IC 555 Timer | Details Explanation And Pin Configuration

IC 555 Timer

The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications.





The 555 Timer, designed by Hans Camenzind in 1971, can be found in many electronic devices starting from toys and kitchen appliances to even a spacecraft. It is a highly stable integrated circuit that can produce accurate time delays and oscillations. The 555 Timer has three operating modes, bistable, monostable and astable mode.

555 Timer Block Diagram

Pin Details

• 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.

Important Features of IC 555 Timer

555 timer is used in almost every electronic circuit today. For a 555 timer working as a flip flop or as a multi-vibrator, it has a particular set of configurations. Some of the major features of the 555 timer would be,

1. It operates from a wide range of power ranging from +5 Volts to +18 Volts supply voltage.
2. Sinking or sourcing 200 mA of load current.
3. The external components should be selected properly so that the timing intervals can be made into several minutes along with the frequencies exceeding several hundred kilohertz.
4. The output of a 555 timer can drive a transistor-transistor logic (TTL) due to its high current output.
5. It has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature which is equivalent to 0.005 %/ °C.
6. The duty cycle of the timer is adjustable.
7. Also, the maximum power dissipation per package is 600 mW and its trigger and reset inputs has logic compatibility.

Working Procedure:

The 555 generally operates in 3 modes:

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

Mono-stable Mode

This configuration consists of one stable and one unstable state. The stable state can be chosen either high or low by the user. If the stable output is set at high (1), the output of the timer is high (1). At the application of an interrupt, the timer output turns low (0). Since the low state is unstable it goes to high (1) automatically after the interrupt passes. Similar is the case for a low stable monostable mode.

Bi-stable Mode:

In bistable mode, both the output states are stable. At each interrupt, the output changes from low (0) to high (1) and vice versa, and stays there. For example, if we have a high (1) output, it will go low(0) once it receives an interrupt and stays low (0) till the next interrupt changes the status.

Application Area

Some common uses and application of 555 timer IC are as follow:

1. PWM (Pulse Width Modulation) & PPM (Pulse Position Modulation)
2. Duty Cycle Oscillator
3. Lamp Dimmer
4. To provide Accurate time delays
5. As a flip-flop element
6. Digital logic probes
7. Analog frequency meters
8. Quad Timer applications