555 timer ic

Understanding 555 Timer

The 555 Timer is one of the most well-known and commonly utilised integrated circuits. It’s a versatile and exceptionally durable integrated circuit that’s employed in a variety of applications, including timers, pulse generators, and oscillators.

The IC555, sometimes known as the 555 Timer, was created by Signetic Corporation’s Hans Camenzind in 1971.

The album was split into two parts: NE 555 and SE 555. The NE 555 parts were developed for commercial use and have a temperature range of 0 to 700 degrees Celsius, whereas the SE 555 parts are meant to fulfil military standards and have a temperature range of -550 degrees Celsius to 1250 degrees Celsius. It was the first commercially available timer IC and is a monolithic IC.


  • Features
  • Different Modes Of Operation
    • Astable Mode
    • Monostable Mode
    • Bistable Mode
  • Pin Configuration of 555 Timer
  • 555 Timer Internal Circuit Diagram
  • 555 Timer Working
  • Introduction to Time Constant RC
    • Choosing Timing Components for RC circuit in Timer
      • Timing Capacitor
      • Timing Resistor
      • Trigger Pulses
  • Applications


  • The 555 timer works with a wide range of power supply, from 5 to 18 volts.
  • It comes in three different packages: an 8-pin Metal Can package, an 8-pin DIP package, and a 14-pin DIP package.
  • Timing might range from a few microseconds to several hours.
  • It has astable and monostable modes of operation.
  • High current output.
  • It features a duty cycle that can be adjusted.
  • Because of its high output current, it is TTL compatible.
  • The output can either source or sink a 200mA current to the load.
  • It exhibits 0.005 percent temperature stability per 0C.

Different Modes Of Operation

Generally, the 555 timer can be operated in three modes: Astable, Monostable (or one-shot) and Bistable.

Astable Mode

In this mode, the 555 work as a free running mode. The output of astable multivibrator will continuously toggle between low and high, there by generating a train of pulse, which is why it is known as pulse generator.

It is a best example for a perfect square wave generator. They are used as an inverter and also used in many of the internal part of the radio. Selecting a Thermistor as a timing resistor allows the use of the 555 in a temperature sensor.

Monostable Mode

It maintains in its stable condition in monostable mode until and unless an external trigger is applied, as the name implies. The 555 acts as a “one-shot” pulse generator in this mode. The most effective use of a monostable is to add a temporal delay to a system.

Timers, missing pulse detection, bounce free switches, touch switches, frequency divider, capacitance measurement, and pulse-width modulation (PWM) are just a few of the applications available.

Bistable Mode

In the bistable mode, the IC 555 acts as a flip-flop as it has two stable states. It can be used to store 1-bit of data. It is not a great choice for implementing a flip-flop.

Pin Configuration of 555 Timer

The 555 Timer is available in 8-pin Metal Can Package, 8-pin Mini Dual in-line Package (DIP) and 14-pin DIP. The 14-pin DIP is IC 556 which consists of two 555 timers.

The 8-pin DIP is most commonly used. The pin out diagrams of 555 Timer in both the 8-pin packages is shown below.

The names and numbers of all the pins along with their descriptions are tabulated below.

Pin 1 – Ground (GND)

Ground reference voltage (low level 0V). All the voltages are measured with respect to this terminal.

Pin 2 – Trigger Terminal

It is responsible for SET and RESET transitions of the flip-flop. The amplitude of the external trigger pulse will influence the output of the timer. The output goes high and the timing interval starts when the input at trigger pin falls below half of Control voltage (i.e. 1/3 of the VCC).

Pin 3 – Output Terminal

This pin can be used to generate an output-driven waveform. It’s pushed down to 1.7 volts below VCC. The output can be connected to two different types of loads. The normally OFF load is connected between Pins 3 and 1 (GND), while the normally ON load is connected between Pins 3 and 8. (VCC).

Pin 4 – Reset Terminal

A negative pulse on this pin will disable or reset the timer. The timer will begin only when the voltage on this pin is above 0.7 V and hence it is normally connected to VCC when not used.

Pin 5 – Control Voltage

It regulates the 555’s threshold and trigger levels, as well as its timing. The control voltage determines the width of the output pulse. An external voltage provided to this pin can alter the output voltage. When not in use, it is usually linked to ground through a 10F capacitor to eliminate any noise.

Pin 6 – Threshold Terminal

The voltage applied at this terminal is compared with a reference voltage of 2/3 VCC. When the voltage at this terminal is greater than 2/3 VCC, the flip-flop is RESET and the output falls from High to Low.

Pin 7 – Discharge

It is connected to the open collector of the internal NPN transistor which discharges the timing capacitor. When the voltage at this pin reaches 2/3 VCC, the output toggles from high to low.

Pin 8 – VCC or Supply

A supply voltage in the range of 5V to 18V is applied to this terminal.

555 Timer Internal Circuit Diagram

  • Two Comparators
  • An SR flip-flop
  • Two transistors
  • A resistive network
555 Timer

The basic Op-amps are the comparators. The threshold voltage is compared to a 2/3 VCC reference voltage by comparator 1, which yields the R input.

The trigger voltage is compared to a 1/3 VCC reference value by comparator 2, which gives the S input to the flip-flop.

A voltage divider circuit will be created by a resistive network comprising three resistors. Each of these resistors has a value of 5K. The name “IC 555” comes from these three 5K resistors.

Out of the two transistors, one transistor is a discharge transistor. The open collector of this transistor is connected to the discharge pin (Pin 7) of the IC. According to the output of the flip-flop, this transistor either goes into saturation or cut-off.

When the transistor is saturated, it provides a discharge path to the capacitor that is connected externally. The base of the other transistor is connected to the reset terminal (Pin 4) which resets the timer irrespective of the other inputs.

555 Timer Working

A voltage divider network is formed by the three 5K resistors. This network supplies two reference voltages to two comparators: 2/3 VCC to the top comparator’s inverting terminal (comparator 1), and 1/3 VCC to the lower comparator’s non-inverting terminal (comparator 2). (comparator 2).

The control input is connected to the higher comparator’s inverting terminal. In most cases, the control input is bypassed and linked to 2/3 VCC. The higher comparator’s other input is threshold, and its output is connected to the flip-R flop’s input.

The flip-flop is RESET and the output becomes LOW when the threshold voltage is greater than 2/3 VCC (i.e. the control voltage). This activates the discharge transistor (which reaches saturation) and creates a discharge route for any externally attached capacitor.

The inverting terminal of the lower comparator is coupled to a trigger input. The lower comparator’s output is high when the trigger input is smaller than the reference voltage (1/3 VCC).

This is linked to the flip-S flop’s input, causing the flip-flop to be SET, the output to go HIGH, and the timing interval to begin. Because the output is high, the discharge transistor is turned off, allowing any capacitor connected externally to be charged.

As a result, the trigger input must be lower than the reference voltage for the output to go HIGH. When the threshold voltage is larger than 2/3 VCC, the output is low, which resets the flip-flop and therefore the output.

Introduction to Time Constant RC

Meeting timing requirements is a high priority task in most of the operations. For example, the heating process of a metal or a material in an industry is time limited.

Hence meeting the specific time requirements can be achieved by timer circuits.

A basic timer circuit is shown below. It consists of a charging circuit, a comparator and an output unit.

A resistor and a capacitor make up the charging circuit. The time it takes for the capacitor to charge to its peak value is regulated by the resistor when an RC circuit is used in series with a DC voltage.

The length of time it takes to charge is related to the resistance value. Time constant is the pace at which the capacitor charges in an RC circuit.

The time taken by the Capacitor to charge through the Resistor by about 63.2 percent of the difference between initial and final values is known as the RC Time Constant (indicated by the symbol ).

It is also equal to the time taken by the capacitor to discharge to 36.8%. Time constant of an RC circuit is equal to the product of R and C.

τ = RC

As mentioned earlier, when the trigger input falls below 1/3 VCC, the output of the timer goes high and the period for which this stays high is determined by the RC time constant.

The pulse width and the frequency of the output of the 555 timer are determined by the RC time constant.

Choosing Timing Components for RC circuit in Timer

Depending on the values of R and C in the charging circuit, a 555 timer can give delays ranging from microseconds to hours. As a result, selecting adequate resistor and capacitor values is critical.

When the 555 timer is in Astable mode, an RC circuit consisting of two resistors and a capacitor is required. The RC circuit consists of a resistor and a capacitor in monostable mode of operation.

Timing Capacitor

It will be difficult to select capacitors with big capacitances. Because electrolyte capacitors with large capacitances frequently have wider tolerance limits, this is the case. As a result, there may be a large disparity between the real and marked values.

Leakage currents in large capacitance electrolyte capacitors are substantial, which can impact timing precision as the capacitor charges. Tantalum capacitors are a superior choice when you need a large capacitance and low leakage current.

Electrolyte capacitors with a high working voltage rating should be avoided since they are inefficient when run at a voltage 10% lower than their rated voltage.

Hence, capacitors with working voltage greater than the VCC of the 555 timer should be chosen.

Timing capacitors with capacitance less than 100pF in order to produce short output pulses may also cause problems.

For capacitors with such low values, stray capacitance around the circuit might affect the capacitance of the timing capacitor.

Timing Resistor

When operating the 555 timer as an Astable multivibrator , the value of the timing resistor should be at least 1 Kilo Ohms. If the idea is to build a low power consumption circuit, then it is better to have higher values for the timing resistors.

But there is a disadvantage in choosing resistors with higher resistances as they lead to inaccuracies in timing. In order to minimize these inaccuracies, the value of the timing resistor shouldn’t be more than 1 Mega Ohms.

Trigger Pulses

The 555 timer’s Pin 2 is a trigger input. The timer’s output is high when the trigger input falls below the reference voltage, which is 1/3 VCC, and the timed period begins.

The trigger pulse should briefly drop below the reference voltage, and its duration should not exceed that of the output pulse.

A narrow negative going spike is commonly used to identify trigger pulses. A capacitor and resistor differentiator circuit produces two symmetrical spikes, but a diode is employed to eliminate the positive going spike.

The duration of the pulse is determined by the differentiator circuit (i.e. it depends on the capacitor and resistor).


Since the introduction of the IC 555 in the early 70’s, it has been employed in numerous circuits and applications by researchers as well as hobbyists. Some of the important areas of applications of the 555 timer are:

  • Pulse Generation
  • Time Delay Generation
  • Precision Timing
  • Sequential Timing
  • Pulse Width Modulation (PWM)

The typical applications of a 555 timer can be differentiated by the mode of operation. Depending on the mode in which it is operated i.e. either in astable or in monostable mode, some of the applications of IC 555 are:

  • Frequency Divider
  • Linear Ramp Generator
  • Missing Pulse Detector
  • Pulse Position Modulation
  • Square Wave Generation
  • Pulse Width Modulation
  • Oscillator
  • Tone Burst Generator
  • Speed Warning Device
  • Regulated DC – to – DC Converter
  • Voltage – to – Frequency Converter
  • Low Cost Line Receiver
  • Cable Tester


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