IC 555

Updated: Jan 10, 2019

Introduction :

IC 555 is a well-known component in the electronic circles but what is not known to most of the people is the internal circuitry of the IC and the function of various pins present there in the IC. The timer got its name from the three 5 kilo-ohm resistor in series employed in the internal circuit of the IC.

IC 555 timer is a one of the most widely used IC in electronics and is used in various electronic circuits for its robust and stable properties. It works as square-wave form generator with duty cycle varying from 50% to 100%, Oscillator and can also provide time delay in circuits. The 555 timer got its name from the three 5k ohm resistor connected in a voltage-divider pattern .

555 is a very commonly used IC for generating accurate timing pulses. It is an 8pin timer IC and has mainly two modes of operation: monostable and astable. In monostable mode time delay of the pulses can be precisely controlled by an external resistor and a capacitor whereas in astable mode the frequency & duty cycle are controlled by two external resistors and a capacitor.

Pin Description:


This pin is used to provide a zero voltage rail to the Integrated circuit to divide the supply potential between the three resistors shown in the diagram.


As we can see that the voltage at the non-inverting end of the comparator is Vin/3, so if the trigger input   is used to set the output of the F/F to ‘high’ state by applying a voltage equal to or less than Vin/3 or any negative pulse, as the voltage at the non-inverting end of the comparator is Vin/3.


It is the output pin of the IC, connected to the Q’ (Q-bar) of the F/F with an inverter in between as show in the figure.



 This pin is used to discharge the timing capacitors (capacitors involved in tThis pin is used to reset the output of the F/F regardless of the initial condition of the F/F and also it is an active low Pin so it connected to ‘high’ state to avoid any noise interference, unless a reset operation is required. So most of the time it is connected to the Supply voltage as shown in the figure.

Control Voltage:

As we can see that the pin 5 is connected to the inverting input having a voltage level of (2/3) Vin. It is used to override the inverting voltage to change the width of the output signal irrespective of the RC timing network.


The pin is connected to the non-inverting input of the first comparator. The output of the comparator will be high when the threshold voltage will be more than (2/3) Vin thus resetting the output (Q) of the F/F from ‘high’ to ‘low’.

he external circuit to make the IC behave as a square wave generator) to ground when the output of Pin 3 is switched to ‘low’.


This pin is used to provide the IC with the supply voltage for the functioning and carrying of the different operations to be fulfilled with the 555 timer.


555 is very commonly used for generating time delays and pulses.

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