555 Timer – A Complete Basic Guide

jojo March 10, 2014 43 Comments

A complete basic tutorial of 555 Timer IC.

This article covers every basic aspect of 555 Timer IC. You may already know that SE/NE 555 is a Timer IC introduced by Signetics corporation in 1970’s. In this article we cover the following information about 555 Timer IC.

1. Introduction to 555 Timer IC

2. 555 Timer IC Pin Configuration

3. Basics of 555 Timer

4. Block Diagram

5. Working Principle

6. Download Data Sheet

If you still need a detailed understanding of the 555 timer IC, we have reviewed 3 books in our online store. These books covers all the aspects of the 555 timer IC, along with its applications. To get reviews and to buy them, click here:- 3 Great Books to Learn 555 Timer Circuits and Projects

1. Introduction

555 timer IC
555 timer IC

The 555 timer IC was introduced in the year 1970 by Signetic Corporation and gave the name SE/NE 555 timer. It is basically a  monolithic timing circuit that produces accurate and highly stable time delays or oscillation. When compared to the applications of an op-amp in the same areas, the 555IC is also equally reliable and is cheap in cost. Apart from its applications as a monostable multivibrator and astable multivibrator, a 555 timer can also be used in dc-dc converters, digital logic probes, waveform generators, analog frequency meters and tachometers, temperature measurement and control devices, voltage regulators etc. The timer IC is setup to work in either of the two modes – one-shot or monostabl or as a free-running or astable multivibrator.The SE 555 can be used for temperature ranges between – 55°C to 125° . The NE 555 can be used for a temperature range between 0° to 70°C.

The important features of the 555 timer are :

  • It operates from a wide range of power supplies ranging from + 5 Volts to + 18 Volts supply voltage.
  • Sinking or sourcing 200 mA of load current.
  • The external components should be selected properly so that the timing intervals can be made into several minutes along with the frequencies exceeding several hundred kilo hertz.
  • The output of a 555 timer can drive a transistor-transistor logic (TTL) due to its high current output.
  • It has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or equivalently 0.005 %/ °C.
  • The duty cycle of the timer is adjustable.
  • The maximum power dissipation per package is 600 mW and its trigger and reset inputs has logic compatibility. More features are listed in the datasheet.

2. IC Pin Configuration

555 timer ic pin configuration and diagram
555 Timer IC Pin Configuration

The 555 Timer IC is available as an 8-pin metal can, an 8-pin mini DIP (dual-in-package) or a 14-pin DIP. The pin configuration is shown in the figures.

This IC consists of 23 transistors, 2 diodes and 16 resistors. The use of each pin in the IC is explained below. The pin numbers used below refers to the 8-pin DIP and 8-pin metal can packages. These pins are explained in detail, and you will get a better idea after going through the entire post.

555 timer dual in line package 14 pin configuration

Pin 1: Grounded Terminal: All the voltages are measured with respect to the Ground terminal.

Pin 2: Trigger Terminal: The trigger pin is used to feed the trigger input hen the 555 IC is set up as a monostable multivibrator. This pin is an inverting input of a comparator and is responsible for the transition of flip-flop from set to reset. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin. A negative pulse with a dc level greater than Vcc/3 is applied to this terminal. In the negative edge, as the trigger passes through Vcc/3, the output of the lower comparator becomes high and the complimentary of Q becomes zero. Thus the 555 IC output gets a high voltage, and thus a quasi stable state.

Pin 3: Output Terminal: Output of the timer is avail­able at this pin. There are two ways in which a load can be connected to the output terminal. One way is to connect between output pin (pin 3) and ground pin (pin 1) or between pin 3 and supply pin (pin 8). The load connected between output and ground supply pin is called the normally on load and that connected between output and ground pin is called the normally off load.

Pin 4: Reset Terminal: Whenever the timer IC is to be reset or disabled, a negative pulse is applied to pin 4, and thus is named as reset terminal. The output is reset irrespective of the input condition. When this pin is not to be used for reset purpose, it should be connected to + VCC to avoid any possibility of false triggering.

Pin 5: Control Voltage Terminal: The threshold and trigger levels are controlled using this pin. The pulse width of the output waveform  is determined by connecting a POT or bringing in an external voltage to this pin.  The external voltage applied to this pin can also be used to modulate the output waveform. Thus, the amount of voltage applied in this terminal will decide when the comparator is to be switched, and thus changes the pulse width of the output. When this pin is not used, it should be bypassed to ground through a 0.01 micro Farad to avoid any noise problem.

Pin 6: Threshold Terminal: This is the non-inverting input terminal of comparator 1, which compares the voltage applied to the terminal with a reference voltage of 2/3 VCC. The amplitude of voltage applied to this terminal is responsible for the set state of flip-flop. When the voltage applied in this terminal is greater than 2/3Vcc, the upper comparator switches to +Vsat and the output gets reset.

Pin 7 : Discharge Terminal: This pin is connected internally to the collector of transistor and mostly a capacitor is connected between this terminal and ground. It is called discharge terminal because when transistor saturates, capacitor discharges through the transistor. When the transistor is cut-off, the capacitor charges at a rate determined by the external resistor and capacitor.

Pin 8: Supply Terminal: A supply voltage of + 5 V to + 18 V is applied to this terminal with respect to ground (pin 1).

3. 555 Timer Basics

The 555 timer combines a relaxation oscillator, two comparators, an R-S flip-flop, and a discharge capacitor.

S-R-Flip Flop

S-R-Flip Flop

As shown in the figure, two transistors T1 and T2 are cross coupled. The collector of transistor T1 drives the base of transistor T2 through the resistor Rb2. The collector of transistor T2 drives the base of transistor T1 through resistor Rb1. When one of the transistor is in the saturated state, the other transistor will be in the cut-off state. If we consider the transistor T1 to be saturated, then the collector voltage will be almost zero. Thus there will be a zero base drive for transistor T2 and will go into cut-off state and its collector voltage approaches +Vcc. This voltage is applied to the base of T1 and thus will keep it in saturation.

S-R Flip Flop Symbol

S-R Flip Flop Symbol

Now, if we consider the transistor T1 to be in the cut-off state, then the collector voltage of T1 will be equal to +Vcc. This voltage will drive the base of the transistor T2 to saturation. Thus, the saturated collector output of transistor T2 will be almost zero. This value when fedback to the base of the transistor T1 will drive it to cut-off. Thus, the saturation and cut-off value of anyone of the transistors decides the high and low value of Q and its compliment. By adding more components to the circuit, an R-S flip-flop is obtained. R-S flip-flop is a circuit that can set the Q output to high or reset it low. Incidentally, a complementary (opposite) output Q is available from the collector of the other transistor. The schematic symbol for a S-R flip flop is also shown above. The circuit latches in either the Q state or its complimentary state. A high value of S input sets the value of Q to go high. A high value of R input resets the value of Q to low. Output Q remains in a given state until it is triggered into the opposite state.

555 IC Timing Circuit

555 IC Timing Circuit

Basic Timing Concept

From the figure above, assuming the output of the S-R flip flop, Q to be high. This high value is passed on to the base of the transistor, and the transistor gets saturated, thus producing a zero voltage at the collector. The capacitor voltage is clamped at ground, that is, the capacitor C is shorted and cannot charge.

The inverting input of the comparator is fed with a control voltage, and the non-inverting input is fed with a threshold voltage. With R-S flip flop set, the saturated transistor holds the threshold voltage at zero. The control voltage, however, is fixed at 2/3 VCC, that is, at 10 volts, because of the voltage divider.

Suppose that a high voltage is applied to the R input. This resets the flip-flop R-Output Q goes low and the transistor is cut-off. Capacitor C is now free to charge. As this capacitor C charges, the threshold voltage rises. Eventually, the threshold voltage becomes slightly greater than (+ 10 V). The output of the comparator then goes high, forcing the R S flip-flop to set. The high Q output saturates the transistor, and this quickly discharges the capacitor. An exponential rise is across the capacitor C, and a positive going pulse appears at the output Q. Thus capacitor voltage VC is exponential while the output is rectangular. This is shown in the figure above.

4. Block Diagram

555 IC Timer Block Diagram

555 IC Timer Block Diagram

The block diagram of a 555 timer is shown in the above figure. A 555 timer has two comparators, which are basically 2 op-amps), an R-S flip-flop, two transistors and a resistive network.

  • Resistive network consists of three equal resistors and acts as a voltage divider.
  • Comparator 1 compares threshold voltage with a reference voltage + 2/3 VCC volts.
  • Comparator 2 compares the trigger voltage with a reference voltage + 1/3 VCC volts.

Output of both the comparators is supplied to the flip-flop. Flip-flop assumes its state according to the output of the two compa­rators. One of the two transistors is a discharge transistor of which collector is connected to pin 7. This tran­sistor saturates or cuts-off according to the output state of the flip-flop. The saturated transis­tor provides a discharge path to a capacitor con­nected externally. Base of another transistor is connected to a reset terminal. A pulse applied to this terminal resets the whole timer irrespective of any input.

5. Working Principle

Refer Block Diagram of 555 timer IC given above:

The internal resistors act as a voltage divider network, providing (2/3)Vcc at the non-inverting terminal of the upper comparator and (1/3)Vcc at the inverting terminal of the lower comparator. In most applications, the control input is not used, so that the control voltage equals +(2/3) VCC. Upper comparator has a threshold input (pin 6) and a control input (pin 5). Output of the upper comparator is applied to set (S) input of the flip-flop. Whenever the threshold voltage exceeds the control voltage, the upper comparator will set the flip-flop and its output is high. A high output from the flip-flop when given to the base of the discharge transistor saturates it and thus discharges the transistor that is connected externally to the discharge pin 7. The complementary signal out of the flip-flop goes to pin 3, the output. The output available at pin 3 is low. These conditions will prevail until lower comparator triggers the flip-flop. Even if the voltage at the threshold input falls below (2/3) VCC, that is upper comparator cannot cause the flip-flop to change again. It means that the upper comparator can only force the flip-flop’s output high.

To change the output of flip-flop to low, the voltage at the trigger input must fall below + (1/3) Vcc. When this occurs, lower comparator triggers the flip-flop, forcing its output low. The low output from the flip-flop turns the discharge transistor off and forces the power amplifier to output a high. These conditions will continue independent of the voltage on the trigger input. Lower comparator can only cause the flip-flop to output low.

From the above discussion it is concluded that for the having low output from the timer 555, the voltage on the threshold input must exceed the control voltage or + (2/3) VCC. This also turns the discharge transistor on. To force the output from the timer high, the voltage on the trigger input must drop below +(1/3) VCC. This turns the discharge transistor off.

A voltage may be applied to the control input to change the levels at which the switching occurs. When not in use, a 0.01 nano Farad capacitor should be connected between pin 5 and ground to prevent noise coupled onto this pin from causing false triggering.

Connecting the reset (pin 4) to a logic low will place a high on the output of flip-flop. The discharge transistor will go on and the power amplifier will output a low. This condition will continue until reset is taken high. This allows synchronization or resetting of the circuit’s operation. When not in use, reset should be tied to +VCC.

Download 555 Timer Data Sheet:

To know more about NE/SE 555 timer IC check out/download the datasheet. – NE-SE 555 Timer Datasheet

To know more about applications of 555 Timer IC take a look at the following posts:

555 Timer IC Packages
555 Timer IC Packages
  • Gaurav Shetty
    March 14, 2015

    In the block diagram,Q of RS Flip flop is connected no where.I think it must be connected to the the discharge transistor.Please get it corrected.

  • Mussawar
    February 27, 2015

    I’ve found another little mistake in “Block Diagram”. The 3R chain, at its lower end, is connected to the discharge transistor. Actually it is connected to the ground. Please correct it. Thanks

  • pushpak jadhav
    October 19, 2014

    good easy to understand …There are so many corrections ..in the working principle it is given that ….at the 2Vcc/3 is connected to the non inverting of upper comparator. But it is connected to INVERTING OF upper comparator ..similarly in lower comparator
    1vcc/3 is connected to NON INVERTINGng …but it is given inverting…plz fix it…

    October 15, 2014


  • shuaib rabiu
    October 1, 2014

    it relly open my mind more about internal architecture of the 55 timer.

  • shivi
    August 14, 2014

    really good

  • lneftw
    August 9, 2014

    Correction: Replace the second occurence of the word “transistor” with “capacitor” in the sentence:
    A high output from the flip-flop when given to the base of the discharge transistor saturates it and thus discharges the transistor that is connected externally to the discharge pin 7.

    Should read:
    A high output from the flip-flop when given to the base of the discharge transistor saturates it and thus discharges the CAPACITOR that is connected externally to the discharge pin 7.

  • parth
    July 4, 2014

    This diagram of IC 555 is wrong plz check the I/p polarity of both comparator……..

  • Dhruv
    May 18, 2014

    Amazing simplicity…very helpful for a high school project I am working on…

    Thank you so much circuitstoday!!

  • May 1, 2014

    sir, please give me easy electronic project circuit

    • Dhruv
      May 18, 2014

      this is a really easy and conceptual topic..you can achieve it easily and it can fetch you attention as well…

  • May 1, 2014

    please give me any electronic theory chapter and easy circuit diagram

  • mangesh patel
    March 20, 2014

    thanx to all of you

  • Anju
    February 25, 2014

    thanx..easy to understand..

  • February 23, 2014

    thank of you.It is very nice.I had learn much from this site

  • February 9, 2014

    nice easy to understand.

  • Nikhil m c
    October 20, 2013

    Superb !!!
    Easily described..even a layman can understand …

  • sasmita
    April 5, 2013

    gud evening sir,plz guides me to make a electronics project & give idea about 555 timer,relay,trasmitter & reciver.

  • divya
    March 16, 2013

    thanking you sir

  • rakesh
    March 7, 2013

    sir, when threshold is < 2/3 vcc, why the output of comparator cannot change the flipflop's output, why it is mentioned that only triggr terminal and comparator 2 can only make output of ic to go heigh? please explain that clearly..

    • ashwin
      August 7, 2015

      look at the truth table of sr ff

  • lacuna
    February 20, 2013

    It was very helpful got to know much about 555 timer IC for my micro project.

  • pooja
    January 12, 2013

    i m really thanking you. its much necessary for me and also useful.

  • atul
    January 8, 2013

    thnx god , its hlp me soo mch

  • Durga Rao
    December 3, 2012

    Everything is good but I think there was a mistake in the inverter connection please correct it if any person newly studying this may get confused…

    • Durga Rao
      December 3, 2012

      sorry not inverter i.e comparator

  • Roney
    November 14, 2012

    thank u its very useful

  • Hamza Arif
    September 8, 2012

    It is realy helpful.!
    Can you make some more projects based on 555 ICs.?

  • Rushirajsinh
    September 2, 2012

    can u mail me the circuit to generate edge output of 1s using 555 ic

  • subhashini
    August 30, 2012

    thank uuuuuuuu

  • amit kumar
    July 23, 2012

    its really good..

  • Ruma Roy
    May 31, 2012

    follow d book “electronic devices and circuits” by J.B Gupta……..a cmplt descriptn abt IC555 is given ovr thr…..

  • Godslight
    May 29, 2012

    I sencerely apreciate this websit and like the sit cos am also an engineer

  • Sahiljain
    January 9, 2012

    Under Head 3. i.e. 555 basics:-
    in the 2th para 3rd line it should be ” keeps the transistor Q1 in cut-off” instead of ” keeps the transistor q2 in cut-off”

  • RICK
    November 16, 2011

    Nice Explanation!!! Mr Hans Camerzind designed The 555 in 1970….It’s the most widely used I.C. that was ever developed for industrial engineering and hobbyist applications alike!!

  • Manoj Yadav
    July 29, 2011

    As DSRAO and IAMZ mentioned, a correction is required in block diagram of 555. comparator connections are not correct.

  • IAMZ
    June 8, 2011

    Same with the trigger…

  • dsrao
    June 5, 2011

    In the Block diagram-555 Timer there is a correction required. The threshold pin 6 must go to the Non-inverting pin of the comparator. In the description, there is no mistake.

    • haba haba
      March 7, 2013

      Dude..your block diagram for step 5 is off.

      The rest is gold

  • February 20, 2011

    Are you able to please deliver me a mail. I truly like your design.

  • […] 555 timer combines a relaxation oscillator, two comparators, an R-S flip-flop, and a discharge […]

Leave a Reply

Your email address will not be published. Required fields are marked *