## Step 3: 555 Timer: Monostable Mode Circuit

As I explained in the last step, a 555 timer in monostable mode will output a high pulse (of voltage ~Vcc) when the trigger pin in pulsed low. The duration of this output pulse is dependent on the values of R and C in fig 4. In the last step we calculated the duration of the pulse output from the 555 in monostable mode to be:

t = 1.1*R*C seconds
where R and C are the resistor and capacitor in series in fig 4.

if we choose R = 10Kohms and C = 470uF
t = 1.1*10000*0.00047
t = 5.17 sec

This means that with a 10Kohm resistor and 470uF capacitor, a pulse low to the 555's trigger pin (pin 2) will cause the output to go high for 5.17 seconds.

I built a circuit which connects the output pin of the 555 to an LED, causing the LED to light up for the duration of the pulse. This way I would have a visual indication that my calculations were correct. I connected the trigger pin of the 555 to a push button momentary switch, connecting it to ground when pressed. Photos of the circuit are shown above, and the schematic is shown in fig 5.

Parts List:
555 timer Digikey LM555CNFS-ND
0.01uF capacitor Digikey 445-5297-ND
470uF capacitor Digikey P5185-ND
(x2) 10Kohm resistor Digikey CF14JT10K0CT-ND
470 ohm resistor Digikey CF14JT470RCT-ND
amber led Digikey C503B-ACN-CW0Y0251-ND
momentary switch Digikey CKN9018-ND
22 gauge jumper wire
5-15V power supply- if you don't have a bench power supply, try using a 9V battery and battery snap or use the 5V output from an Arduino

Wiring Info:
The schematic is shown in fig 5. Connect power and ground to pins 8 and 1 of the 555 timer (red and black wires). I used a 9V supply and battery snap for my circuit. As indicated in the schematic in fig 5, connect a 0.01uF capacitor between pins 5 and 1. Connect a 440uF capacitor between pins 1 and 6, make sure that the negative lead of the capacitor is connected to pin 1. Connect pins 6 and 7 with a jumper wire (green). Connect a 10K resistor between pins 7 and 8. I left the reset pin floating, you could connect it to Vcc as well.

Connect an LED and current limiting resistor in series from the output of the 555 to ground. The output pin of the 555 will output Vcc-1.2V maximum (the -1.2V comes from some transistors in the circuit that drop the voltage slightly). My circuit was driven by a 9V supply, so the max output is 9-1.2V = 7.8V. I used a 470ohm current limiting resistor for my setup, if you use a 5V supply you can use a lower current limiting resistor (like 220ohm), and for higher Vcc use a higher resistance (maybe even up to 1K).

Wire the momentary push button switch in series with a 10K resistor between Vcc and ground. Connect a wire (yellow) from the junction between the switch and resistor to the trigger pin so that when the switch is not pressed the trigger pin is held high. When the switch is pressed the trigger pin will drop to low. See the schematic if this does not make sense.

Operation:
Press the button. The LED should light up for a time and then turn off. If you time the LED, you'll find that it lights up for exactly 5.17 seconds, just as I calculated above.

You can experiment with switching out the 10k resistor or the 470uF capacitor (connected to the 555) to see how they affect the duration of the output pulse. Remember, since t = 1.1*R*C seconds, increasing resistance or capacitance will always increase the duration of the pulse.

I wired up a 10Kohmpotentiometer as a variable resistor and put it in my circuit in place of the 10K resistor between 555 pins 7 and 8 (fig 9). This way by turning the knob all the way to one side, the LED stays on for 5.17 seconds, but when turned to the other extreme the LED turns off immediately after I released the button. Turning the potentiometer to any position in between will cause a pulse duration anywhere from 0 to 5.17 seconds.

<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>
Thanks!
<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?