electroSome

Astable Multivibrator using 555 Timer

555 Timer

555 Timer

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. Astable Multivibrator using 555 Timer 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

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.

Circuit with Internal Block Diagram

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

Astable Multivibrator using 555 Timer – Working

Design

Capacitor Charges through Ra and Rb.

Capacitor Discharges through Rb

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

  Seconds
  %
  μF



Video