Introduction: Astable Multivibrator Using BJTs

About: Electronics Hobbyist and Creator

Today we will Learn about a simple,easy to make Astable Multivibrator circuit which generates square waves as an output.And also make an LED flasher the same Circuit.

Step 1: List of Components

1. 10 uF Electrolytic Capacitors x 2
2. BC 547 Transistor (Any General Purpose) x 2
3. LEDs x 2
4. Resistors :-
5. 27k x 2
6. 220 ohm x 2
7 .Jumper Wires and Breadboard.

Step 2: Let's Understand the Circuit

It's an Astable Multivibrator Circuit which consists Two Transistor ,whose collectors are cross Connected to each other's Base via the 10 uF Capacitors.
The Charging and Discharging of the Capacitor Leads to Alternate Triggering of Transistors and hence we Obtain a square Wave at the Point L1 and L2.

So How does All these Happens???

If you Consider,the transistor ideal then both led to should light up and not blink.
So why it's Blinking ??
It's because of the fact that Transistor are not ideal and differ due to manufacturing defects.
Hence one of the two Transistor will Conduct First.

Let's say Q1 conducts first ,which means that current must flow through C2 into base of Q1.
And the Voltage drop Across C2 will be (9-0.7)
8.3 V { 9 volt of battery and 0.7 V drop across Transistor Juntion BE).
Similarly the voltage at point L1 will be nearly 0 as Q1 is short Circuited to Ground.

Now because of that C1 starts charging though the 27 k resistor ,and once it reaches 0.7 volt then Q2 starts Conducting.
As Q2 is Short Circuited potential at L2 is zero as it is Shorted to ground.
So the voltage on left side of Capacitor C2 will also drop by -8.3.
This negative voltage wi be then applied to base of Q 1 and it will slam Shut.
This cycle repeats again and again and we obtain Square Wave at L1 and L2.

Step 3: Calculation of Frequency

The frequency of the Square Wave generated can be calculated by the formula.

F = 0.72/ R*C

Which in our Case :-
R = 27k
C = 10uf
F = 2.666 Hz

The Formula is Calculated using Transient Analysis.For more info look at the Image.

Step 4: Complete Circuit and Video

If you have connected everything right , your Circuit will look something like that, here is an demo video from my Youtube channel.