How Astable Multivibrator using IC 555 works? – Detailed study

What is an astable multivibrator?

The multivibrator which not have any stable state is called an astable multivibrator. In an astable multivibrator, there will be no stable state. So it means that the output will be automatically swinging between high and low state and vice versa continuously.

Why it is called a free-running multivibrator?

As the output of astable multivibrator changes automatically that’s why it is also called a free-running multivibrator or self-triggering multivibrator.

             Before starting to study about the working of an astable multivibrator I like to suggest you read all about the IC 555. let us discuss the working of an astable multivibrator with the help of its construction, circuit diagram, waveforms, and all other details.

Construction of an Astable multivibrator

The following circuit diagram shows an astable multivibrator using IC 555 in which the pin no. 2 (trigger) and pin no. 6 (threshold) are shorted together. A timing capacitor C is connected at the lower arm of short of pin no. 2 and 6. The resistor R_B is connected at the upper arm to pin no.7, it is used to provide a discharging path for capacitor C. The resistor R_A is connected between pin no. 7 & 8, it is used to provide a charging path for capacitor C.

A small value capacitor of 0.01µF is connected at pin no. 5 to prevent the upper comparator from noise disturbances. The power supply or battery is connected between pin no. 8 (+Vcc) and 1 (Ground). The pin no. 4 (Reset) is connected to +Vcc to avoid accidental rest of IC 555. The output of an astable multivibrator is taken between pin no. 3 (output) and ground. An LED is connected at an output to indicate the output voltage level.

How does an Astable multivibrator work?

When the power supply is switched on the capacitor is in discharge condition so the voltage across the capacitor is zero. It gets applied to trigger and threshold (pin no 2 and 6) inputs of comparators. The lower comparator is inverting comparator now its input voltage is less than its reference voltage of 1/3Vcc so it produces a high output voltage.

The upper comparator is noninverting now its input voltage is less than its reference voltage of 2/3Vcc so it produces a low output voltage. The outputs of both comparators are applied to R and S inputs of the flip flop. The inputs of the flip flop are R =1 and S=0, so the flip flop goes into Reset condition and its outputs become Q = 0 and   = 1.

On-time of an Astable multivibrator or Charging of Capacitor

The flip flop output is the actual output of the IC 555 and for now, it is high i.e.  = 1. The LED connected at the output starts glowing and indicates IC 555 output waveform started it’s on time. The output Q of the flip flop is connected to the base of a discharge transistor. The base of discharge transistor is low (Q =0) therefore transistor goes into the cut off condition.

Therefore the capacitor starts charging exponentially towards the +Vcc voltage through the resistor R_A and R_B. This is called charging time (tc) of a capacitor and it is given by t_on = tc = 0.693 (R_A +R_B) C. The charging time (on time) is shown in the waveform.

While charging the voltage across the capacitor increases towards +Vcc. When the voltage across the capacitor is slightly greater than +2/3 Vcc the input applied to the upper comparator gets more than a reference voltage of +2/3 Vcc. So, therefore, the upper comparator produces a high output voltage.

The voltage across the capacitor is also applied to the trigger input of the lower comparator. The input voltage for the lower comparator gets more than a reference voltage of +1/3 Vcc. So, therefore, the lower comparator produces a low output voltage. The outputs of both comparators are applied to R and S inputs of the flip flop. The inputs of the flip flop are R =0 and S=1, so the flip flop goes into the Set condition and its outputs become Q = 1 and  = 0.

Off-time of an Astable multivibrator or Discharging of Capacitor

The flip flop output is the actual output of the IC 555 and for now, it is low i.e. = 0. The LED connected at the output gets off and indicates IC 555 output waveform started it’s off time. The output Q of the flip flop is connected to the base of a discharge transistor. The base of the discharge transistor is high (Q =1) therefore transistor goes into saturation condition.

The discharge transistor provides a path for discharging a capacitor and it starts discharging towards the ground through the resistor R_B and transistors collector to emitter terminal. This is called a discharging time (td) of a capacitor and it is given by tc = 0.693 (R_B) C. The discharging time (off time) is shown in the waveform.

During discharging of a capacitor the voltage decreases towards the ground. When the voltage across the capacitor is slightly less than +1/3 Vcc the input applied to the upper comparator gets less than the reference voltage of +2/3 Vcc. So, therefore, the upper comparator produces a low output voltage.

The voltage across the capacitor is also applied to the trigger input of a lower comparator. The input voltage for the lower comparator gets less than the reference voltage of +1/3 Vcc. So, therefore, the lower comparator produces a high output voltage. The outputs of both comparators are applied to R and S inputs of the flip flop.

The inputs of the flip flop are R =1 and S=0, so the flip flop goes into Reset condition and its outputs become Q = 0 and  = 1.

Again the capacitor starts charging and an astable multivibrator produces a high output voltage. After charging the capacitor discharges and astable multivibrator produces a low output voltage. This process will continue automatically, therefore, it is called a free-running multivibrator.

Applications of an Astable multivibrator

Astable multivibrators can be used to generate a continuous train of rectangular pulses called a clock for microcontroller or microprocessor circuits and also for other digital circuits. It can be also used in communication.

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