555 timer IC
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The 555 timer IC is an integrated circuit which can be used to perform a number of timer and multivibrator functions. It continues to be one of the most versatile integrated circuits that are seen in use today. This IC is known for its ease of use, low cost, versatility and excellent stability.
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Development of the 555 timer
The 555 timer was invented by Swiss design consultant Hans Camenzind. It was designed in 1970 and introduced in the market by Signetics Corporation in 1971. Signetics was later acquired by Philips. Originally called the SE555/NE555, this device was referred to as ‘The IC Time Machine’. As electronic technology flourished over the years, the IC timer device became an important part of many electronic systems.
The use of the 555 timer has risen because of the numerous advantages it has to offer. The Korean company Samsung today manufactures over a billion such chips every year to be sold in the global electronics market.
The 555 timer and its variants
The 555 timer essentially consists of 23 transistors, 2 diodes and 16 resistors integrated on a silicon chip. This chip is installed in an 8-pin dual-in-line package (DIP). The timer may also be available in different forms such as the 14-pin 556 DIP that combines two 555s on a single chip, the 16-bit 558 that has four 555s on a single chip and other ultra-low power variants such as the 7555. The low power versions have a wiring which is slightly different as compared to the conventional types. Additionally they use less external components and hence require reduced power.
Modes of operation of the 555 timer
The 555 has three modes of operation:
- Monostable mode: The timer has two states in this mode, only one of which is stable. A control input is used to push the circuit into the unstable mode. The charging and discharging of a capacitor through a resistor decides the time period for which a certain state is maintained. This mode is useful for timer applications and missing pulse detection. Touch switches and bouncefree switches can also be implemented in this mode.
- Astable mode: In this mode the 555 operates as an oscillator. The multivibrator in this mode has two states, neither of which is stable. The circuit thus works as an oscillator. The time period here too is controlled by a capacitor and resistor. Applications of this form may include logic clocks, security alarms and LED flashers.
- Bistable mode: In the bistable mode, the 555 works as a flip-flop. The DIS pin is left unconnected and no capacitor is employed. There exist two states with the help of which applications such as bouncefree latched switches are possible.
Specifications of the NE555
The NE555 chip has the following specifications: Supply voltage (VCC) 4.5 - 15 V Supply current (VCC = +5 V) 3 - 6 mA Supply current (VCC = +15 V) 10 - 15 mA Maximum output current 200 mA Power dissipation 600 mW Operating temperature 0 - 70° C
There are several variants of the 555, each of which may have improved specifications depending on the type of application that is desired.