Overview :

The 555 Timer module is used to generate pulses for various purposes.  It is a highly stable integrated circuit that can produce accurate time delays and oscillating pulses at regular intervals.  The 555 Timer was designed by Hans Camenzind in 1971, can be found in many electronic devices starting from toys and kitchen appliances.


555 pin configuration diagram

  • Pin 1. – Ground, The ground pin connects the 555 timer to the negative (0v) supply rail.
  • Pin 2. – Trigger, The negative input to comparator No 1. A negative pulse on this pin “sets” the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a “LOW” to a “HIGH” state.
  • Pin 3. – Output, The output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately Vcc – 1.5V so small speakers, LEDs or motors can be connected directly to the output.
  • Pin 4. – Reset, This pin is used to “reset” the internal Flip-flop controlling the state of the output, pin 3. This is an active-low input and is generally connected to a logic “1” level when not used to prevent any unwanted resetting of the output.
  • Pin 5. – Control Voltage, This pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise.
  • Pin 6. – Threshold, The positive input to comparator No 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This pin connects directly to the RC timing circuit.
  • Pin 7. – Discharge, The discharge pin is connected directly to the Collector of an internal NPN transistor which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”.
  • Pin 8. – Supply +VCC, This is the power supply pin and for general purpose TTL 555 timers is between 4.5V and 15V.

555 Timer Modes:

The 555 timer generally operates in 3 modes

  1. A-stable
  2. Mono-stable
  3. Bi-stable modes.

Astable mode:

This means there will be no stable level at the output. So the output will be swinging between high and low. This character of unstable output is used as a clock or square wave output for many applications. To make 555 Timer work as Astable mode make circuit as give below.

555 Timer Astable Mode

555 Timer Astable mode circuit diagram

The frequency of the oscillation can be adjusted by changing the values of the resistors R1 and R2 and the capacitance of the capacitor C.

The frequency can be calculated using the following expression:


Frequency calculation in 555 timer Astable mode

With the output voltage that comes from pin 3, you can control anything you want (like an LED, speaker, motor, etc.).

Application of Astable Multivibrator :

  • The astable multivibrator is used as the wave generator.
  • It is used as voltage-frequency converter.
  • It is used in pulse synchronization.
  • Since it produces square waves, it is a source of production of a harmonic frequency of higher order.
  • It is used in the construction of voltmeter and SMPS.
  • I can be operated as an oscillator over a wide range of audio and radio frequencies.

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