Astable Multivibrator


To design and study of Astable Multivibrator using 555 Timer ICs.


A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors or capacitors. The name "multivibrator" was initially applied to the free-running oscillator version of the circuit because its output waveform was rich in harmonics.

    There are three types of multivibrator circuits depending on the circuit operation:
  • Astable: in which the circuit is not stable in either state —it continually switches from one state to the other. It functions as a relaxation oscillator.
  • Monostable: in which one of the states is stable, but the other state is unstable (transient). A trigger pulse causes the circuit to enter the unstable state. After entering the unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot.
  • Bistable: in which the circuit is stable in either state. It can be flipped from one state to the other by an external trigger pulse. This circuit is also known as a flip flop. It can be used to store one bit of information.
The 555 timer is a chip that can be used to create pulses of various durations, to output a continuous pulse waveform of adjustable pulse width and frequency, and to toggle between high and low states in response to inputs. By wiring the 555 timer with resistors and capacitors in various ways, you can get it to operate in different modes.

Fig.1 Timer-555

Fig.2 Internal structure of Timer 555