How to Control the Duty Cycle of a 555 Timer

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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13 Responses to “How to Control the Duty Cycle of a 555 Timer”

  1. LAWRENCE HANSON December 23, 2015 at 10:05 am #

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

    • Jason December 29, 2015 at 8:07 am #

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

  2. ivica3730k February 6, 2016 at 12:58 pm #

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

    • Adam June 23, 2016 at 11:01 am #

      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

  3. Adam June 23, 2016 at 10:58 am #

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

    • Jason June 29, 2016 at 8:22 am #


  4. Jason September 1, 2016 at 1:51 pm #

    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)

    • Jason September 10, 2016 at 9:07 pm #

      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.

  5. Ola November 9, 2016 at 3:27 pm #

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

    • Ola November 9, 2016 at 3:30 pm #

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

      • Jason November 16, 2016 at 7:53 pm #

        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!

        • Hsing-Yu June 3, 2017 at 1:40 pm #

          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) ?

          • Jason June 18, 2017 at 9:51 pm #

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

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