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.

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>hey. i planning on building a chess timer. could you guide me a bit in the process? thank you very very much in advance</p>
<p>Hi great circuit! like it so much I made three! swapped the 10k to Vcc for 1M and the 470uF cap for 10uF = 10sec on time. wanted to use just one 1M resistor for all three but LED stays on forever cant figure out why! Is there a way to just use one resistor for all three?</p><p>thanks</p><p>lasl</p>
<p>I'm working on making a mobile outlet with on/off buttons. The buttons would be from the bistable configuration leading to a relay. As of right now, I'm looking to have the 555 run off a 9V battery. My question is, does the 555 timer draw current even when it is in the off position of the bistable configuration?</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>You should use an microcontroller, like and MSP430 or something like that..... It'll be IMPOSSIBLE to use an 555 for that...</p>
<p>Impossible??</p><p>Nah, pretty easy - just add a 4017 and Bob's your uncle ;)</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>
<p>You could always use an jumper wire instead of an button</p>
<p>Thanks a lot!!!!. It was very much helpful. I wanted to understand 555 from basics and it helped me a lot. Thanks :-)</p>
<p>Can any of the pin be used to give an output logic 1 unless I open the switch??</p>
Can any one help me to calculate the resistance of PWM to make astable pulse 50% duty cycle time on/off 5 second.. <br>The formula T=0.693 x R x C or T= 1.1 x R x C ?
<p>so if say a triangular input is applied at the trigger input of monostable 555 timer is it still possible to get rectangular waveforms from the output of the timer ? and how does the period of trigger input affect the output? thank you</p>
<p>The output on pin 3 is always a switched ON/OFF output. Pin 7 is the same except that it is open collector (no internal or active pull-up). That is, the rise and fall time are fast. The output is from a comparitor which produces an instant ON and OFF. The trigger level is when the voltage on pin 2 and pin 6 reach their respective trigger voltages (1/3 and 2/3 of the voltage that is on pin 5). The dead-band or hysteresis is the voltage difference between 1/3 and 2/3 of the voltage on pin5 and the difference in voltage of pins 2 and 6. Most simple timers/oscillators have pins 2 and 6 tied together. </p><p>The 555 is a very versatile device and can be operated in many nonstandard ways. The 7555 is a little better equivalent because it is 100% CMOS and can operate far more symmetrically than the 555 at lower operating currents. </p><p>The input on pins 2 &amp; 6 can have any shape from square to sine depending on what you are designing.</p><p>good Luck </p><p>Pixeltamer</p>
<p>like in this picture ???</p>
<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>For what I have found in the datasheets, the minimum triggertime should be about 25nS. So... quite fast I think ;-)</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>Johanna, you have to amplifie the output of that micro first. Then you can connect the amplified output to the trigger input .</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>You can do that, Brad. Although you have to take care about your capacitators. When using ELCO's, high values give high leakage-currents. So the pulses might not be stable. For very long timing, it is possible to use a 10x divider (or more than one).</p>
<p>Great information, you put a lot of work into this to help people learn useful info.</p>
<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>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>
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?

About This Instructable




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 »
More by amandaghassaei:OTCA Metapixel - Conway's Game of Life "9 Degrees of Freedom" IMU Twitter Controlled Pet Feeder 
Add instructable to: