Adjust the Duty Cycle of the 555 Timer without Changing the Frequency

This tutorial explains how to control the duty cycle of the 555 timer’s output. Not only does this design control the duty cycle but the frequency remains relatively stable during the adjustment. This is helpful for a number of applications such as working with pulse width modulation.


555 Duty Cycle Schematic

Formulas in the Video:

555 Duty Cycle Formulas

14 thoughts on “Adjust the Duty Cycle of the 555 Timer without Changing the Frequency”

  1. Hi Jason,
    I found this very useful. Not just the schematic, but a good explanation about how it functions. And thanks for posting the jpeg images. It’s lots easier to grab these, rather than get a screen-grab from the video. I’ve learned a lot from your tutorials. Hope you continue.

  2. Stanciu Stefan Cristian

    Hy. first of all sorry for my english.
    using your info i built some timer using 555 for different time combination for high and low periods..tolerances for seconds atr ok but for more than one minute for high the drift is around 2 to 3 time more. i changed the fixed resistors wirh potentiometers for tuning intervals . the drift amplifies . please help me fix this.
    best regards,

    1. You English is just fine and I understand your question. First, if you need high precision timing, the 555 timer is not the best choice. It would be better to go with a oscillator. Second, did you construct it on a breadboard or on a circuit board? If a breadboard then the board itself has small capacitive differences between each row. This can affect the output. Also, if your hand, or body, or anything gets close to the circuit, you can affect the timing with your body capacitance. The 555 timer is a sloppy little beast and can shift for all kinds of reasons. Anytime you use a circuit for timing that requires and RC time constant, it’s going to be a painful experience. Sorry I couldn’t help more.

  3. Hello Jason, T nk you for helping me understand the maths behind this. my current problem is that i have a circuit that goes from 1-54%. I am using a 100K POT. I am considering to ignore the circuit completely and adapt your circuit however I am not sure this will combat my problem. May you please tell me how I

  4. Hi. I have to simulate a PWM generator using Pspice. And I have a problem. Using different types of diodes leads to very different frequencies. I need a fixed frequency. I get the most accurate results when I use a Schottky rectifier. Can you tell me why this is a good solution. Thanks

    1. I’ve not looked deeply into the circuit analysis yet, but it probably has something to do with the fact that Schottky diodes have a lower forward bias voltage.

  5. Thank you for the video. I have built this circuit a while ago and using it as a dimmer for an actively cooled LED. Can you think of a way to automatically adjust duty time depending on supply voltage in order to keep LED light output constant ? Across a reasonable range of 1S Li-ion battery, let’s say 4 to 3.5V based on a lowest acceptable value. While it’s probably much easier to achieve with ADC, analogue solution would be much more elegant as well as likely cheaper and save more battery power.

  6. Jason, greetings. Thank you for taking the time to share your knowledge/videos. I’ve successfully put together the other circuit on your site that allows for using the 555 timer as a means of controlling the frequency of an led flasher using pin 3 on the timer at the base of an NPN transistor. Worked great. Next I will be putting together your circuit for controlling the duty cycle of the 555 without changing frequency, as you’ve shown above. I’m curious, is it possible to combine these circuits and their functions? For my purposes, what I really want to do is adjust both parameters independently within the same circuit. Apologies if I am missing an obvious conflict that makes this impossible. My electronics theory is still a bit shaky but I’m having a ton of fun frying components and discovering. Thanks again! Brian

    1. Both functions should work together without a problem. Just know that if it looks like the light is always on, you might have the frequency set too high.

  7. I have been trying to create a signal with an output of 45Hz. and then change the duty cycle via the potentiometer. The issue is that I always get high. Please help. Can you please recommend components for a 45Hz signal output. Thank you

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