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Blog, Tutorials  /  November 7, 2015

How to Control the Duty Cycle of a 555 Timer

by Jason

Continuing with a focus on the 555 timer, this video tutorial looks at how to vary the pulse width of a 555 timer’s output. It’s all possible with just a a hand full components. By varying the duty cycle one may vary the blink rate on an LED or the voltage being fed to a servo. Enjoy!

If you are interested in the schematic, it can be found here: 555 Timer Duty Cycle Variation Circuit

 

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21 comments

  • LAWRENCE HANSON
    December 23, 2015

    Please can ì use 555 to design 220 VOLT SINE WAVE POWER SUPPLY FOR AC TOOLS

    Reply
    • Jason Baird
      December 29, 2015

      No you can’t. It would not work for many reasons.

      Reply
  • ivica3730k
    February 6, 2016

    Must C1 cap be 1uF,and must it be ceramic?

    Reply
    • Adam
      June 23, 2016

      No as you can change c 1 to a higher capacity and it will take longer to charge it can be use full to change c 1 if u don’t have any resistors which are high ohm

      Reply
  • Adam
    June 23, 2016

    I want to use a 100 uf cap and a resistor to get .9 hz what is the formula to find the resistor value???

    Reply
    • Jason Baird
      June 29, 2016

      1.44/((Ra+2Rb)xC)

      Reply
  • Jason
    September 1, 2016

    Hey Jason,

    Love the videos, definitely great for someone who has an EE degree and forgot to much.

    Could you elaborate on the diode’s you used? I find them intimidating when I build circuits and am interested in setting up a variable duty cycle 555 timer as you have in this video.

    Thanks so much.

    -Jason (my name is also Jason)

    Reply
    • Jason Baird
      September 10, 2016

      Hi Jason, glad you liked the videos. The diodes I use in the circuit are just general purpose diodes. They are labeled 1N4001. You can buy then at Digikey with the following part nunber, 1N4001FSCT-ND. You’ll find a white band wrapped around one side of the diode. You can match the band up with the small bar you see on the diodes schematic symbol. Hope this helps.

      Reply
  • Ola
    November 9, 2016

    Hi..iam electrical poower engineering ciriciut and iwant to design a pulse generator circiut for a specific duty cycle ..so can u help me ?!

    Reply
    • Ola
      November 9, 2016

      Duty cycle is 0.6 f:1khz
      So how i choose the right capacitors. And variable R ? And should i use the diode!?

      Reply
      • Jason Baird
        November 16, 2016

        Duty cycle is determined by the following formula:

        Duty Cycle = Rb / (Ra + 2Rb)

        The forumula for frequency is mentioned in the video. I would start by working the duty cycle formula backwards to determine Ra and Rb. Next use those values to determine what C should be. If that capacitance value is not to your liking, pick new values for Ra and Rb that give you the same ratio of 0.6 but will allow for a different value of C. See the comment below!

        Reply
        • Hsing-Yu
          June 3, 2017

          Hi Jason,

          Thank you for a series of excellent videos about timer 555. The concept is lucidly explained and easy to understand. However, my question is in this circuit (with 2 diodes) why the duty cycle is Rb/(Ra+2Rb) ?

          t1= 0.693*R1*C1
          t2= 0.693*R2*C1

          duty cycle = t1/(t1+t2) = R1/(R1+R2) ?

          Reply
          • Jason Baird
            June 18, 2017

            I accidentally posted the previous comment thinking it was for another video. You are correct in your calculations.

  • RAVI
    July 12, 2017

    Hello sir,
    how we can choose value of c2.

    Reply
  • bader
    August 29, 2017

    how can i make this duty cycle 100 hz ?

    Reply
  • Dr. T
    December 18, 2017

    Hi Jason. You mentioned in your YouTube post that a schematic could be downloaded from your site. Could you direct me to the URL? Thanks.

    Reply
  • Kaushal Ambasana
    April 15, 2018

    How can I vary frequency along with duty cycle?

    Reply
  • Andre
    July 25, 2018

    where is the PDF shematic

    Reply
    • Jason Baird
      July 25, 2018

      Click on the link under the video. It’s hard to see because the link isn’t underlined. I need to change my webpage’s code to make it easier to see.

      Reply
  • Kuldeep
    May 15, 2019

    Hi..what didode i use D1 and D2

    Reply
    • Jason Baird
      May 19, 2019

      They are 1N4001 Diodes.

      Reply

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