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Schematic Circuit Diagram Astable Multivibrator using 555-Timer proteus simulation

The monostable circuit is modified to re-trigger itself by connecting the trigger terminal and threshold terminal. As in the monostable circuit, the output is set during the charging period of a capacitor. When the voltage across the capacitor tends to exceed 2/3 Vcc, the output is reset. The capacitor discharges through Rb. As the voltage tends to fall below 1/3 Vcc the timer is triggered again. Thus, the output is reset during the discharging period of the capacitor.

Astable Multivibrator using 555

Schematic Circuit Diagram Astable Multivibrator using 555-Timer proteus simulation

However, the Set and Reset time durations will not be equal to this circuit. The resistors Ra and Rb are involved in charging the capacitor but, only resistor Rb is involved in discharging period. So, charging time will definitely be more than discharging time i.e.., TON is more than TOFF. Hence, this circuit can be used to generate clock pulses for digital circuits or where the frequency is the parameter rather than the duty ratio.

Astable with Equal Ton and Toff

Schematic Circuit Diagram Astable Multivibrator using 555-Timer proteus simulation 2

This is called a Square wave generator, which can be used to generate musical notes by setting the frequency of the wave at the desired musical note. For example, setting the frequency at 440 Hz and connecting the speaker at the output in proper configuration gives the musical note ‘A’ of the musical piano. This note is used as a tuning standard.

Simulation – Waveforms of Astable Multi-vibrator

Astable Multivibrators are the most commonly used type of multivibrator circuit. An astable multivibrator is a free-running oscillator that has no permanent “meta” or “steady” state but is continually changing its output from one state (LOW) to another state (HIGH) and then back again.

Astable multivibrators are used in the applications like pulse position modulation, frequency modulation, etc. as they are simple, reliable, and easy to construct. Get electrical articles delivered to your inbox every week.
The basic principle of the Astable multivibrator is a slight variation in the electrical properties or characteristics of the transistor. This difference causes one transistor to turn ON fast than the other when power is applied for the first time, thereby triggering oscillations.

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