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555 Timer – A Complete Basic Guide
September - 8 - 2009
A complete basic tutorial of 555 Timer IC.

This article covers every basic aspect of 555 Timer IC. You may already know that SE/NE 555 is a Timer IC introduced by Signetics corporation in 1970′s. In this article we cover the following information about 555 Timer IC. 1. Introduction to 555 Timer IC

2. 555 Timer IC Pin Configuration
3. Basics of 555 Timer
4. Block Diagram
5. Working Principle
1. Introduction

555 timer IC
555 timer IC

One of the most versatile linear ICs is the 555 timer which was first introduced in early 1970 by Signetic Corporation giving the name as SE/NE 555 timer. The 555 is a monolithic timing circuit that can produce accurate and highly stable time delays or oscillation. Like general-purpose op-amps, it is very much reliable, easy to use and cheaper in cost. It has a variety of applications including monostable and astable multivibrators, dc-dc converters, digital logic probes, waveform generators, analog frequency meters and tachometers, temperature measurement and control devices, voltage regulators etc. The timer basically operates in one of the two modes either as a monostable (one-shot) multivibrator or as an astable (free-running) multivibrator.The SE 555 is designed for the operating temperature range from – 55°C to 125° while the NE 555 operates over a temperature range of 0° to 70°C. The important features of the 555 timer are :

* It operates from a wide range of power supplies (+ 5 V to + 18 V supply voltage). * Sinking or sourcing 200 mA of load current.
* Proper selection of only a few external components allows timing intervals of several minutes or frequencies as high a several hundred kHz. * It has a high current output; the output can drive TTL. * It has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or equivalently 0.005 %/ °C. * It has an adjustable duty cycle; the maximum power dissipation per package is 600 mW and its trigger and reset inputs are logic compatible.

2. IC Pin Configuration

555 Timer IC Packages
555 Timer IC Packages

The 555 Timer IC is available as an 8-pin metal can, an 8-pin mini DIP (dual-in-package) or a 14-pin DIP.

This IC consists of 23 transistors, 2 diodes and 16 resistors. The explanation of terminals coming out of the 555 timer IC is as follows. The pin number used in the following discussion refers to the 8-pin DIP and 8-pin metal can packages.

555 timer ic 14 pin configuration
555 timer ic 14 pin configuration

Pin 1: Grounded Terminal: All the voltages are meas­ured with respect to this terminal.

Pin 2: Trigger Terminal: This pin is an inverting input to a comparator that is responsible for transition of flip-flop from set to reset. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin.

Pin 3: Output Terminal: Output of the timer is avail­able at this pin. There are two ways in which a load can be connected to the output terminal either between pin 3 and ground pin (pin 1) or between pin 3 and supply pin (pin 8). The load connected between pin 3 and ground supply pin is called the normally on load and that connected between pin 3 and ground pin is called the normally off load.

Pin 4: Reset Terminal: To disable or reset the timer a negative pulse is applied to this pin due to which it is referred to as reset terminal. When this pin is not to be used for reset purpose, it should be connected to + VCC to avoid any possibility of false triggering.

Pin 5: Control Voltage Terminal: The function of this terminal is to control the threshold and trigger levels. Thus either the external...
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