555 Timer – An Introduction

555 Timer

In this article I will explain about popular IC 555 and it is commonly called the “555 timer”. This chip was introduced in the Year 1971 by Signetic Corporation and it was the first commercial IC available in the market as SE 555/NE 555. The name comes from the three 5 kilo ohm resistors used to generate two reference comparator voltages.SE/NE 555 specifies the temperature range SE 555 is designed to operate over range from 55 to 125 degrees C and NE 555 is operates from 0 to 70 degrees C. It is basically a monolithic IC which is used for highly accurate and stable time delays. This chip can be used both as a Timer and oscillator. As a timer it produces a single pulse and as an oscillator it gives a continuous stream of pulses producing stabilized wave forms of duty cycles from 50 to 100% this is done by using external components resistor and capacitor. It is relatively cheap, stable one used for monostable and astable applications. Apart from its applications as monostable and astable it is used in waveform generators, temperature measurements, voltage regulators. A dual version called 556(14 pin) is also available which contains two independent 555 IC’s.

Available Packages:

555 Timer is available in two types of packages 1. 8 pin metal can or ‘T’ package 2. 8 pin DIP (Dual Inline package).  Mostly in these days DIP type was preferred as it is easy to fabricate and use. This configuration can be used with an IC socket and prevents the damage caused by the heat. The IC can be replaced easily during repair.

555 Timer DIP and Metal Can Package
555 Timer DIP and Metal Can Package

Here are some of the features of 555 Timer taken from the data sheet.

Features of 555 Timer:

  • 555 timer is used in Monostable and astable modes.
  • It can withstand high amount of temperature.
  • Wide range of operating voltages ranging from +5v to 18v.
  • It produces High current output that can sink or source 200 mA.
  • It is Compatible with different types of logic families DTL, TTL, and CMOS circuits.
  • The Duty cycle can be adjusted form 50 to 100%..
  • It provides good Timing from few micro seconds through several hours.
  • Inputs (TRIGGER and RESET) are logic compatible.


  • It operates with a supply voltage of 4.5 to 18 v.
  • Operates with a supply current of 3 mA.
  • It has output voltage (LOW) of 0.1v.
  • It has output voltage (HIGH) of 12.5(15v VCC) and 3.3v (5v VCC).
  • It has Maximum operating frequency of 500 KHZ.

555 Timer IC Pin Configuration: How it works?

The IC consists of 23 transistors, 16 resistors, and two diodes.

PIN 1: Ground pin connects the 555 timer to the negative supply rail i.e. Ground (0 V). All the voltages are measured with respect to this terminal.

PIN 2: Trigger pin when applied with a negative pulse (a voltage less than 1/3 of VCC) triggers the internal flip flop through comparator 2. This makes the pin 3 (output) to switch from Logic “LOW” to Logic “HIGH “state. The output remains in the high state until the trigger terminal is maintained at low voltage.

PIN 3: Output terminal can be connected to the load in two way: Between output and ground or output and supply rail (+ VCC). When the output is low, the load current (sink current) flows through the load connected between pin 3 and VCC into the output terminal. Similarly when the output is high the current through the load connected between pin 3 and VCC is zero and the output pin supplies current (source current) to the normally off load.

PIN 4: Reset pin is used to disable the flip flop which controls the output state of the pin 3 this can be done by applying a negative pulse to this pin. Generally this pin is active low input connected to a logic “HIGH” when it is not in use. This is done to prevent the unwanted triggering of the output pin. When it is connected to positive terminal voltage it works normally however when it is connected to ground it stops working completely.

PIN 5: Control voltage pin is used to control the trigger and threshold levels. The timing of the IC can be controlled by applying a voltage on this pin or by connecting a POT between ground the pulse width can be varied and hence the comparator switches from one state to another state modulating the output. When this pin is not used it should be connected to ground by using a 0.01µF capacitor to eliminate noise interferences.

PIN 6: Threshold pin is the non-inverting terminal of the comparator 1 this pin traces out the voltage available across the external capacitor. When the voltage at this pin is greater than the threshold voltage 2/3 VCC the output of the comparator 1 becomes high this makes the timer off.

PIN 7: Discharge pin is connected internally to the collector of the transistor and a capacitor is connected between the pin and ground. When the output of this pin goes high Q1 is off and acts as open circuit to the capacitor connected to it. Similarly when the output is low transistor Q1 gets saturated and acts as short circuit at his makes capacitor shorted to ground.

PIN 8: VCC pin is connected to the supply of voltage ranging from +5v to +18v applied to this pin with respect to ground (pin 1).

The main functioning of the IC comes with its operating modes.

Internal Structure of 555 Timer
Internal Structure of 555 Timer

Modes in 555 Timer:

It operates in three modes. They are monostable or one shot mode, bistable mode, astable or free running mode according to multivibrator configurations.

MONOSTABLE MODE: This mode has one stable state when a triggering pulse is applied with push button. The output stays in low state until a triggering pulse is applied hence the name monostable.

ASTABLE MODE: In this mode the circuit has no stable state that is the circuit switches between the two Quasi (temporary) states automatically without applying any type of triggering pulse.

BISTABLE MODE: In this mode the circuit has two stable states. The circuit switches from one state to another state only when another trigger pulse is applied.

We will discuss about the modes clearly in the next tutorial.