Timer IC-555


Timers help in generating time delays and pulses of required duty cycle in the required mode. A timer IC is a form of oscillator. The length of the pulse is adjusted by means of external resistors and capacitors. In this tutorial, we’ll discuss IC555. NE555 is one of the ICs which facilitate timer usage. This IC has one timer set.


Pin configuration

NE555 is the most commonly used IC for timer usage. Here is the pin configuration of NE555.



Pin description:

Vcc(pin8), Gnd(pin1): Power supply

Discharge (pin7):  This pin is used to discharge an external capacitor that works together with a resistor to control the timing interval.

Trigger(pin2): Triggering is accomplished by taking the pin below 1/3 of rail voltage - in digital terms, this is called a LOW. When it is taken low, it causes the output to go high.

Threshold (pin6): It detects 2/3 voltage of Vcc to make output LOW only when pin 2 is high.

Control (pin5): By applying a voltage to this pin, it is possible to vary the timing of the chip. If the control-voltage pin is not used, it should be bypassed to ground, with a 10n capacitor to prevent noise entering the chip.

Reset (pin4): The reset pin is an overriding function. It will force the output pin to go low regardless of the state of the trigger pin (Pin 2). It can be used to terminate an output pulse at once.

Output (pin3): Output of the pulses is obtained from this pin. To make the output high, the trigger pin (pin 2) is momentarily taken from a high to a low.  This causes the output to go high. This is the only way the output can be made to go high.

The output can be returned to a low by making the threshold pin (Pin 6) go from a low to a high.

The output can also be made to go low by taking the reset pin to a low state.


Modes of operation

Using the features of the pins, 555 IC can be used in different modes. There are 3 modes in which you can use this IC. They are:

  1.      Monostable mode: In this mode the output is stable in only one (mono) state i.e. ‘off’ state. Thus it can stay only for a finite time, if triggered, to the other state i.e. ‘on’ state. This time can be set choosing appropriate values of resistances in the formula:

T = 1.1 x R x C

Here is the circuit diagram:


Here is how the output varies:



  1.      Bistable mode: In this mode, we can set the output in both the modes as per our requirement. When triggered, the output goes high and when reset, the output goes low.

Here is the circuit diagram:


Here is how the output varies:




  1.      Astable mode: In this mode; the output is stable neither in ‘high’ state nor in ‘low ‘ state. Hence it oscillates from one state to another giving us a square wave or clock. We can set the clock frequency and Duty cycle by the formulae:


Time High (secs) = 0.693 * (R1 + R2) * C1

Time Low (secs) = 0.693 * R2 * C1

Frequency = 1/(Th+Tl) = 1.44 / ((R1 + 2 R2) * C1)

Duty Cycle Percentage = ((Th / (Th + Tl)) * 100


Here is the circuit diagram:




Here is how the output varies:


The fact that timer can be used in different modes makes it open for various usages in the real world.