Astable Multivibrator using Transistors

In the world of electronics, astable multivibrators serve as fundamental circuits for generating continuous oscillating waveforms. By utilizing transistors, these circuits can produce a continuous square wave without a stable state, making them pivotal in various electronic applications where a continuous oscillation is required. Here astable multivibrator circuit with transistor is explained but see also astable multivibrator designed with op-amp and astable multivibrator with 555 Timer.

Understanding Astable Multivibrators:

Astable multivibrators, often referred to as free-running oscillators, are circuits that don't have a stable state, constantly switching between two quasi-stable states without any external triggering. This continuous oscillation generates a square wave output with equal high and low durations.

Other well know multivibrator include the monostable multivibrator using transistors and bistable multivibrator using transistors.

Transistors in Astable Multivibrators:

Implementing astable multivibrators using transistors involves employing the switching properties of these semiconductor devices to create the oscillating waveform.

Basic Components:

  1. Transistors: Utilized for their ability to switch between conducting and non-conducting states.
  2. Resistors and Capacitors: These components control the timing characteristics of the oscillations.
  3. Power Supply: Provides the necessary voltage for the circuit operation.
  4. Output: The resulting square wave signal.

Circuit Design:

The design of an astable multivibrator using transistors typically involves configuring the transistors in a feedback loop, creating a continuous switching action:

  1. Transistor Switching: The arrangement of transistors and the feedback network causes continuous switching between states.
  2. Charging and Discharging Capacitors: Capacitors charge and discharge through resistors, determining the high and low durations of the output square wave.
  3. Oscillating Output: The circuit generates a continuous square wave output without any external triggering, showcasing a constant oscillation.
  4. No Stable State: Unlike monostable and bistable circuits, the astable multivibrator has no stable state, enabling it to continuously oscillate.

Here two circuit implementation of bistable multivibrator with transitors.

One LED flasher with Transistor

Here we will flash a LED with a general purpose transistor 2N3904. The following shows the circuit diagram of one LED based LED flasher.

One LED flasher with Transistor

The following shows the one led flasher circuit implemented on breadboard.

The rate at which the LED blinks is determined with the resister R3 and C2. The RC time constant is given by,

\( \tau = R_4 C_1 = 68K\Omega \times 10 \mu F = 1s\)

So the frequency of blinking led is,

\( f = \frac{1}{1s} = 1Hz\)

 Two LED flasher with Transistor

 Next, the following circuit shows how to make a two LED flasher circuit with 2N3904 transistor. 

Two LED flasher with Transistor

The two led flasher circuit is just modification of the one led flasher circuit. We can just place another LED on the other side of the circuit.

The following shows the two led flasher circuit implemented on breadboard.

 The component values can be calculated using the online astable multivibrator calculator.


  1. Continuous Oscillation: Provides a consistent square wave output without the need for external triggers.
  2. Simplicity: The circuit design, although involving active components like transistors, is relatively simple and straightforward.
  3. Frequency Control: Changing resistor and capacitor values allows control over the frequency of oscillation.


Astable multivibrators using transistors find applications across various domains:

  • Signal Generators: Used in generating clock signals, tone generation in audio circuits, and triggering operations.
  • Pulse Width Modulation (PWM): Valuable in controlling the width of pulses for regulating power in electronic systems.
  • Timing and Control Circuits: Applied in electronic timers and in scenarios requiring continuous oscillation.


Astable multivibrators, employing transistors in their circuitry, offer a versatile and reliable means of generating continuous square wave signals. Their ability to produce a consistent oscillating output without external triggering finds widespread use in numerous electronic applications, showcasing the importance and utility of transistor-based oscillators.

Monostable multivibrator has one stable state while the bistable multivibrator has two stable state unlike astable multivibrator which has no stable state.

By leveraging the inherent properties of transistors, astable multivibrators serve as essential building blocks in electronics, providing a continuous and stable oscillating waveform crucial for a myriad of applications across industries. Understanding these circuits and their operational principles is essential for engineers and enthusiasts delving into the world of electronic oscillators and signal generation.

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