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 Astable Multivibrator using 555 Timer B Y L I G O G E O R G E ( H T T P S : / / E L E C T R O S O M E . C O M / A U T H O R / L I J O P P A N S / ) / 8 C O M M E N T S

( H T T P S : / / E L E C T R O S O M E . C O M / A S T A B L E - M U L T I V I B R A T O R - 5 5 5 - T I M E R / # D I S Q U S _ T H R E A D )

 An Astable Multivibrator is an oscillator circuit that continuously produces rectangular wave

without the aid of external triggering. So Astable Multivibrator is also known as Free Running

Multivibrator. I have already posted about Astable Multivibrator using Transistors

(https://electrosome.com/astable-multivibrator-transistors/). Astable Multivibrator using 555

Timer (https://electrosome.com/555-timer-ic/) is very simple, easy to design, very stable and

low cost. It can be used for timing from microseconds to hours. Due to these reasons 555 has

a large number of applications and it is a popular IC among electronics hobbyists.

 Astable Multivibrator using 555 – Circuit

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(https://electrosome.com/wp-content/uploads/2012/11/555-timer-astable-

multivibraotr.jpg)

 Astable Multivibrator using 555 Timer Circuit Diagram

 Above figure shows the circuit diagram of a 555 Timer wired in Astable Mode. 8th pin and 1st

pin of the IC are used to give power, Vcc and GND respectively. The 4th pin is RESET pin

which is active low and is connected to Vcc to avoid accidental resets. 5th pin is the Control

Voltage pin which is not used. So to avoid high frequency noises it is connected to a capacitor 

C’ whose other end is connected to ground. Usually C’ = 0.01μF. The Trigger (pin 2) and

Threshold (pin 6) inputs are connected to the capacitor which determines the output of the

timer. Discharge pin (pin 7) is connected to the resistor Rb such that the capacitor can

discharge through Rb. Diode D connected in parallel to Rb is only used when an output of 

duty cycle less than or equal to 50% is required. For the sake of explaing the working Circuit

Diagram with Internal Block diagram is shown below.

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Circuit with Internal Block Diagram

(https://electrosome.com/wp-content/uploads/2012/11/astable-multivibrator-using-555-

timer-internal-diagram.jpg)

 Astable Multivibrator using 555 Timer Block Diagram

Since the Control Voltage (pin 5) is not used the comparator reference voltages will be 2/3

Vcc and 1/3 Vcc respectively. So the output of the 555 will set (goes high) when the capacitor 

voltage goes below 1/3 Vcc and output will reset (goes low) when the capacitor voltage goes

above 2/3 Vcc.

Working

When the circuit is switched ON, the capacitor (C) voltage will be less than 1/3 Vcc. So

the output of the lower comparator will be HIGH and of the higher comparator will be

LOW. This SETs the output of the SR Flip-flop.

Thus the discharging transistor will be OFF and the capacitor C starts charging from

Vcc through resistor Ra & Rb.

When the capacitor voltage will become greater than 1/3 Vcc ( less than 2/3 Vcc ), the

output of both comparators will be LOW and the output of SR Flip-flop will be same as

the previous condition. Thus the capacitor continuous to charge.

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(https://electrosome.com/wp-content/uploads/2012/11/Astable-Multivibrator-using-

555-Timer-Working.gif)

 Astable Multivibrator using 555 Timer – Working 

When the capacitor voltage will becomes slightly greater than 2/3 Vcc the output of the

higher comparator will be HIGH and of lower comparator will be LOW. This resets the

SR Flip-flop.

Thus the discharging transistor turns ON and the capacitor starts discharging through

resistor Rb.Soon the capacitor voltage will be less than 2/3 Vcc and output of both comparators will

be LOW. So the output of the SR Flip-flop will be the previous state.

So the discharging of capacitor continuous.

When the capacitor voltage will become less than 1/3 Vcc, the output SETs since the

output of lower comparator is HIGH and of higher comparator is LOW and the capacitor 

starts charging again.

This process continuous and a rectangular wave will be obtained at the output.

Design

Capacitor Charges through Ra and Rb.

Thigh = 0.693(Ra + Rb)C

Capacitor Discharges through Rb

Tlow = 0.693RbC

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Output Frequency = 1/(Tlow + Thigh) = 1.44/((Ra + 2Rb) * C)

Duty Cycle = Thigh/(Thigh + Tlow)

Where Thigh and Tlow are the time period of HIGH and LOW of the output of 555.

From this we can find that Duty Cycle less than or equal to 50% cannot be obtained. There

are two ways to obtain this.

1. Inverting the output

2. Using a Diode Parallel to resistor Rb

Inverting the Output

In this method, just compliment the output. Thus the Duty Cycle = Tlow/(Tlow + Thigh).

Where Tlow and Thigh are the time period of LOW and HIGH of output of 555. In this method

the duty cycle of the output of 555 will be greater than 50% and that of inverter will be less

than 50%.

 

Duty Cycle of 555 = 1 – Duty Cycle of Inverter 

Using a Diode Parallel to resistor Rb

In this method we will connect a diode parallel to resistor Rb as shown dotted in the first

circuit diagram above. Thus the charging current of capacitor will bypass the resistor Rb.

Thus Thigh = 0.693RaC Thus a Duty Cycle less than or equal to 50% can easily obtained.

Calculator 

Time Period :

0.5 Seconds

Duty Cycle :

75 %

Capacitance C :

10 μF

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