of the RC constants, while the low state is determined by the other.

Theoretically, you should be getting a perfect square wave on the collectors. However, due to the charging process of the capacitor, the edge of the output is slightly curved. An additional diode is placed between the capacitor and collector, with an extra resistor connecting the anode of the diode to Vcc to sharpen the edges of the pulse. This is done for both sets of capacitors to prevent charging current flowing through R1 and R4.

## Designing Astable Multivibrator Circuit On A PCB

With only a handful of components, designing an astable multivibrator on a PCB shouldn’t be a complicated process. You’ll need to ascertain the value of the RC constant to produce the desired square wave.

RC values determine pulse wave frequency.

The calculation is fairly simple as the period for the off state of the transistor is given by the 0.69 x RC. An equal value for the RC constant of both sets of resistors and capacitors will produce a 50% duty cycle of the waveform.

To ensure that the astable multivibrator is functional, you could add a couple of LEDs to the collector of the transistors. These LEDs are good visual indicators to ensure that the circuit does oscillate as intended.

Alternatively, you could use simulation tools that are included with modern PCB designer software. OrCAD PSpice Simulator can simulate the performance of an astable multivibrator circuit. Thus, providing a simple cost-effective means of confirming the intended operation prior to circuit construction and of verifying new ideas that could lead to improve circuits performance. Computer programs like PSpice have revolutionized the electronics industry by giving the user an ability to perform various types of analysis to study their circuit better.