This tutorial provides sample circuits to set up a 555 timer in monostable, astable, and bistable modes as well as an in depth discussion of how the 555 timer works and how to choose components to use with it.  The 555 timer is a chip that can be used to create pulses of various durations, to output a continuous pulse waveform of adjustable pulse width and frequency, and to toggle between high and low states in response to inputs.  By wiring the 555 timer with resistors and capacitors in various ways, you can get it to operate in three different modes:

Monostable Mode is great for creating time delays.  In this mode an external trigger causes the 555 timer to output a pulse of an adjustable duration.  Jump straight to an example circuit for monostable mode here.

Astable Mode outputs an oscillating pulse signal/waveform.  In this mode the output of the 555 timer is switching between high and low states at a tunable frequency and pulse width.  Jump straight to an example circuit for astable mode here.

Bistable Mode causes the 555 timer to toggle its output between high and low states depending on the state of two inputs.  Jump straight to an example circuit for bistable mode here.

Some applications that come to mind include:

- a steady clock/trigger to keep time in a circuit (astable mode)
- the core oscillator of an analog synthesizer, with the addition of some op amps and other components this pulse wave can be shaped into a triangle, saw, and even sine shapes
- a very basic chiptune style noise maker (see atari punk console)
- time delay for an incoming signal (monostable mode)
- very basic storage of input data/management of two button control system (bistable mode)

The 555 timer is flexible, cheap, and easy to find (you can even pick them up at Radioshack).  It's also a great starting point for audio projects because its output can be wired directly to a speaker.  Feel free to use any of the info or example circuits I've provided in this tutorial as a starting point for an entry in the DIY Audio Contest! We're giving away an HDTV, DSLR cameras and tons of other great stuff!  (ends November 26)

Step 1: 555 Timer Pin Diagram

Fig 1 shows the pin connections to the 555 timer, it was take directly from the 555 timer datasheet.  The power connections to the chip are through pins 1 (ground) and 8 (+Vcc).  The positive supply voltage (+Vcc) should be between 5 and 15V.

The second image is a close up of the diagram depicting the internal functional components of the chip.  This consists of a few different elements: resistors, transistors, comparators, a flip flop, and an output stage.

All three resistors diagrammed in fig 2 are 5kOhm (see image notes in fig 3).  The purpose of these resistors is to set up a voltage divider between Vcc and ground.  Since all resistors are the same value we know that the voltage at the junction between the resistors are 2/3Vcc and 1/3Vcc (see image notes in fig 2).  These voltages are used as reference voltages for the comparators.

A comparator is a circuit which compares an input with a reference voltage and outputs a LOW or HIGH signal based on whether the input is a higher or lower voltage than the reference.  The 555 timer uses several transistors to construct its comparators (see the image notes in fig 3), so in the simplified functional diagram in fig 2 they are represented by boxes labelled "comparator."  The comparator connected to pin 2 compares the "trigger" input to a reference voltage of 1/3Vcc and the comparator connected in pin 6 compares the "threshold" input to a reference voltage of 2/3Vcc from the voltage divider.

A flip flop is circuit that switches between two stable states based on the state of its inputs.  The 555 flip flop outputs a high or low based on the states of the two comparators.  When the trigger comparator is outputting a low signal (regardless of the state of the threshold comparator), the flip flop switches high, when both comparators are outputting a high signal, the flip flop switches low.  The timing of a high pulse output from the flip flop can also be manually reset (the beginning of a pulse can be triggered) by pulsing the reset pin low.

The functional diagram in fig 2 also includes two transistors.  The transistor attached to pin 7 is an NPN transistor.  Since pin 7 is connected to the collector pin of the NPN transistor, this type of configuration is called open collector or open drain.  This pin is usually connected to a capacitor and is used to discharge the capacitor each time the output pin goes low.  The transistor attached to pin 4 is a PNP transistor.  The purpose of this transistor is to buffer the reset pin, so the 555 does not source current from this pin and cause it to sag in voltage.

The output stage of the 555 timer is indicated in the image notes of fig 3.  Its purpose is to act as a buffer between the 555 timer and any loads that may be attached to its output pin.  The output stage supplies current to the output pin so that the other functional component of the 555 timer don't have to.
<p>Hi,</p><p>Thanks for the detailed instructable!</p><p>I was wondering if anyone knows the minimum input pulse length in monostable mode (for catching a <em>very</em> short sensor input pulse in a ballistic chronograph, for example).</p><p>Thanks</p>
<p>Great information, you put a lot of work into this to help people learn useful info.</p>
Thank you for this instructable! Question: if I set up the resistors and capacitors right, would I be able to create a 5 minute pulse? Or would I need a microcontroller? Probably a stupid question but I am very new to electronics
<p>Very detailed instruction set, Thank You for the explanation, I was struggling with trying to figure out how to get a 10% duty cycle on a &quot;blinker&quot; and this worked great. Again Thank You very much</p>
<p>Thanks sir, very nice and very clear.</p>
<p>Hi!<br>Is there any way to mix the mono and bistable mode? I mean pressing 1 button to turn it on the led, and keep it on untill press the other button (or the same button, this is not a critical issue) then after a time, the led goes off?</p>
<p>In the diagram pin 7 is connected directly to the pot and then to Vcc, but in your breadboard the pot goes to pin 6. Can you explain this discrepancy?</p>
<p>Nice article. One mistake though. You use a non standard layout for your astable circuit diagram. This is very amateurish.<br><br>http://www.555-timer-circuits.com/common-mistakes.html</p>
<p>put the mic at the trigger but light will only stay on for 5 seconds lol, i suppose this circuit is to trigger something for only 5 seconds after that its an off circuit</p>
<p>Thank you so much for the information you provided and used simple words.Can you please tell what is the use of capacitor in this circuit? Can anyon please...............answer?</p>
<p>Well, you could specify a little bit better what circuit you mean, but in general the use of the capacitors in these examples is because they need a very stable Voltage. The capicator insures this, ( just because of how a capacitor is build and where it is placed). Hope this helps.</p>
<p>hi . If i'm gonna change the momentary switch to a electret mic to make it a clap switch circuit. is it possible? what should be change on the connections?</p>
<p>You have an error in one of your equations. In point 3 you have:<br><br><strong>2/3*Vcc/</strong><strong>(Vcc - V0)</strong><strong> = 1- e^(-t / [(RA+RB)*C])</strong></p><p><strong>1/3*</strong><strong>Vcc/(Vcc - V0)</strong><strong> = e^(-t / [(RA+RB)*C])<br><br></strong>But this is not correct, it just happens to work for V0 = 0. This becomes a much larger problem in point 5 when you reuse the result to determine the amount of time the output is HIGH. <br><br>The way to calculate the time the output is HIGH is to subtract the time it takes to repeat the calculation from point 3, except solve for 1/3*VCC. Then subtract this result from the solution to point 3.<br><br>Your answer coming out correct is actually quite surprising. </p>
<p>does anyone know how to configure the 555 timer as a monostable multivibrator with a delay of about 100-500ms?? thx</p>
<p>T = 1.1 RC select C= 1uf @T= .1 sec R= T/1.1xC,,Do the same with T= .5 sec</p>
<p>This is exactly what i was looking for! I have little to no base in electronics and i'm trying to conceive a led dimmer in order to control a rgb led color. Your article mad me understand how a 555 timer worked and your equations will come in very handy! Thank you!</p>
<p>I'm planning on creating an astable multivibrator with as close to 50% duty cycle as possible. Thank you for the equations!</p>
<p>I followed your Astable guide to make a 200 Hz tone using Ra=2700, Rb=2200, and C=1uF. Was going crazy at first because I had two broken 555 chips in a row and didn't know. I wanted to say that this instructable is such good documentation to have, it is both thorough and informative -- thank you!</p>
<p>thanks for the post... the bistable circuit was exactly what i needed to toggle 2 hall effect sensors (A3144E) when all i had laying around were some 555 chips.</p>
<p>Im working on a model train layout, what i want is for a kid to press a button the train goes for 2 <strong>min</strong> then stops. the child does this 4 more times. after the train stops for the 5th time it activates a cool down clock for 10 min. it would have to work with electricity from an out lit. Any ideas? </p>
<p>I want this circuit, but I have 741 opamps in stead of the momentary switches. Can I connect the 741 output to the bottom of the pull-up resistors to set or reset the 555? or is some sort of buffering or isolation necessary between the 741 and the 555?</p>
question looking a building a 555 monostable mode can I use a switch that when trigger high, it will delay off after x seconds and stay off without the trigger input going back to low?
<p>Nice! Well written.</p>
<p>Hey everyone, I need to run a motor for a 3 second pulse, but only once, not a continuous on-off situation. and i need it to run in both directions. Is it possible to use this circuit for that or will i need something else? </p>
<p>sounds like you will need a microcontroller</p>
<p>Hi.. I have tried to make the monostable mode of 555 timer exactly as it is shown here for 5.17s pulse width. But i m not getting the output.The pulse becomes high when the push button is on but it comes to low only when the push button is switched off. I have checked the circuit connections numerous times but there is no error. Can you please tell me what might be the problem?</p>
<p>i had the same problem on my first trial,try changing your breadboard,and see it is works. :)</p>
<p>Thank you :)</p>
hi,pls am new in d electronic world but understand the basis.how can i increase or amplifer dc 2v to 12v dc to charge 12v battery.pls state all d component require and the diagram to build dis circult,and i want the 12v dc to constant. <br>thanks
For astable mode,when I added a load to output, the frequency increase. But it seems like the frequency is independent of load. Anyway to overcome it?
I'm not sure why that's happening, try putting a buffer between the load and the 555, use an op amp or transistor wired up like this: <br />http://en.wikipedia.org/wiki/Buffer_amplifier <br />
Thanks a million :)
Is there a way to adjust the low pulse so it's not 0?
nope sorry, these are digital circuits so 0 and 5V is all you can get. Do you have something specific in mind that you'd like to do?
I was looking for a way to make a pulse feature for my TIG welder. It's an inexpensive one and does not have a feature that helps control the amount of heat input to a weld. For my welder, it takes an input between 0 and 10v and, based on your amp settings, applies a percentage of the current to the weld. I'm looking for a way to take the voltage input and pulse it with the ability to adjust frequency, duty cycle and background (what % of the max voltage the low should be). I thought I might be able to use an op amp on the output of an arduino, but I am not well versed in electronic circuitry.
you can do this, but you will need to learn about biasing (also called dc offset) and amplification
plz, if i want to find the water level by using 2 plates capacitor, then i will connect the output of the timer to a f/v converter and display the voltage <br>which capacitor i should change with the 2 plates??
do you have any idea how to install into a XBOX360 Controller? can it be a two or three wire hookup?
no idea, can you give me any more info? what are you trying to do?
trying to hook it up so i have a rapid fire controller using a momentary on button, but no idea where to wire in the 555 timer at :/ since my controller doesn't have a crystal oscillator.
I've never tried modding a controller like that, but I know it's possible, do you have a schematic?
no lol i don't, that sure would be helpful though.
&quot;In the equation above, when RA is much larger than RB (you can ignore the RB terms) you end up with a duty cycle ~= 1 and when RB is much larger than RA (you can ignore the RA terms) you get a duty cycle =~ 1/2. So the limits of the duty cycle with the circuit shown in fig 2 are 50% to 100&quot; <br> <br>Note that this is not true. <br> <br>You can set r1 to 0, and this gives you a %50 duty cycle. The only reason to have a resistor at R1 is to modify the duty cycle, and to keep a straight VCC from burning up the internal transistor at pin 7. This can be rectified by adding a resistor at GND. giving you a pure 50% duty cycle for the full range of r2. <br> <br>Note that output is HIGH while charging through R1 + R2, and low while discharging through R2 only. IE the High duty time can never be smaller than the low duty time. limiting the duty cycle to greater than %50.
Why doesn't the &quot;print pdf&quot; or save file to .pdf functions ever work for this site!???
scroll down on the page...were you signed in?
Thank you for this. This compliments the datasheet nicely... <br>Does step 6 have the wrong schematic? How you have it wired and how the schematic says to wire it, appear to be different. (I am still a complete noob when it comes to a lot of this, so I could be completely wrong) <br>Shouldn't the variable resistor be tied directly to to pin 6 without the 10k ohm resistor in series? And the 10k ohm resistor should be between pin 6 and pin 7. At least that is how fig 14 of the datasheet appears to wire it.
great question, this schematic is for the 50% duty cycle oscillator, which is a type of astable mode. This circuit is great if you really need a square wave output (50% high, 50% low) as opposed to a pulse output, but one annoying thing about it is that the frequency is given by: <br /> <br />f = 1/(Ra+Rb) <br /> <br />so in order to change the frequency of the timer you have to increase both Ra and Rb, in the circuit I provided you increase Ra and/r decrease Rb and vice versa to change the frequency, which makes it easy to use a single potentiometer as both Ra and Rb so you can physically dial in your frequency by turning a knob. But if you want to learn more about the 555 timer I'd recommend trying this schematic out and seeing what happens when you start changing things!
Great 'ible, even if the part is older than the hills. <br> <br>All these circuits can be easily simulated using the free (as in beer) LT Spice circuit simulator (get it here: http://www.linear.com/designtools/software/ ) . You'll find the NE555 chip in the &quot;misc&quot; catalog of components. <br> <br>In recent years it has been possible to do most of the 555 's functions (plus many more) using low pin count (6-8 pins) uCs. It frequently takes fewer components and performance is less variable with temperature than a 555 circuit. Of course, uCs require a programmer of some sort, but once you have that you usually find all sorts of uses for uCs. <br> <br>
Dear sir, <br> In LT spice link u given it has many appli to download. in that which i need to download for this type simulation.(my id r.navaneethan1991@gmail.com)

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Bio: I'm a grad student at the Center for Bits and Atoms at MIT Media Lab. Before that I worked at Instructables, writing code for ... More »
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