Electronic circuits that are used to measure time intervals are known as 555 TIMERs, as the name suggests. The topic of 555 timers will be discussed in this article. The primary application for this integrated circuit is to provide precise and consistent delays in electronic circuits. Additionally, it comes in 8 pin and 14 pin DIP configurations. Electronics projects frequently use 555 timers. Some of them perform microcontroller functions and are utilized in place of microcontrollers.

555 Timer IC Introduction

555 is a timer oscillator IC introduced by an American company named Signetics and is intended for use in timing applications for generating long time delays, pulse generation, and frequency division. It has a simple circuit design, easy to use and cost-effective due to which it is used in a wide variety of applications. Most importantly,  it has three different modes of operation namely astable, monostable and bistable mode. All these modes are discussed in later sections.

555 Timer Pinout Diagram

Furthermore, it is also available in the 14 Pin DIP package. This diagram represents pinout for 8-pin DIP and 14-pin DIP-packages. But, there is no difference between them except No connection of pins for 14 pins package.

Pin Configuration Details

This IC is composed of a comparator, RS flipflop and a transistor. The description of pins is mentioned below:

  • The supply pins, pins 1 and 8, are attached to the ground and a source of positive voltage, respectively.
  • Pin2 is the chip’s inbuilt comparator’s trigger input pin, which is a negative pin. The output is a function of the value applied to this pin’s amplitude. It sets the internal flip flop that changes the output from low to high when its value is less than 1/3 of the supply voltage.
  • An output pin on the 555 timer IC is pin 3.
  • Regardless of the trigger input, the timer can be reset or disabled using the reset pin. It is connected to a high voltage when it is in typical use. It will reset the output if a low signal is applied.
  • A control input pin is pin number 5. It is used to regulate trigger and threshold inputs, as its name suggests. Additionally, it is used to regulate the output pulse’s width. In order to control the output pulse’s width or modulate it, an external voltage is attached to it. To prevent any noise problems, attach a capacitor to any pins that are not in use.
  • The positive pin of a comparator is used as the threshold input. Its amplitude is what causes the flip-flop to be in its set state.
  • Internally, the discharge pin is coupled to the low logic. When the transistor reaches saturation, it is used to discharge the capacitor.

Features of 555 timer

It is used as

  • Timers-To provide time delays
  • Pulse generation-To toggle between high and low states
  • Oscillator applications.

Other important features are:

  • Wide range of power supply (5-18)V
  • Sinking High current output (200mA of load current)
  • Variable duty cycle
  • Logic compatible trigger and reset inputs
  • High-temperature stability
  • The operating power supply range is from 5 Volts to 18 Volts.
  • It can drive TTL as its output current is high.
  • It has a capability of sourcing or sinking up to 200mA current
  • Trigger and reset inputs are logic compatible.
  • It has an adjustable duty cycle
  • It has two outputs which are normally on and normally off.
  • Turn off time is less than 2µ

555 timer Internal Circuit

The internal circuit of this 555 timer consists of comparators, flipflops, transistors, resistors, and output stages.

555 Timer Working Principle

In this section, we will see the working of each internal component of the 555 timer IC.


Three 5kOhm resistors were utilized as a voltage divider between Vcc and ground in this circuit. The voltage at the junction of the resistor is 2/3 Vcc and 1/3 Vcc because all resistors are the same value, and this voltage is utilized as a reference voltage for the comparators.

Comparator Circuit

It is a circuit that determines if an input voltage is higher or lower than a reference voltage and outputs a low or high signal based on the comparison. For this comparison purpose, many transistors are used. Trigger is the comparator 1’s inverting input. The comparator 2 receives a threshold as a non-inverting input. The trigger input is compared to a reference voltage that is 1/3 of Vcc by the comparator 1 attached to pin 2. The threshold input is compared to a reference voltage that is 2/3 of Vcc by the comparator 2 attached to pin 6.

Flip Flop Function

Depending on the state of the inputs, the circuit can be in either of two states. The inputs of a flip-flop are the outputs of the two comparators. The flip flop changes to a high output when the trigger comparator outputs a low signal (regardless of the threshold comparator’s output). The flip flop shifts from a high output to a low output when trigger and threshold, both comparators, are producing a high signal. By manually pulsing the reset pin to low, the time of the high pulse output can be reset.


Two transistors are also shown in the figure above. One transistor is N-P-N and its collector is connected to pin 7. This configuration is known as an open-drain or open collector. Usually, this pin is connected to a capacitor and is used to discharge the capacitor each time the output pin goes low. The second transistor is P-N-P which is connected to pin 4. The purpose of this transistor is to buffer the reset pin and so the 555 IC does not source current from this pin and causes it to sag in voltage.

Output Stage

It acts as a buffer between the 555 timer and the loads that may be attached to its output pin. This stage supplies current to the output pin.

Where to use 555 IC?

Due to its three operating modes, it can be used in a wide variety of applications for pulse generation, creating long time delays, voltage regulators, analog frequency meters and tachometers, control devices, multivibrators, oscillator circuits and many more.

555 Timer Different Modes of Operation

By wiring 555 timers with resistors and capacitors in different ways, It can operate in 3 modes:

  1. Monostable mode
  2. Astable mode
  3. Bistable mode


To create time delays, use this mode. The 555 timer produces a high pulse in this mode. One stable state, or the off state, is referred to as a monostable. The monostable enters its transient state each time an input pulse triggers it. An RC network determines the duration of time that it stays in that condition. Then, it resumes its steady state.

  • Less than 1/3Vcc) is the trigger pin’s low setting. The flip flop is set by a comparator attached to the trigger pin, which has a low output and changes the flip flop’s output from low to high.
  • Pin 7 will cause the discharge transistor to turn off. The capacitor won’t experience a short circuit. When the voltage across the capacitor reaches 2/3 of the supply voltage (Pin 6 voltage=2/3 Vcc), a charge will build up across the capacitor.
  • The comparator’s output now changes to “HIGH” at this moment. The flip-flop will be reset to its initial condition in this way (low).
  • The transistor now turns on, discharging the capacitor through pin 7 to ground. The result is that the output returns to its initial stable state.
  • The output remains low until the trigger pin is pulsed low again.
  • The amount of time that the output voltage remains “HIGH” or at a logic “1” level is given by the following time constant equation.


This mode produces a pulse signal that oscillates. With a configurable pulse width and frequency, the output alternates between high and low states. The values of the two resistors (R1 and R2) and capacitor C1 affect frequency. When the trigger pin is linked to the threshold pin, the output alternates between the high and low states continually.

Frequency of Output = 1/[0.7*(R1+2*R2)*C1]

duty cycle = (R1+ R2) / (R1 + 2*R2)

  • Initially, there is no charge on the capacitor C1, so the voltage across the capacitor is zero. Since capacitor C1, Threshold pin and Trigger pin are connected at the same, so these both will also be at zero volts. This drives the output high.
  • Vcc is applied, the current flowing through resistors start to accumulate charge on the capacitor C1. This causes the voltage across capacitor C to increase.
  • When the voltage across the capacitor C1 equals 2/3Vcc, it makes the threshold pin high, this flips the comparator attached to the threshold pin. This drives the output low and enables the discharge pin.
  • When the discharge is pin enabled, the capacitor will discharge through R2.
  • Once the voltage across the capacitor C1 equals 1/3Vcc, it makes the the trigger pin low. This flips the comparator attached to the trigger pin and drives the output high. By repeating above steps, the output switches between the high and low states to produce a continuous pulse wave.


This mode causes the 555 timers to toggle its output between high and low states depending on the state of two inputs. The circuit is stable in both states. It remains in the same state (either HIGH or LOW) until an external trigger is applied.

  • There are no equations in this mode because, unlike the other two 555 modes, there is no RC network.
  • In reality, this circuit is a flip-flop.
  • The trigger (pin 2) and reset pins are the two inputs (Pin 4).
  • Both are by default kept high by pull-up resistors operating in bistable mode.
  • The trigger pin produces a strong output when pulsed low (less than 1/3(Vcc)). Even if the trigger pin is set to high once more, the output will still be high.
  • The output becomes low when the reset pin pulses low. Even if the reset pin goes high once more, the output will stay in this position.

555 Timer Applications

Some common applications of this IC are:

  • Pulse generation
  • Pulse width and pulse position modulation
  • Sequential Timing circuits for precise timing or pulse generation
  • Oscillators and as a flip flop
  • Analog frequency meters.

2D diagram

It has 8-lead PDIP, SOIC, and VSSOP packages available. The dimensions and 2d diagram of its PDIP package is given below.