The 555 timer IC is the most commonly used in electronics projects. It is an integrated circuit including a single 8-bit microcontroller and some complex system on a chip. It provides time delays as an oscillator and as a flip-flop element in other projects. Ic 555 was introduced in 1971 by an American company Signets. It is being used in most projects of electronics due to its low price, ease to use, and stability. Most electronics companies make ic 555. It is mostly used in LED flasher circuits for the fun of learners.


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The standard 555 timer IC consists of 25 transistors,  15 resistors, and 2 diodes on a silicon chip. It consists of an 8-pin mini dual-in-line package (DIP-8). We try to learn the pins with functions. The circuit diagram of 555 is given below.

Circuit Diagram of Timer ic 555

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Pin diagram and description of 555

Pin 1 (Ground):
Connects to the 0v power supply it is also called a ground pin.

Pin 2 (Trigger):
Timer IC 555 makes output HIGH as detects 1/3 rail voltages. Pin 2 trigger has control over pin 6 thresholds. When pin 2 is LOW and pin 6 is also LOW then output goes and stays HIGH. When pin 6 is HIGH and pin2 goes LOW then output goes LOW while pin2 is LOW. This pin has a high impedance of about 10 m and will trigger about 1uA.

Pin 3 (Output):
The output of the timer IC is available at this pin. There are two style outputs (positive and negative) so the out can be connected in two ways. One way is to connect the output pin (pin 3) and ground pin (pin 1) or between pin 3 and pin 2.

Pin 4 (Reset):
Pin 4 of timer ic is used to reset or disable, and a negative pulse is applied to pin 4 for resetting. You must be taken below 0.8v DC to reset the chip.

Pin 5 (Control):
Threshold and trigger levels are controlled using this pin 5. The pulse width of the output is controlled by using this pin.

Pin 6 (Threshold):
Pin6 of 555 detects 2/3 of rail voltage to make output LOW only if pin 2 is HIGH. Pin 6 has a very high impedance of about 10 M and will trigger.

Pin 7 (Discharge):
Open collector output discharge a capacitor between intervals and phase output is controlled with pin 7.

Pin 8 (Supply):
PIN 8 is also called a supply pin. A positive supply between 3 and 16 volts can be applied by using this pin.

Features of Timer ic

555 timer is used in almost every electronic project today. As a flip-flop or as a multi-vibrator, it has a specific set of configurations. Some major features of the 555 timer IC are given below.

  • It operates from +5 Volts to +18 Volts supply voltage.
  • Sourcing or Sinking 200 MA of load current.
  • The external components are selected to change the timing intervals as needed.
  • The output of a 555 can drive a transistor-transistor logic due to its high output.
  • Temperature stability of 50 parts per million (ppm) per degree Celsius changes in temperature which is equivalent to 0.005 %/ °C.
  • The duty cycle of the 555 timers is adjustable.

555 timer IC working

This Timer IC generally operates in 3 modes:

  • A-stable
  • Mono-stable
  • Bi-stable modes.

Astable mode

Astable mode means there is no stable level at the output and the output will be swinging between high and low. This character is used as a clock or square wave output for many applications.

Mono-stable mode

This means that there is one stable and one unstable state. The stable state can be chosen as either high or low according to the requirement.

Bi-stable mode

In bistable means, both the output states are stable, and the output changes from low (0) to high (1) and vice versa.

Applications of 555 Timer circuits

To understand the full working of 555 is visits the following Projects base on this is.