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What is IC 555?

by Sonali

The IC 555 timer is an 8 pin chip. It is used in many applications such as multi-vibrators. This chip operates in three modes 

  • A-stable mode
  • Mono-stable mode
  • Bi-stable mode

Features of IC 555

  • It can operate in the range of +5v to +18 V of supply voltage.
  •  It sinks or sourced 200 mA of load current. 
  • In order to maintain the timing intervals it is important to have a proper selection of external components along with frequency exceeding several hundred kilohertz.
  • As output of this IC is high current it can easily drive the transistor transistor logic (TTL).
  • It can handle a temperature ranging upto 50 ppm (parts per million)/ °C. It is equivalent to 0.005%/°C
  • We can easily adjust the duty cycle of this IC.
  • It has trigger and reset inputs with logic compatibility. The maximum power dissipation can be upto 600mW /package.  

Working of IC 555

Astable mode

In this case the output is not stable and always awnings between the high and low states. Hence, the output is not stable. Therefore, the unstable output is widely used as clock or square wave output.

Mono-stable mode

There are two states in this configuration: one is stable and the other is unstable. Decision of making a stable state high or low depends upon the user. If we set stable state output high then the output of the timer will be high. The timer output is low when an interrupt is applied. Since, the low state is unstable it can change to high immediately after an interrupt passes through it. 

Bi-stable mode

Unlike monostable the Bi-stable mode both states are stable. When an interrupt is applied the output changes from low to high. For instance if output is high it will change to low on arrival of an interrupt and stays in that state until another interrupt arrives.

Pin Configuration

  • Pin 1: This is ground pin.
  • Pin 2: This pin shows as TRIG. The output stays high till the trigger is low.
  • Pin 3: This is the output pin
  • Pin 4: It is a reset pin. We can also reset the timing interval by grounding this pin.
  • Pin 5: When the internal voltage divider gets the control access through this pin.
  • Pin 6: When voltage at threshold is greater than control it ends the timing interval.
  • Pin 7: Open collector output which may discharge a capacitor between intervals and also it is in phase with output.
  • Pin 8: It is positive voltage supply and is in between 3V-15V

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