555 Timer Internal Circuit Operation Tutorial

555 timer internal circuit Operation Tutorial:

How the 555 Timer IC Operates Internally?

                       Before working with 555 timer IC based mini projects, it is essential to know and understand thoroughly about the internal circuitry of the IC.

In this post we will discuss in detail about internal components of the 555 timer IC. If you want to refresh about 555 timer IC pin configuration read the 555 Timer IC Pinout Details post. The 555 timer internal circuit diagram is shown below: 


555 Timer Internal Circuit Diagram

 555 timer internal circuit Operation

  • As shown , the IC consists of two Operational Amplifliers (IC1 and IC2) in the comparator mode, along with an R-S bistable element (IC3). 
  • Additionally, an inverting buffer (IC4) is incorporated at the output stage. 
  • To understand easily, the circuit diagram is separated in to 5 sections (A,B, C, D, E).


  • It consists of three resistors R1, R2 and R3 connected in series.
  • Since R1 = R2 = R3, the supply voltage VCC will be divided equally across the three resistors.
  • It means, at the junction of R2 and R3 (1/3) of VCC will appear.
  • A the junction of R1 and R2 (2/3) of VCC will appear.


  • In this section, two operational amplifiers used in comparator mode.
  • The OP-AMPs are used to check the voltages at the threshold and trigger inputs.
  • They will compare these inputs with the fixed voltages set by potential divider(The fixed values are 2/3 and 1/3 of VCC respectively).

Section – C:

  • This section comprises of an R–S bistable stage.
  • It can be either set or reset depends on the output from the Section-B (comparator stage).
  • In addition to that, an external reset input is provided.


  • In this section, an open-collector transistor switch (TR1) is available. 
  • This section is used to rapidly discharge an external timing capacitor. 
  • Whenever the base of the transistor is driven positive, the external capacitor will be shorting. 
  • Since the resistors R1, R2 and R3 all have equal values, the VCC is divided into three equal values.
  • So the voltage at the non inverting input of IC1 is (1/3) of VCC and the voltage at the inverting input of IC2 is (2/3) of VCC.
  • So if the VCC is 9 V then 3 V will appear at each resistor(R1, R2, R3). 
  • The upper comparator (IC1) will have 6 V applied to its inverting input and the lower comparator (IC2) will have 3V at its non-inverting input.


  • This stage consists of an inverting power amplifier.
  • It is capable of sourcing / sinking required current to drive a small relay or a resistive load connected to the output pin.
  • The 555 timer IC can source as well as sink the current (Remember that standard TTL logic devices can not do this).
  • When sourcing the current, the IC’s output (at pin no-3) is in the high state. The current will flow from the output pin to ground through the load.
  • When sinking the current, the IC’s output (at pin no-3) is in the low state. Here the current will flow from VCC to Pin-3 through the load. 

 Different 555 Timer IC family Versions:

Standard 555 Timer:
The standard 555 timer is in a 8-pin dual-in-line (DIL) package.
It operates from supply voltages of 4.5V and 15V.
As the normal working voltage is of 5V, it will work well in TTL circuitry. 

Several versions of the 555 timer are available in the market. They are listed below:

Low power (CMOS) 555 Timer 
It is CMOS version of the 555 timer.
Both the pin & function of this version is compatible with standard 555 timer IC.
As it is made up of CMOS technology it operates from 2V to 18V of supply voltages.
It will consume less operating current (120mA 
for an 18V supply). Example IC- ICM75561PA

Dual 555 timer 
This is a dual version of the standard 555 timer in a 14-pin .
The two devices 
can be worked independently.
They have the same electrical characteristics as the standard 555.
Example IC – NE556A

Low-power (CMOS) dual 555 Timer
This is a dual version of the low-power CMOS 555 timer in a 14-pin Dual-In-Line package.
The two 
devices can be worked completely independently.
They have the same electrical characteristics as the 
low-power CMOS 555.

Read More related to 555 Timer:
Monostable Multivibrator using 555 timer IC
Astable Mulitivibrator circuit design based on 555 IC
555 Timer based warning Light Project
555 Timer based simple project ideas

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