Application of IC 555 as an Astable Multivibrator

 Application of IC 555 as an Astable Multivibrator

         An astable multivibrator can generate vibrating output between high and low voltage hence called as astable output as there is no stable state. It is also known as a free running multivibrator.

     That means it is oscillating from one state to another hence it is called as astable mode, and it produces rectangular output signal.

Fig. – Circuit Diagram of IC 555 as Astable multivibrator

Fig. Working of internal transistor with external capacitor

Working :When Q is low, transistor is cut off and the capacitor charges through R1 and R2.

As the capacitor charges, the threshold voltage increases, when it reaches through 2/3 Vcc the upper comparator has a high output and sets of the flip-flop making Q high.

This saturates transistor and the capacitor discharges through it. The discharge takes place through resistor R2 only when the capacitor voltage drops slightly less than 1/3 Vcc. The lower comparator has high output which resets the flip-flop and makes Q low.

This brings dicharge transistor again in cut-off condition and the capacitor again charges through R1 and R2.

The output waveforms are shown in fig.


The capacitor charges through R1 and R2and discharges through R2 only, therefore,Charging

time of capacitor, TON = 0.693 (R1 + R2) C

And the discharge time will be, TOFF = 0.693 R2 C

The period T is, T = TON + TOFF

                          = 0.693 (R1 + R2) C + 0.693 R2 C

The frequency of oscillations is given by F= 1/T

                                                                   = 1/ ( 0.693 (R1 + R2) C + 0.693 R2 C )

                                                                   = 1.44/ ( R1 + 2R2) C Hz

Duty Cycle

The charging time constant is greater than discharge time constant.

Hence, the output waveform is not symmetric square wave.

The high output remains for longer time than low output.

The ratio of high output period and low output period is given by duty cycle.

It is defined as “ the ratio of ON time to the total time of one cycle”.

D = TON / T = TON / (TON  + TOFF )

D = 0.693 (R1 + R2) C / 0.693 (R1 + 2R2) C

    = (R1 + R2) / (R1 + 2R2)

Percentage duty cycle is D’ = ((R1 + R2) / (R1 + 2R2) ) X 100

The duty cycle is always greater than 50% if R1is much greater than R2the duty cycle approaches 50% and output waveform will be symmetric square wave.


■       Application of Astable M/V : Square Wave Generator

To get the square wave output, it is necessary to adjust the 50% duty cycle.

One way for getting it, is the diode connected in series with R2 as shown in following fig.

                                                      Fig.- Square Wave Generator

The capacitor C charges through R1only, as a diode D is forward biased and discharges throughR2.

TON = 0.693 R1 . C

TOFF = 0.693 R2 .C

The period wave of T will be – T = TON+ TOFF = 0.693 R1 . C + 0.693 R2 .C

But R1 = R2 = R

T = 0.693 RC + 0.693 RC

    = 1.386 RC

Hence, The frequency, F = 1/T = 1/ 1.386 RC

                                         = 0.72 / RC

The percentage duty cycle, D’ = (TON/ (TON+ TOFF) ) X 100

                                                 = (0.693 RC / 2 X 0.693 RC) X 100

                                            D’ = R/ 2R X100

                                                  = 50%

This gives square wave output.


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