Timing Circuits

555 Timer: Astable Operation

In Astable operation, we have no stable states. Hence we say that timer doesn’t stay in any of the two states indefinitely i.e. vibrates between the two states. Hence we don’t need trigger in this case. This is also called Astable Multi-vibrator. This is also called free-running multi-vibrator. When-ever we give power to the timer, we get the rectangular oscillating output signal.

The following diagram would explain the working of the Astable Multi-vibrator:


Working of this circuit is similar to Monostable multi-vibrator. In this circuit voltage of the capacitance oscillates between Vcc/3 and 2 Vcc/3.

Suppose initially we have Q=0 & Q’=1. As Q is 0, transistor is turned OFF and hence capacitor starts charging through R1 + R2. When the voltage of capacitor goes greater than 2 Vcc/3, output of upper Op-amp gets 1 and hence S=1 & R=0 and due to this Q becomes HIGH and Q’ goes LOW.

Now as we have Q=1 & Q’=0. As Q is 1, transistor is turned ON and hence capacitor starts discharging through R2. When the voltage of capacitor becomes less than Vcc/3, output of lower Op-amp gets 1 and hence S=0 & R=1 and due to this Q becomes LOW and Q’ goes HIGH.

Now again we have Q=0 & Q’=1 and whole procedure is repeated. Hence we get the oscillating output as illustrated follow:


In the figure above W is equal to the time in which capacitor is charged to 2 Vcc/3 from Vcc/3 and P is equal to the time in which capacitor is discharged from 2Vcc/3 to Vcc/3. Hence

W= 0.693 (R1+R2) C

P= 0.693 R2 C

So the time period of output is T= 0.693 (R1+2 R2) C

We can vary the duty cycle of output pulse by changing the value of R1 & R2 and duty cycle is defined as

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

And frequency of the timer is F= 1/T = 1.44/ (R1+2 R2) C

2 Replies to “555 Timer: Astable Operation

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