How to Control the Duty Cycle of a 555 Timer

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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


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

    1. 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

  1. 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)

    1. Jason Baird

      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.

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

      1. Jason Baird

        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!

        1. 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) ?

  3. You can use a CMOS 555 timer to get the full output voltage. You can also control the duty cycle without changing the frequency by use of a CMOS 555 timer a potentiometer and 2 diodes between the output pin 3 and the timing capacitor pin 6 (connected to pin 2).

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