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


19 thoughts on “How to Control the Duty Cycle of a 555 Timer”

    • 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

  • 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)

    • 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.

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

      • 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!

        • 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) ?

    • 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.

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