Astable Multivibrator using Transistors and NE555






Astable (Free Running oscillator) Multivibrator

I hope as the name itself gives you a clear view about the "Free Running Oscillator"
They didn't posses any Stable state.
By giving the supply voltage Vcc, the astable multivibrator starts to oscillate.
The frequency of oscillation is determined by the RC time constant employed in the circuit.

The Astable Multivibrator generates nonsinusoidal output wave forms.
ie; Square/Rectangular Pulse or saw tooth.

The astable multivibrator finds too many applications in Electronics.
It is commonly used to produce sound waves of various frequencies.


The output of astable multivibrator may be used as input for various electronic circuits.

Working of Astable Multivibrator

The circuit for astable multivibrator employs two identical transistors Q1 and Q2.

It is basically a two stage RC Coupled Amplifier with positive feedback.

ie; The output from Q2 is given as input to Q1 and viceversa.

Initially, both the transistors are in ON condition. Due to the variations in Transistor parameters one transistor may conduct more than the other so the less conducting Transistor gets turned OFF(go to cutoff region) and after some time the other transistor gets turned off.

The alternate Turn ON and Turn OFF process of these two identical transistors results Oscillations in the output voltage.

TRY THIS TOOL
Astable Multivibrator Frequency Calculator


We can replace the circuit for astable multivibrator with a single IC called as NE555.
Here is the Pin diagram of NE555

Applications of Astable Multivibrator
using NE555

- Voltage controlled oscillator
- Ultrasonic Repellants



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