555 Timer IC: Working, Pin Diagram, Features, and Different Operating Modes


The 555 Timer is a commonly used IC designed to produce a variety of output waveforms with the addition of an external RC network The 555 timer IC is an integrated circuit that is use in a variety of timer, pulse generation, square wave generation, delay operations and oscillations etc.  The 555 timer was introducing by Signetic Corporation in 1970. This is available in 8, 14 and 16 pin dual in package. The internal structure of IC is depending upon the manufacturing company the IC has around 23 transistors, 2 diodes and 16 resistors arranged to form two comparators, a flip-flop and a high current output stage as shown in block diagram.

555 Timer Tutorial

The 555 timer is an IC that produces various types of wave forms with the combinations of external resistance and capacitor network. We have notices that the CMOS Oscillators and Multivibrators can be made of different type of component for generation of relaxation oscillators for generating basic square wave output waveforms with the combination of few timing component.

On such component comes on integrator circuit that is “555 timer”. The naming of 555 timer is come from the three resistor at the in input of IC and the value of resistors is 5kΩ that resistance is use to generate the two comparators reference voltages. It is most popular for commercial use because it is less expansive, generate single pulses, long time delays and relaxation oscillator producing a string of stabilised waveforms of varying duty cycles from 50 to 100%.

It is extremely robust device that can be operated either as a very accurate Astable Multivibrator, Bistable Multivibrator and Monostable Multivibrator to produce a variety of applications such as one-shot or delay timers, LED and lamp flashers, logic clocks, pulse generation, frequency division, alarms and tone generation, power supplies and converters etc, in fact any circuit that requires some form of time control as the list is endless.

The 555 is also available in the NE556 Timer Oscillator. The NE556 Timer is the combination of two individual 555 timers in 14-pin DIP package. The CMOS 555 timer is 7555 and MOSFET timer is LMC555.

555 Timer Block Diagram

The block diagram represent the internal connection of 555 is given below. The internal circuit consists of three resistance, two comparator, one flip flop and one output driver.

555-Timer -Block-Diagram

555 Timer PIN Diagram

  • Pin 1. – It is a Ground pin whose connects the 555 timer to the negative (0v) supply.
  • Pin 2. – It is Trigger pin, 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. – It is Output pin, that 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, the function of this pin to reset the Flip-flop controlling the state of the output, pin 3.
  • Pin 5. – Control Voltage, It control 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.

Functional Diagram

The functional diagram of 555 timer is shown below. With the functional diagram we can easily understand the internal function.


This circuit contains three 5k resistance, two comparator one flip flop, two transistors and one an inverter. We are going to discuss these functions in detail.

Voltage Divider Network

The voltage divider consists of three 5k resistors and these are connected in series between the supply voltage Vcc and ground. The voltage divider network provides a voltage of Vcc/3 between point and ground when only one 5k resistance exists. Similarly it provide 2 Vcc/3 between point and ground when only two 5k resistance exists.


The timer circuit has two comparators one is Upper Comparator (UC) and a Lower Comparator (LC). These comparators compare the two inputs that are applied to it and produce an output. The non inverting input of Op-amp is greater than inverting input than comparator output will be +Vsat. And this considers logic “1” in digital representation.

If the voltage at non inverting terminal is less than inverting terminal of Op-amp than the comparator output will be −Vsat. In digital electronics, it is consider logic “0”.

SR Flip-Flop

The SR flip flop may be positive or negative edge trigger. It has two inputs S-R and two outputs Q(t) and Q(t)’. Both outputs are compliment with each others. The state table of SR flips is shown:

S R Q(t+1)
0 0 Q(t)
0 1 0
1 0 1
1 1

Here, Q(t) & Q(t+1) are present state & next state respectively. So, SR flip-flop can be used for one of these three functions such as Hold, Reset & Set based on the input conditions, when positive (negative) transition of clock signal is applied.

The outputs of Lower Comparator (LC) and Upper Comparator (UC) are applied as inputs of SR flip-flop as shown in the functional diagram of 555 Timer IC.

555 Timer Circuits

The circuit of 555 timer is use to generate square wave and also use in many circuit. The transistors, resistor diode flip flop and capacitor are the basic comports. It has three main part that are voltage divider, comparator and flip flop. This circuit use 4.5-15V DC supply. The function of this circuit is depends on the comparing of inputs single like inverting and non inverting.

If voltage of non inverting terminal is low with respect to inverting terminal of Op-amp than the comparator output will be −Vsat. In digital electronics, it is consider logic “0” and non inverting input of Op-amp is greater than inverting input than comparator output will be +Vsat. And this considers logic “1” in digital representation.

Operating Modes of 555 Timer IC

The operation mode of 555 timer are astable, bistable and monostable. Each mode of operation signifies with a circuit diagram and its output.

Astable operation mode

In this operation mode the circuit will produce continuous pulse with exact frequency based on the value of the two resistors and capacitors. Due to continuous pulse generation it is known as free running mode and self triggering mode. It has no any stable state and only has two quasi stable states that automatically changes from one to another without any trigger pulse. The capacitor charging and discharging of this mode is depends upon the specific voltage. The circuit diagram of Astable is shown below. If the voltage applied at input, the capacitor start charging through two resistor and produce pulses continuously. The pins 2 and 6 are short with each other for endless re-activate the circuit. If output is high, than capacitor start discharging. The applications of this mode are to generate square wave oscillations, clock pulse, PWM wave etc.


Monostable Mode Operation

This mode of operation, the circuit generates only single pulse. Due to this it is also known as single shot mode. It has one quasi stable state and one stable state. It changes state from quasi stable state to stable state when trigger pulse is applied and comes back to stable state after pre determine time automatically. The pulse width of this mode is depending on value of resistor and capacitor. If an activating pulse is applied to the i/p of the circuit through a push button, then the capacitor gets charge and the timer circuit extends a high pulse, then it remains high until capacitor totally discharges. If it is necessary to enhance the time delay, then higher rate of capacitor and resistor are required. Its application is generating pulses, time delay etc.


Bistable Mode Operation

The bistable mode is also called as flip flop mode because it has only two stable states. It uses two different pulses for changing the state for low to high and high to low. It is most use in generate pulse and automatic switching applications.


Important Features of the 555 Timer

  • It’s Sourcing or sinking load current is 200mA.
  • Adjustable duty cycle.
  • The output of a 555 timer IC can drive TTL due to its high current output.
  • 555 timer IC works from a wide range of power supplies ranging from +5V to +18V.
  • It takes a temperature stability of 50 ppm/oC change in temperature (ppm means parts per million)
  • The max power dissipation per package is 600 milliwatts and its reset and trigger inputs has logic compatibility.


The 555 timer IC is mostly use in digital electronics. The application of 555 timer is given below:-

  1. It is use in generation of non sinusoidal wave like square, triangular rectangular etc.
  2. For time delay generation, precision timing, and sequential timing.
  3. It can be used as a mono-stable multi-vibrator and a-stable multi-vibrator.
  4. It is use in digital commendation for generation of PWM (Pulse Width Modulation) & PPM (Pulse Position Modulation).
  5. For maintain DC Voltage in circuit.
  6. Used in Tachometers & temperature measurement.
  7. Commonly used in DC Voltage Regulators.
  8. Used for conversion like Voltage to Frequency Converter.
  9. It is also used in Frequency Divider.
  10. It is used in the Pulse detector.
  11. It can be used to design a Timer Switch.

555 Timer Datasheet

The timer IC is most usable circuit in various applications. For all given parameters I have taken reference of LM555 /NE555/SA555 datasheet provided by “Fairchild semiconductor”.

parameter condition Min value Max value Explanation
Supply voltage (Vcc) 4.5 V 16 V The biasing voltage limit should not exceed these two limits
Supply current (Icc) Vcc = 5V & R= ? 6 mA For 5V supply and output open circuited the maximum supply current is limited to 6 mA
Power dissipation (PD) 600 mW When IC555 is in running condition it dissipates 600 mW
Operating temperature 0oC 70oC IC555 should be operated within this temperature range. Beyond it performance will deteriorate
Soldering temperature For 10 sec 300oC If IC555 is directly soldered in circuit the maximum soldering temperature should not exceed 300 oC
Storage temperature -65 oC 150 oC IC555 should not be stored beyond these storage temperature limits
Control voltage (Vc) Vcc = 5 V 2.6 V 4 V The input voltage at pin number 5 is limited to 2.6 V to 4 V for 5 V supply
Threshold voltage (Vth) Vcc = 5 V 3.33 V 3.33 V The input voltage at pin number 6 should be slightly higher than this value
Trigger voltage (Vtr) Vcc = 5 V 1.1 V 2.2 V The input voltage at pin number 2 must be less than 2.2 V for 5 V supply. Its typical value is 1.67 V
Reset Voltage (Vrst) 0.4 V 1 V The reset input voltage at pin number 4 should be higher than 0.4 V for proper operation
Threshold current (Ith ) Vcc = 5 V 0.25 µA This current will limit the value of total resistance (R1+R2) = 6.7 M?
Trigger current (Itr) Vtr = 0 V 2 µA The trigger input requires maximum 2 µA current
Reset current (Irst) 0.4 mA
Low output voltage (Vol) Vcc = 5 V Isink = 5 mA 0 V 0.35 V For 5 V supply the maximum output low voltage is 0.35 V
High output voltage (Voh) Vcc = 5 V Isource = 100 mA 2.75 V Vcc For 5 V supply the minimum output high voltage is 2.75 V
Sinking current (Isink ) Vcc = 5 V 5 mA For 15 V supply when output is low chip can maximum sink 50 mA current from supply
Vcc = 15 V 10 mA 50 mA
Sourcing current(Isource) Vcc = 5 V 100 mA For 15 V supply when output is high chip can maximum source 200 mA current to load
Vcc = 15 V 100 mA 200 mA
Rise time (Tr) 100 nS Time required for output to rise from low value to 50 % of maximum value is 100 n sec only
Fall time (Tf) 100 nS Time required for output to fall from high value to 50 % is 100 n sec only


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