Astable Multivibrator Using Transistors

This may be the fist step of electronics in the most of the newbies. Though this is a very simple circuit, it takes a little bit of time to understand the working principle of the unit. Lets see how it works.

First we will make the circuit.

Astable Multivibrator
Astable Multivibrator

How does it really works,

To understand this circuit it is important to note that both sides of the circuit are not identical. Due to some asymmetry one transistor starts conduction (On State) and the other is not in the beginning. Lets assume that T1 is on and T2 is off. Voltage at the collector of T2 (Vc) is in high state (closer to Vcc) and the Capacitor C2 is charging. Voltage across the capacitor is Vcc on the positive and, 0.7V on the negative side. Once it is charged, it will stop the current flow through the base of T1 transistor which will cause T1 to go to off state. Then Capacitor C1 will have a similar condition like C2 in the previous state and it will start charging. This will cause T2 to go to on state where positive side of the C2 capacitor will be grounded through transistor. C2 starts discharging, but the voltage on the negative side will be on the negative potential until it increases through the discharge cycle up to 0.7V which will cause T1 to go to on state. This cycle repeats. 

In this video, electronsapdotcom explains how it works,

For further studies we can connect an oscilloscope like this, (refer the video for graphs)

Astable Multivibrator with Oscilloscope
Astable Multivibrator with Oscilloscope

Since I wanted to make the On/Off time of the LEDs variable, I've added a variable resistor to the discharging node.

Astable Multivibrator with Variable Speed
Astable Multivibrator with Variable Speed

Few tips on Proteus in designing the circuit.
  1. Use power and ground from terminal rather than a battery to power the circuit. (Can change the Vcc voltage from (Design >> Configure Power Rails >> Select Vcc/Vdd from "Power Supplies" and put the desired voltage.
  2. Do not use generic components, (ex: use transistors like BC548, 2N2222 etc.)

Here I've added drawings for a PCB for the Transistor Astable multivibrator.

PCB TOP Astable Multivibrator
PCB Top Layer - Astable Multivibrator

PCB BOTTOM Astable Multivibrator
PCB Bottom Layer - Astable Multivibrator

Download the Gerber files from here.
Project file in EasyEDA

PCB Manufacturers (you can easily upload the gerber files to one of these web sites and order)
  1. JLC PCB
  3. PCB WAY