literature review on 555 timer

555 Timer Tutorial: How It Works and Useful Example Circuits

In this 555 Timer tutorial, you’ll learn how to use the 555 timer to do fun things.

One of the first things many do with it is to create a blinking light. But that’s just one simple example of the many things you can do with this chip. You can also control motors, create an alarm, create a musical instrument, and much more.

Let’s start with an overview of the pins.

555 Timer Pinout

Pin 1 Ground This pin connects to the negative side of the battery.

Pin 2 Trigger When this pin goes low (less than one-third of VCC), the output goes high.

Pin 3 Output The output voltage from the chip is around 1.5 V lower than VCC when high and around 0 V when low. A 555 timer can give out only 100 to 200 mA in total. Check your chip’s datasheet for the exact value.

Pin 4 Reset This pin resets the whole circuit. It’s an “inverted” pin, which means it resets when the pin goes low. This means the pin must be high normally so that the chip isn’t in a “reset” state.

Pin 5 Control Voltage This pin is used to control the threshold voltage of the Threshold pin. This can be useful when you want to adjust the frequency of the circuit without changing the values of R1, R2, and C1. Sometimes you’ll see this pin connected with a capacitor (0.01 µF/10 nF) to ground; this is a way to keep any noise on it from influencing the frequency. Sometimes you’ll see it disconnected.

Get the 555 Timer Cheatsheet

A super helpful reference that makes it easy to design circuits, so that you can build oscillators, timer circuits, and more in no time.

Note: I’ve heard from people not able to get their circuit working without this capacitor. So you can try adding a capacitor between this pin and ground if your circuit is not working.

Pin 6 Threshold This pin sets the output back to low when the voltage goes high (above two-thirds of VCC).

Pin 7 Discharge This pin is unconnected when output is high, and it’s connected to ground when output is low.

Pin 8 VCC Supply This is the positive power supply pin and can take a voltage between 5 and 15 V.

To learn more about the circuit on the inside, check out the article How Does a 555 Timer Work?

Astable Mode

When the 555 Timer is in  astable  mode it means that the output will never be stable. The output will keep switching between HIGH and LOW forever. That means it works as an oscillator.

You can use this to blink a light, create sound, control motors, and much more!

555 Timer Astable Circuit Example

Our first example is how to blink an LED with the 555 Timer. This is like the “hello world” equivalent of this IC.

Component List

This circuit is simple enough to build on a breadboard . To build it, you need the following components:

• 555 Timer IC
• R1-R3: Resistor, 1 kΩ
• LED1: Red 5mm LED or similar
• C1: Capacitor, 1000 µF
• C2: Capacitor, 10 nF (it usually works without this)

You don’t need exact values for the resistors and capacitors. But if you use the values listed above, your LED should blink about once every other second. Use the 555 Timer calculator to find the blinking frequency for other values.

Monostable Mode

Monostable means the output is stable in one state, and it will always come back to this state. You can push it out of that state, but it will always return back to its stable state after a certain time.

The output from the 555 Timer in monostable mode is normally LOW. When you trigger the circuit, the output goes HIGH for a certain amount of time before it goes back to LOW again.

This is sometimes called a one-shot circuit .

The time it stays HIGH is decided by the size of a resistor and a capacitor. The higher the values, the longer it stays HIGH.

If you connect a buzzer to the output, you can create an alarm circuit that is triggered for example by a window being opened.

555 Timer One-Shot Example Circuit

The following circuit turns on an LED when you push the button. After about 10 seconds, the LED turns off.

• R1: Resistor, 100 kΩ
• R2: Resistor, 5kΩ to 1 MΩ (this is a  pull-up resistor )
• R3: Resistor, 1 kΩ
• C1: Capacitor, 100 µF
• C2: Capacitor, 10 nF
• S1: Pushbutton, normally open

For longer delays, increase C1 and/or R1. If you want an adjustable delay, replace R1 with a potentiometer. Use the 555 Timer calculator to find the values you need.

The output is connected to control an LED, but could easily be modified to control a motor, a lamp, a coffee maker, or anything else. Just replace R3 and the LED with a transistor . See how in the section Driving Higher Loads below.

Bistable (Flip-Flop) Mode

Bistable means the output is stable in both states (HIGH and LOW). It will stay in one state until you push it over to the other state. Then it stays in the other state. You push it from one state to the other with the Trigger and Threshold pins.

This mode isn’t a timer function at all, and it’s not a common way to use the 555 Timer. In this mode, the 555 Timer works as a flip-flop .

You can for example use it to reverse the direction of a robot when it bumps into a wall. Or separate the ON and OFF switches for a machine.

555 Timer Bistable Example Circuit

The following example shows the 555 Timer in bistable mode. Here you have separate ON and OFF buttons to control an LED.

• S1, S2: Pushbutton, normally open
• R1, R2: Resistor, 5kΩ to 1 MΩ (these are  pullup resistors )
• Resistor (R3): 1 kΩ
• LED: Red 5mm LED or similar
• Capacitor (C1): 10 nF

The output is connected to control an LED, but could easily be modified to control a motor, a lamp, or anything else by connecting a transistor. See the section Driving Higher Loads below for examples.

555 Timer Output Current

The output of the 555 Timer can sink and source current of up to 200 mA.

Sourcing is when the output is HIGH and you’ve connected something from the output down to ground:

In the above circuit, the LED turns on when the output is HIGH.

Sinking is when the output is LOW and you’ve connected something from V CC to the output:

In the above circuit, the LED turns on when the output is LOW.

If you use both sourcing and sinking in your circuit, you can make a cool emergency vehicle light effect by connecting two LEDs; a blue one that is sourcing current and a red one that is sinking current.

Or how about connecting two buzzers with different frequencies to create a siren?

Circuit Example: Police Car Emergency Lights

• R1-R2: Resistor, 1 kΩ
• R3: Resistor, 470 Ω
• R4: Resistor, 330 Ω
• LED1: Red LED
• LED2: Blue LED

R1, R2, and C1 control the blinking speed. R3 and R4 set the brightness of the LEDs.

Driving Higher Loads

If you want to control motors, LED strips, or other things that need more than 200 mA of current, you can connect a transistor to the output.

If you want to use an NPN transistor, you’ll need to connect a resistor between the output and the base to limit the base current. 1 kΩ will probably work fine as a starting point.

If you want to use a MOSFET on the output, make sure you use a MOSFET with a V GS that is lower than the output voltage from your 555 timer.

The resistor is there to protect the output pin from high spikes of current when the MOSFET is being turned on. But given that the 555 Timer supports 200 mA, it will most likely work without it in most cases.

555 Timer Datasheet

Looking for the datasheet for the 555 timer? Each manufacturer has their own. Here’s a list of a few of the most common ones:

• LM555 datasheet by Texas Instruments
• NA555/NE555/SA555/SE555 datasheet by Texas instruments
• NE555 datasheet by Diodes Inc
• LM555C datasheet by National Semiconductor

More 555 Timer Tutorials

• Complete 555 Timer Tutorial with Circuits
• 555 Timer Calculator
• How Does a 555 Timer Work?
• 555 PWM Circuit
• Project: Railroad Lights
• Project: Police Siren Circuit
• Project: Flashing Police Lights
• Project: Metronome Circuit
• Project: Reaction Game
• Project: Machine Gun Sound Effect
• Project: Music Box
• Project: TV Remote Jammer
• Project: Atari Punk Console
• Project: Knight Rider LED Bar
• Project: Continuity Tester Tool
• Project: Blinking Christmas Lights
• Project: The Useless Halloween Machine

Applications of 555 Timer for Development of Low-Cost System

• Conference paper
• First Online: 04 January 2022
• Cite this conference paper

• Shadan AlShidani 41 ,
• Salim Alshabibi 41 ,
• Md Tabrez 42 ,
• Kanak Kumar 43 &
• Farhad Ilahi Bakhsh 44  

Part of the book series: Lecture Notes in Electrical Engineering ((LNEE,volume 820))

356 Accesses

- NE555 is a low cost chip that can be programmed to act as Mono-stable, Bi-stable, or astable multivibrator. This chip is widely used to design electronics circuits that require a single-bit data to function. In this paper, the application of NE555 to develop a low-cost system is discussed. The objective of the work is to design and develop a low cost and Automatic Irrigation Water system and a Clap-Switch using NE555 and mic sensor. These days automatic water irrigation system is used in Gardens, Parks, Stadiums, roadside gardens to minimize the labor cost water losses. Low cost automatic irrigation system shall help automation of gardens/fields irrigation at larger level. NE555 is also found its application to develop a system that can work by clapping sound, especially in design of household appliances switch. This system (clap-switch) might be very useful for differently abled person or children. This paper is a work of under-graduate student project that discuss the design, simulation, and prototype development of low cost automatic irrigation system and a clap-switch using NE555.

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Shadan AlShidani & Salim Alshabibi

EEE Department, Motihari College of Engineering, Motihari, 845401, India

Electronics Engineering Department, IIT(BHU), Varanasi, India

Kanak Kumar

National Institute of Technology Srinagar, Srinagar, India

Farhad Ilahi Bakhsh

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Chandan Kumar Chanda

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Jerzy R. Szymanski

Instrumentation and Control Engineering, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India

Afzal Sikander

Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati, India

Pranab Kumar Mondal

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AlShidani, S., Alshabibi, S., Tabrez, M., Kumar, K., Bakhsh, F.I. (2022). Applications of 555 Timer for Development of Low-Cost System. In: Chanda, C.K., Szymanski, J.R., Sikander, A., Mondal, P.K., Acharjee, D. (eds) Advanced Energy and Control Systems. Lecture Notes in Electrical Engineering, vol 820. Springer, Singapore. https://doi.org/10.1007/978-981-16-7274-3_18

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3 Great Books to Learn 555 Timer Circuits and Projects

Any one who is interested in electronics would have come across this IC – 555. It is the one component that is used in many wide range of applications. 555 Timer is used in a variety of applications including  monostable  and  astable multivibrators ,  dc-dc converters , digital logic probes,  waveform generators , analog frequency meters and tachometers,  temperature measurement  and control devices,  voltage regulators  etc. So it is essential that you should have a deeper understanding of 555 IC and how it is used in these applications.  The best way to learn about anything in this world is to – ‘get your hands wet’. You should read,understand and do things on your own with 555 IC and that is the best way to learn more and more. So here I am introducing you to 3 great books that covers lots of 555 timer circuits and applications. All of them are from the most popular author in electronics – Forrest M Mims . He is well known for his book on basic electronics- “Getting started in Electronics” , which has been sold many million copies. We have reviewed his book about basic electronics and 3 other books to ‘Get started in electronics’ in a previous article.  You can read them here:-

4 Great Books to learn and Get started in Basic electronics

Note:- All these 4 books have a sound explanation about theoretical concepts of 555 IC and you can learn a lot about the design side concepts, especially in time delay design.

Before going deep into the book reviews of 555 , we have written some good articles about 555 IC – focused on theory. You can read them here:-

1. 555 Timer – A complete basic guide

2. A collection of 555 Timer Circuits and Projects done by CircuitsToday folks!

Now lets get to the book review part. All the 4 books can be bought from Amazon by using the direct BUY links given in this article. These are Global editions in the range of 10 to 12 USD, so I hope any one can afford the price! After all these books are really worth your money.

Book #1 –  Timer, Op Amp, and Optoelectronic Circuits & Projects by Forrest M Mims

This book is a combination of Mims 3 “Engineers Notebook” series. The publisher has combined 3 books written by Mims, namely 555 Timer Circuits, Op Amp projects and Optoelectronics. So this book contains more than 25 circuits based on 555 IC, more than 50 Op amp circuits and a good collection of optoelectronic circuits. All of them are hand tested by Forrest Mims himself and you can build all of them by using the associated components.  So by reading this book you will be able to build many applications like – “Sound Effects Generator”, LED Flasher, Chirp Generator, Photophone, Frequency Meter etc etc.

To Buy This Book

BUY – Timer, Op Amp, and Optoelectronic Circuits & Projects from Amazon-Global

Book #2 – Science and Communication Circuits & Projects by Forrest M Mims

Like the first book, this is also a compilation of 3 of Mims “Engineers Notebook” series, Science projects+Environmental projects+Communication projects. The applications and circuits listed in this book are closely linked with real life and real world – like applications which measure solar light, temperature, pressure, rain drops, solar radiation etc. These are quite interesting and fun to build type applications listed in this book.  You can make a seisometer, you can make a photometer to study sun light and its effects, you can learn more about rain and rain fall, and there is a good collection of radio communication projects too. Most of these circuits are build using 555 and 741 Op Amp.

BUY- Science and Communication Circuits Projects-from Amazon-Global

Book #3- Electronic Sensor Circuits & Projects, Volume III by Forrest M Mims

Well this is the third one and a really useful book because it deals with sensors. You can learn in great detail about how real world data such as temperature, pressure, light etc are sensed and converted to electrical signals for processing by circuits. Like the other 2 books listed above, this book also is a compilation of 3 “Engineers Notebook” series from Forrest Mims. They are Sensor projects+Solar cell projects+Magnet sensor projects. Any one who reads this book can learn lot about Solar cells, Photo resistors, thermistors, magnet switches which all can be then used to build circuits that can measure light, heat, pressure, sound etc.

BUY – Electronic Sensor Circuits & Projects, Volume III – from Amazon-Global

Related Posts

What is iot (internet of things), the history of 555 timer ic – story of invention by hans camenzind, 555 timer circuits in proteus, 10 comments.

Mims is a Genius!!!!!!! I Have used his books since 1974 I Have Lost Track of the number of Books He Has Written But I Do own 5 of his Books

thank you sir,you have been of help

Is this a review of 3 books about the 555 timer? Or is this a review of 3 books by Forest Mims II that have rehashed and repacked by the publisher to include the 555 timer? I don’t know, it just seems kind of bogus.

@Kevin – There is no good books on 555 other than one written by MiMs

its very helpful for b.tech students

i need book 555 project hindi pdf e book

If i want know about different ic`s.refer some books.

Thanks for sharing these books information. But the links given in “To Buy This Book” are showing 404 error pages. So, kindly once check it and give the proper links so that it would be helpful to many people like me.

Good day, pls were can i get does books in Nigeria?.

thanks we are graetfull

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The Biggest Little Chip: An Intro to the Versatile 555 Timer

By Charles Platt

Charles platt.

Charles Platt is a contributing editor to Make magazine, which has published more than 50 of his articles. Six of his books are available from Make: Books

Make: Electronics, an introductory guide, now available in its second edition.

Make: More Electronics, a sequel that greatly extends the scope of the first book.

Encyclopedia of Electronic Components, volumes 1, 2, and 3 (the third written in collaboration with Fredrik Jansson).

Make: Tools, which uses the same teaching techniques as Make: Electronics to explore and explain the use of workshop tools.

Back in 1970, when barely half a dozen corporate seedlings had taken root in the fertile ground of Silicon Valley, a company named Signetics bought an idea from an engineer named Hans Camenzind. It wasn’t a breakthrough concept, just 23 transistors and a bunch of resistors that would function as a programmable timer. The timer would be versatile, stable, and simple, but these virtues paled in comparison with its primary selling point. Using the emerging technology of integrated circuits, Signetics could reproduce the whole thing on a silicon chip.

This entailed some handiwork. Camenzind spent weeks using a drafting table and a specially mounted X-Acto knife to scribe his circuit into a large plastic sheet. Signetics then reduced this image photographically, etched it into tiny wafers, and embedded each wafer in a half-inch rectangle of black plastic with the product number printed on top. Thus, the 555 timer was born.

It turned out to be the most successful chip in history, in both the number of units sold (tens of billions, and counting) and the longevity of its design (unchanged in almost 40 years). The 555 has been used everywhere from toys to spacecraft. It can make lights flash, activate alarm systems, put spaces between beeps, and create the beeps themselves. Today, you can buy a single chip online for about 25 cents.

For the introductory project described below, you can use the 555CN, Fairchild LM555CN or KA555, Texas Instruments NE555P, or STMicroelectronics NE555N. The brand makes no difference. Each manufacturer offers a Complimentary Metal-Oxide Semiconductor (CMOS) version, a dual version, and a surface-mount version in addition to the old-style chip that stands on eight metal legs spaced 1/10″ apart. For various reasons, you should use the old-style version.

First I’ll show how the 555 can make an LED flash on and off. Then I’ll adapt it to generate a musical tone, and finally I’ll chain three 555s together to create a gadget you can use to impose a time limit in non-video games such as checkers or Scrabble. At the end of a preset interval, the timer will make a groaning sound to tell a tardy competitor that his time’s up and his turn is over.

Turn a 555 Chip into a Noisemaker

Figure 1 shows a 555 chip seen from above, with its pins identified. The circular mark stamped in its body is adjacent to Pin 1. Figure 2 shows a basic light-flashing circuit using the astable mode of the 555, meaning that the output on Pin 3 flips to and fro between positive and negative for as long as the power is switched on. The cycle time is determined by a capacitor and two resistors. A capacitor has electrical storage capacity (hence its name), while resistors reduce the flow of electricity. If you put a resistor in sequence with a capacitor, the resistor slows the charge and discharge times of the capacitor, thus offering a simple way to use electricity to measure time.

When you close switch S1 in the circuit, current flows through R1 and R2 and gradually starts charging capacitor C1. IC1 (the 555 timer) monitors this process. When C1 acquires 2/3 of the positive voltage powering the circuit, the 555 reverses its output on Pin 3 from positive to negative and forces C1 to discharge itself through R2. When the charge on C1 diminishes from 2/3 to 1/3, the chip flips back to its original state, resets its output from negative to positive, and repeats the cycle.

Using a 0.1 microfarad (μF) capacitor for C1, a 120 kilohm (kΩ) resistor for R1, and a 1 megohm (MΩ) resistor for R2, the LED flashes about 5 times each second. (The other components in the circuit have no effect on timing: R3 protects the LED from excessive current, while C2 protects the 555 timer from random electronic noise.)

Suppose you use a 1μF capacitor instead of the 0.1μF capacitor as C1. Now each cycle lasts 10 times as long. Conversely, if you use a 0.01μF capacitor for C1, the cycles are 1/10 as long. You can also change the timing by adjusting the resistor values. The value of R1+R2 affects the “on” cycle, while R2 alone determines the “off” cycle.

With high resistance and a small capacitor, the 555 will cycle very fast indeed — easily fast enough for its pulses to make musical noises through a loudspeaker. Figure 3 shows a modified version of the circuit. The LED and its series resistor have been replaced with a different resistor, capacitor C3, and L1, a 1″ RadioShack miniature loudspeaker. (Note: You cannot drive a full-size loudspeaker with a 555 timer unless you add an amplifier.) Make sure you update the values of R1, R2, and C1, which have been changed to make the 555 run faster. Now when you connect power, you should hear a low-pitched drone.

Add a Second 555 to Trigger the Noisemaker for a Fixed Interval

We have a noisemaker; now we need to trigger it for a fixed interval. This can be done with a second 555 wired in monostable mode, meaning that it emits only one pulse. Figure 4 shows it added to the circuit. S2 is a pushbutton, although you can improvise just by touching 2 wires together. When this happens, IC2 emits a single pulse lasting about 1 second. This illuminates D2, an LED, to provide visual confirmation. The pulse also goes through D1, a signal diode, and activates IC1, which makes a sound as before, except that C4 prolongs it and causes it to diminish in frequency, creating a groaning effect.

Make sure this version of the circuit works before you continue.

Add a Third 555 to Time the Wait Period

We have a noisemaker that can be triggered for a fixed interval; now we need to measure an interval of time before the sound occurs. A third 555 timer can impose this wait period, if we adjust it with higher-value resistors and a larger capacitor.

In Figure 5 , C7 is charged through P1, a potentiometer (variable resistor). You can “tune” P1 to adjust the wait interval, and increase the value of C7 to make the interval even longer. At the end of the interval, the output on Pin 3 goes negative. This connects with the trigger pin of IC2 and tells it to emit its brief pulse, which tells IC1 to make its sound.

Note that S2 has been moved so that it controls IC3. When you use the circuit to impose a time limit during a game, hit S2 at the beginning of each person’s turn.

So the circuit won’t make its rude noise if a player does move within the allowed time, a cancel-reset button, S3, has been added. You hit this button when a player makes a move. The “NC” beside it tells you that it is a normally closed pushbutton. You still need the power switch, S1, to disconnect your power supply when the gadget is not in use.

What’s Next?

You can substitute other components instead of the timing resistors to make the 555 behave in interesting ways. In Figure 3 , if you use a thermistor or a photoresistor instead of R2, you can control the audio frequency with heat or light. A photoresistor and the 555 in monostable mode can function as a motion detector. Search makezine.com for “555″ or check out Doctronics for even more project ideas.

Hans Camenzind never imagined that his timer would become such a universal utility. He now thinks the internal design of the 555 isn’t particularly elegant and should have been given a makeover decades ago. Elegance in design can be a big deal for engineers, but for end users, utility is usually more important. The 555 is simple, accurate, and robust, tolerating a wide range of power supplies and able to drive not only LEDs and loudspeakers but also relays and even small motors.

For 25 cents, that’s more than enough.

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555 Timer IC – Working Principle, Block Diagram, Circuit Schematics

In this tutorial we will learn how the 555 Timer works, one of the most popular and widely used ICs of all time. You can watch the following video or read the written tutorial below.

How It Works, Internal Schematic and Block Diagram

Let’s take a closer look what’s inside the 555 Timer and explain how it works in each of the three modes. Here’s the internal schematics of 555 Timer which consists of 25 transistors, 2 diodes and 15 resistors.

Represented with a block diagram it consists of 2 comparators , a flip-flop , a voltage divider, a discharge transistor and an output stage.

Next are the two comparators. A comparator is a circuit element that compares two analog input voltages at its positive (non-inverting) and negative (inverting) input terminal. If the input voltage at the positive terminal is higher than the input voltage at the negative terminal the comparator will output 1. Vice versa, if the voltage at the negative input terminal is higher than the voltage at the positive terminal, the comparator will output 0.

The Q-bar output of the flip-flip goes to the output stage or the output drivers which can either source or sink a current of 200mA to the load. The output of the flip-flip is also connected to a transistor that connects the “Discharge” pin to ground.

555 Timer – Bistable Mode

The Trigger and the Reset pins of the IC are connected to VCC through the two resistors, and it that way they are always high. The two pushbuttons are connected between these pins and the ground, so if we hold them pushed the input state will be low.

Initially, the two comparators outputs are 0, thus the flip-flop output as well as the output of the 555 Timer are 0.

If we press the Trigger pushbutton, the state at the Trigger input will become Low, so the comparator will output High and that will make flip-flip Q-bar output go Low. The output stage will invert this and the final output of the 555 Timer will be High.

The output will remain high even when the trigger pushbutton is not pushed because in that case the flip-flop inputs R and S will be 0 which means that the flip-flop won’t change the previous state. For making the output Low we need press the Reset pushbutton, which resets the flip-flop and the entire IC.

Related tutorial: What is Schmitt Trigger | How It Works

555 Timer – Monostable Mode

Next, let’s see how the 555 Timer works in a monostable mode. Here’s an example circuit.

The trigger input is held High by connecting it to VCC through a resistor. That means that the trigger comparator will output 0 to the S input of the flip-flop. On the other hand, the Threshold pin is Low and that makes the Threshold comparator out 0 as well. The Threshold pin is actually Low because the Q-bar output of the flip-flop is High, which keeps the discharge transistor active, so the voltage coming from the source is going to ground through that transistor.

In order to change the 555 Timer output state to High we need to press the pushbutton on trigger pin. That will ground the trigger pin, or the input state will be 0, thus the comparator will output 1 to the S input of the flip-flip. This will cause the Q-bar output to go Low and the 555 Timer output High.  At the same time, we can notice that the discharge transistor is turned off, so now the capacitor C1 will start charging through the resistor R1.

The 555 Timer will remain in this state until the voltage across the capacitor reaches 2/3 of the supplied voltage. In that case, the Threshold input voltage will be higher and the comparator will output 1 to the R input of the flip-flip. This will bring the circuit into the initial state. The Q-bar output will become High, which will activate the discharge transistor as well as make the IC output Low again.

So we can notice that the amount of time the output of the 555 Timer is High, depends on how much time the capacitor needs to charge to 2/3 of the supplied voltage, and that depends on the values of both the capacitor C1 and the resistor R1. We can actually calculate this time with the following formula, T=1.1*C1*R1.

555 Timer – Astable Mode

Next, let’s see how the 555 Timer works in an astable mode. In this mode the IC becomes an oscillator or also called Free Running Multivibrator. It doesn’t have a stable state and continuously switches between High and Low without application of any external trigger. Here’s an example circuit of the 555 Timer operating in astable mode.

We only need two resistors and a capacitor. The Trigger and Threshold pins are connected to each other so there is no need of external trigger pulse. Initially, the voltage source will start charging the capacitor through the Resistors R1 and R2. While charging the Trigger comparator will output 1 because the input voltage at the Trigger pin is still lower than 1/3 of the supplied voltage. That means that the Q-bar output is 0 and the discharge transistor is closed. At this time the output of the 555 Timer is High.

Once the voltage across the capacitor reaches 1/3 of the supplied voltage, the Trigger comparator will output 0 but at this point that won’t do any change as both R and S inputs of the flip-flop are 0. So the voltage across the capacitor will keep rising, and once it reaches 2/3 of the supplied voltage, the Threshold comparator will output 1 to the R input of the flip-flop. This will active the discharging transistor and now the capacitor will start discharging through the resistor R2 and the discharging transistor. At this moment the output of the 555 Timer is Low.

We can calculate the time the output is High and Low using the shown formulas.  The High time depends the on the resistance of both R1 and R2, as well as the capacitance of the capacitor. On the other hand, the Low time depends only on the resistance of R2 and the capacitance of the capacitor. If we sum the High and Low times we will get the Period of one cycle. On the other hand, the frequency is how many times this happens in one second, so one over the Period will give use the frequency of the square wave output.

If we make some modifications to this circuit, for example, change the R2 resistor with a variable resistor or a pototentiometer, we can instantly control the frequency and the duty cycles of the square wave. However, more on this in my next video where we will make a PWM DC motor speed controller using the 555 Timer .

I hope you enjoyed this tutorial and learned something new. Feel free to ask any question in the comments section below.

3 thoughts on “555 Timer IC – Working Principle, Block Diagram, Circuit Schematics”

Thanks very much, I am an internationally experienced industrial control geek and have had the honour of teaching basic Electronics at the local community college but, while of course I have some 555s in one of the scores – perhaps hundreds – of little plastic drawer cabinets in the basement (labeling has faded with the years but I had no trouble reading 555 and 556) I must confess that I’d forgotten what the internal block diagram looks like and my wife is building the Flola table lamp project (a young Czech designed a lamp with flower petals that are opened and closed by a stepper motor under the control of an Arduino that reads the position of a joystick in the lamp base) and the brightness is controlled by a 50K rotary potentiometer which is built into the Arduino joystick but actually has nothing to do with it: it varies the duty cycle of the 555-based astable which drives a MOSFET that sinks the LED current. My wife has gone to bed so I did a quick search for a refresher on the 555 (I am 69 years old and I did not want to admit that I could not draw the block diagram of a 555 from memory to show her how the brightness control circuit works!) I had already told her no need to download the circuit board mask or to order it through the project website, I could see right away that I can breadboard it relatively easily but I wanted to be able to explain what the parts that I solder to the 8-pin IC socket actually do, especially wrt the capacitor charge-discharge, so thanks for your work with this tutorial (and others that I’ll check out later) I now feel confident to draw the 8-pin “box” at the center of the schematic showing her the two comparators and the resistor ladder, the RS latch and the discharge switch (transistor) I would have been very embarrassed if I could not do that and explain for her it very simply. Very good of you to make these tutorials available in the spirit of community that is still graces the Internet, much appreciated.

I’m so glad you found this tutorial useful. Cheers!

Nicely Explained with diagrams. It will be appreciated if you also provide step by step derivations also.

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Home > Electronics > Digital Electronics > 555 Timer IC- Types, Construction, Working & Application – Circuit & Pinout

555 Timer IC- Types, Construction, Working & Application – Circuit & Pinout

555 timer ic – mode of operation – schematic, internal, block diagram & applications.

Table of Contents

Digital Timers

Timers are those circuits, which provide periodic signals to a digital system which change the state of that system. In other words, those circuits, which work on the base of multivibrator changes or a device, which can be used as multivibrator is called Timer . The 555 Timer IC is one of them, and we will discuss it in detail in the following article.

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What is 555 Timer IC?

555 Timer is a digital monolithic integrated circuit (IC) which may be used as a clock generator . In other words, 555 Timer is a circuit which may be connected as a stable or monostable multivibrator . In more simple words, 555 Timer is a monolithic timing circuit , which can produce accurate timing pulses with 50% or 100% duty cycle. It was developed in the year 1970 by Signetic Corporation and designed by Hans Camenzind in 1971.

555 Timer is a versatile and most usable device in the electronics circuits and designs which work for both stable and monostable states. It may provide time delay from microseconds up to many hours.

555 timer is a very cheap IC which works for wide range of potential difference (typically, from 4.5 to 15V DC) and the different provided input voltages do not affect the timer output.

555 Timer is a linear device and it can be directly connected to the CMOS or TTL (Transistor – Transistor Logic) digital circuits due to its compatibility but, interfacing is must to use 555 timer with other digital circuits.

Being an integral part of electronics project, 555 Timer IC is very often used in simple to complex electronics projects. The standard 555 timer IC is made of 2 diodes , 25 transistors, 15 resistors installed in an 8 pin dual in-line package.

Good to know:

This timer is called 555 Timer due to the fact that it contains three 5 Kilo-Ohm resistors in series to form the voltage divider pattern.

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Features of 555 Timer IC

• There are two types of 555 timer based on its nomenclature – NE 555 Timer and SE 555 Timer . While NE 555 timer can be used in the temperature range from 0 to 70°C, the SE 555 Timer can be used in the temperature range from -55°C to 125°C and has a temperature stability of 0.005% per 0C ..
• it can be operated of different power supplies ranging from 5 Volts to 18 Volts .
• It can be used either as a pulse generator or an oscillator by operating it in different modes.
• The name 555 comes from the fact that it contains three 5 Kilo-Ohm resistors in series to form the voltage divider pattern.
• It can drive both Transistor-Transistor Logic (TTL) due to its high output current and CMOS logic circuits.
• It has high output current and adjustable duty cycle .
• 555 timer can be operated in both astable and monostable modes .
• The output of 555 timer can source or absorb current up to 200mA sinking or sourcing current to the load.
• It contains 24 transistors , 2 diodes and 17 resistors .
• 555 timer is available as an 8-Pin Dual in Line Package ( DIP ), 8-Pin Metal Can or 14-Pin Dual in Line Package ( DIP ).

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555 Timer Construction & Block Diagram

There are lots of manufacturers who manufacture 555 timer which included the number 555 e.g. NE555 , CA555 , SE555 , MC14 555 etc. typically, two 555 timers sandwiched inside a single chip which is called 556 . Nowadays, chips are available with four 555 timers in it. These devices are available in circular IC with eight (8), DIP (Dual inline Package) with 8 pins or DIP with 14 pins.

Below is the pin diagram of DIP (Dual inline Package) 555 timer with 8 pins.

A simple 555 timer circuit is shown above in fig 3 which shows the internal construction of 555 timer. According to fig 1 & 3, the timer contains on two comparators , an RS flip flop , an Output stitch (output buffer) and a Discharge Transistor Q 1 .

In addition, there are three 5kΩ resistors are connected in series with 5kΩ resistor which first end is connected with V CC (Pin 8 = Supply voltage) and the other end is connected with ground (GND = Pin 1).

In above fig 1 and (as well as below fig 2 & 3), As given in the block diagram, heart of the IC lies in the two comparator circuits. While inverting terminal of the upper comparator is connected to a point with DC potential of 2/3 V CC (where V CC can be +5V to +18V), the non-inverting terminal is connected to the threshold pin.

The inverting terminal of the lower comparator is connected to the external trigger input pin whereas the non-inverting terminal is connected to the point with DC potential of 1/3 V CC . The three 5 Kilo-Ohm resistors are connected in series to form voltage divider circuit. Output from both the comparators is given to the R-S Flip Flop whose state depends on the output from the two comparators.

Output from the R-S Flip-Flop is connected to the two transistors – Q 1 and Q 2 . Q1 is the discharge transistor and provides discharge path to the external capacitor , when saturated. Q 2 is the reset transistor, where a pulse applied will reset the whole timing circuit. The output from the flip-flop is amplified by the power amplifier block.

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555 Timer Pinout Configuration

Here is the simple explanation of the 8 pins of 555 Timer IC (Fig 1 & 2). Let’s understand this IC with its pin configuration and circuit diagram.

1.   Ground (GND)

It’s the common ground point of the circuit. The ground terminal of external circuit as well as power supply (V CC ) ground terminal is connected to the GND ( Ground ) terminal of 555 timer.

This pin is either grounded or connected to the negative rail. Connection using a resistor is not recommended to avoid heating up of the IC because of the stray voltage accumulated inside it.

2.   Trigger

When Trigger terminal gets one –third (1/3) of the supply voltage i.e. V CC /3 equal amplitude’s negative trigger pulse, then the circuit output changes form Low to High.

This pin is the input trigger pin to the IC and activates the timing cycle. A low signal at this pin triggers the Timer. Required current at this pin is 0.5 uA for a period of 0.1 uS . To avoid false triggering due to noise, the pin requires a pull up connection. Voltage at this pin is 1.67 Volts for a supply voltage of 5 Volts and 5 Volts for a supply voltage of 15 Volts .

3.   Output

This terminal is used for getting output and connected with load. At any instant, its value is low or high. i.e. This is the output pin of the Timer. Output of the Timer depends on the duration of timing cycle of the input pulse. The output can either sink or source current, at maximum 200mA.For LOW output, it sinks current, voltage being slightly greater than zero and for HIGH output, it sources current, voltage being less than V CC .

Without taking into account the previous state of output, by providing a trigger pulse to this terminal resets the device. I.e. Its output becomes low.

The reset pin is either not connected or connected to the positive rail. A logic LOW signal at this pin resets the Timer regardless of its input. The required reset voltage is 0.7 Volts, at current of 0.1mA

5.   Control Voltage

There are two third positive voltages of the total Supply voltages (V CC ) at control voltage terminal. Thus, it becomes a part of the comparator circuit. Generally, a capacitor is connected between ground and voltage control terminals.

This is also a generally non-connected pin or connected to ground through a 0.01uF capacitor . For some applications, this pin is required to control the threshold voltage at the upper comparator and is connected to an external DC signal, to change the duty cycle.

6.   Threshold Voltage

Threshold voltage and control voltage is the two inputs of comparator circuit. The circuit compares the available voltage at threshold voltage terminal to the available reference voltage at control terminal.

If the available voltage at threshold terminal (Pin 6) is greater than the control voltage i.e. two-third of V CC , then the output would be low, otherwise, it would be high.

This pin provides threshold voltage to the upper comparator. When voltage at this pin is greater than 2/3 V CC , the duty cycle is changed. It is connected to non-inverting terminal of the upper comparator. Required current is 0.1 mA , with pulse duration of 0.1 uS .

7.    Discharge

When output is low, then Discharge terminal provides a low resistance discharge path to the externally connected capacitor. However, it acts an open circuit, when output is high.

This pin provides a discharge path for the timing capacitor through the NPN transistor . A discharge current less than 50 mA is required to avoid damage. It can also be used as open collector output.

8.   +V CC (Supply Voltage Terminal)

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Schematic & Working Principle of 555 Timer IC

In the 555 Timer block or functional diagram, comparators are those devices which output is high, when their positive input voltage is greater than their negative input voltage and vise versa.

Internal Function Diagram of 555 Timer

The voltage divider in the circuit (which contains on three series connected 5kΩ resistors ), which provides the trigger level of one-third of V CC (V CC /3) and two-third (2/3) of threshold voltage. To understand this point, suppose the input value is 15V . In this case, the value of trigger level would be 5V as ( V CC /3 = 15V/3 = 5V ). And the value of threshold level would be 10V as ( V CC  x 2/3 = 15V x (2/3) ) = 10V .

When needed, the trigger level and threshold can be adjusted by using the Control Voltage terminal (Pin 5) i.e. by changing the control voltage at Pin 5, we may change the trigger level and threshold voltage according to the required specification. However, in this case, the value of trigger and threshold would be remain equal to 1 /3 V CC and 2/3 V CC respectively.

Coming to the working part of 555 Timer IC, this circuit generally operates in three different modes namely A-stable, Mono-stable and Bi-stable modes. For a better understanding of 555 Timer IC and its different states, check the below circuit diagram.

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555 Timer Internal Schematic Diagram

When the normal high trigger input value instantaneously reduce then the 1/3 V CC , Then the output of Comparator B becomes High from Low, as a result, RS latch or RS Flip flop goes to “set”. When flip flop goes to set, then Output (at Point 3) becomes high. Simultaneously, the discharge transistor Q 1 gets off and The output remains high until the value of normally low threshold input does not increase then the 2/3 V CC .

As soon as the threshold input increase than the 2/3V CC , then the output of comparator A becomes Low, as a result, RS flip flop get reset (because the output of comparator is directly connected to the RS flip flop’s input R as shown in the fig). When flip flop gets reset, output becomes low and discharge transistor Q 1 goes to on.

The flip flop can be reset by applying external input reset without threshold circuit. Note that, the trigger and threshold inputs (Pin 2 and Pin 6) are controlled by externally components and the 555 timer can be used as stable , monostable or bi-stable operation by controlling the trigger and threshold inputs with the help of those external components.

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Types of 555 Timers & Operating Modes

There are three basic types of 555 Timer with respect to mode of function and operation.

• 555 Timer as Astable Multivibrator
• 555 Timer as Monostable Multivibrator
• 555 Timer as Bi-Stable Mode

555 Timer can be operated in three modes – Monostable Mode, Bi-Stable Mode and Astable Mode .

Astable Mode:

In this mode, there will be no stable level at the output and the output will keep swinging between high and low. i.e.- It doesn’t have any stable state and keeps switching between high and low without application of any external trigger.

Working of 555 timer in A-stable Mode

The trigger and threshold pin are connected together so, there is no need of external trigger pulse. The comparator will output 1 while charging the trigger because the input voltage at trigger pin is still lower than 1/3 of supplied voltage.

This time, the output of timer is high. Once the voltage across reaches 1/3 of the supplied voltage, the trigger comparator will output 0,keeping the situation unchanged as both R and S input of flip flop are 0.Once the voltage across the capacitor reaches 3/7 of applied voltage, the threshold comparator will output 1 to R input of the flip-flop.

You can easily calculate the output of this configuration using the below mentioned formula. The high time depend on resistors R 1 , R 2 and capacitor. On the other hand, low time depends only on resistor R 2 and capacitor.

T H = 0.693 x (R 1 x R 2 ) XC 1

T L = 0.693 x (R 2 ) X (C 1 )

Period for one cycle:

T = TH +TL x (R 1 + 2R 2 ) C1

f =1.44 / (R 1 + R 2 ) C 1 ) HZ

It is also known as self-triggering mode, the Timer is used in this mode as clock pulse generator or oscillator . The Timer switches between two quasi stable states and without any external trigger input.

Given below is 555 Timer circuit in Astable mode.

(Ref Fig 2 as well) As the Timer is switched ON, i.e. the output is HIGH, the transistor Q 2 will be in cut off region on receiving a LOW input signal . The capacitor charges through both the resistors R 1 and R 2 toward V CC . The capacitor charging time being

τ 1 = 0.693 (R 1 + R 2 )*C.

This capacitor voltage is the threshold voltage to the upper comparator.

As the voltage exceeds 2/3 V CC , the upper comparator output resets the Flip-Flop, which turns the Timer output to OFF state (provided reset pin is in LOW state) The transistor τ will in saturation region, i.e. will be turned ON, providing a discharge path for the capacitor through resistor R 2 , the discharge time being – 0.693 R 2 *C .

 As the capacitor voltage falls below -1/3V CC , the second comparator output sets the Flip-Flop, which makes the Timer output LOW and the whole process starts again. Thus the Timer output oscillates between HIGH and LOW state, generating oscillations.

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Monostable Mode :

This configuration consists of one stable and unstable state. If the stable output is set at high then the output of timer is high.

Working Of 555 Timer In Mono-Stable Mode

The trigger input is held high by connecting it to V CC through resistor. Threshold pin is low making the threshold comparator out 0. IN result, voltage coming from source is going to ground through the transistor. Press the pushbutton on trigger to change the 555 timer output to high.

At the same time, capacitor C 1 will start charging through resistor R 1 . The 555 timer will remain in this position until the voltage across capacitor reaches 2/3 of the supplied voltage. Comparator will output 1 to R input of the flip flop bringing the circuit into initial state. The amount of time the timer output will remain high; depend entirely on value of both the capacitor C 1 and resistor R 1 .

To calculate the time, use the below formula:

T = 1.1 * C 1 * R 1

It is also known as single shot mode or pulse generating mode. In this state, the 555 Timer is normally in a stable state until triggered, after which it jumps to the quasi stable state.

Given below is 555 Timer circuit in a monostable mode.

(Ref Fig 2 as well). Initially the Timer output is LOW and the transistor Q 2 is in saturation mode, i.e. Fully ON. As a negative trigger pulse, more negative than -1/3 V CC , is applied to the second comparator, the Flip Flop sets to HIGH, turning the Timer output to HIGH state and the Transistor τ is turned OFF.

The output remains HIGH for time Tou i.e τ = 1.1 RC , i.e. the time taken for Capacitor C to charge (Also known as Time Constant RC) . As Capacitor voltage exceeds 2/3 V CC , output from the upper comparator resets the Flip-Flop to zero and the discharge transistor Q 2 gets again saturated, providing a discharge path to the capacitor. As the capacitor voltage comes back to zero volts, the circuit comes back to its normal state.

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Bi-Stable Mode :

In this configuration, both the output states are stable. At each interrupt, the output changes from low to high and vice-versa If we have a high output, it will go low once it receives an interrupt and stays low until the next interrupts changes the status.

Working of 555 Timer in Bi-stable mode:

After pressing the trigger button, the trigger input state will become low. Consequently the comparator will output High and that will make flip-flip Q-bar output go Low. The final state of the timer state will be high .The output will remain high even when the trigger pushbutton is not pushed because in that case the R and S input of flip-flop will be 0 which means that the flip-flop won’t change the initial state. To make the output low, we need to reset the pushbutton which eventually Resets the entire 555 Timer IC.

This is also known as Flip-Flop mode and the Timer remains in two stable states in this mode. It does not require any external timing circuit as the time delay between two states depends upon the timing of application of external pulses.

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Given below is 555 Timer circuit in Bi-Stable mode.

Two switches are connected as such that while Switch S 1 is connected to reset pin with V CC , switch S 2 is connected to trigger pin with ground. A negative going pulse, at the trigger input, at voltage more negative than -1/3V CC , triggers the lower comparator output to set the Flip-Flop and thus the Timer output to be HIGH. As the threshold pin is grounded, a positive going pulse at the reset pin triggers the

Since the Timer remains in one stable state until an external pulse is applied and then changes to another stable state, this mode is termed as Bi-stable mode. An important application is the Schmitt Trigger circuit.

555 Timer Calculator

The calculator can see under the title “ 555 Timer Calculator with formula & Equations “

Applications of 555 Timer

555 timer is most important  integrated circuit (chip) used widely in digital electronics. Some common uses and application of 555 timer IC are as follow:

• PWM (Pulse Width Modulation) &  PPM (Pulse Position Modulation)
• Duty Cycle Oscillator
• Lamp Dimmer
• To provide Accurate time delays
• As a flip-flop element
• Digital logic probes
• Analog frequency meters
• Quad Timer applications
• Pulse, Waveform, and square wave generation
• Stepped tone & tone burst generator & linear ramp generation
• Tachometers & temperature measurement
• It can be used as monostable multivibrator and astable multivibrator
• DC to DC Converters
• DC Voltage Regulators
• Voltage to Frequency Converter
• Frequency Divider
• Schmitt trigger
• Cable tester
• Pulse detector
• Wiper speed control
• Timer Switch
• Time delay generation, precision timing and sequential timing
• The 555 Timer IC are widely used in most of interesting electronic circuits and project like Traffic Light Circuit using 555 Timer , LED Flashing circuits, police siren, LED dice, Music Box, Metal detector, Joystick and game paddles, & low cost line receiver, Clap switch activated circuit and lots of other projects and circuits designs.

This is the basic tutorial about 555 Timer IC. Any other information regarding 555 Timer is welcome in the comments section. We hope that you have got a better understanding of 555 Timer IC and its working in different configurations. 

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14 comments.

that is great.how can singleph,twophase and threephase changeover be fix.

i want to information of generated electrical energy by piezoelectrical tansducer

Marvelous, what a blog it is! This web site presents valuable facts to us, keep it up.

where i found circuit of traic ???? ohhhh i for get to say thankkkkkkkkkkkkkkkkks a lot

You can see the full article with circuit diagram on TRIAC here .

It appears that the pin diagram for the 555 chip is incorrect.

Your pinout of the555 is wrong. the pins 1-8 run around the chip anticlockwise.

Thank you for your correction. It has been modified now.

Pinout numbers are incorrect for Fig 2, they are correct for other pics. nice web page – thanks!

Thank you for correction. It has been modified now.

Fig. 3. Q21 is NPN. Drawing shows wrong PNP. This way it would short the supply directly to ground and burn down Q22 and Q24.

Fig.5. is not symmetric oscillator. For less part count and guaranteed precise equal 50% duty cycle see https://bit.ly/3FsOFkZ regardles the RC factor determining the frequency. Note that load cannot exceed 200mA otherwise it needs external transistor.

Haw many voltage is the construction take?

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