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Project Notes

#445 555Timer/VariableDutyCycle/ControlAdjust

Using the control pin to adjust duty cycle of a 555 Timer astable oscillator.



The standard 555 astable configuration uses a built-in 2/3 x VCC upper threshold reference.

Given R1 = 4.7kΩ, R2 = 47kΩ and C1 = 1µF this produces an oscillation at 14.590 Hz with a 52.38% duty cycle.

The control pin (5) provides the opportunity to adjust the upper threshold, allowing the duty cycle to be vary above and below 50%.

While this provides access to duty cycles below 50% (unlike the standard astable configuration), the disadvantage of this approach is that both duty cycle and frequency are affected.


The circuit is essentially the standard astable, but with the addition of adjustable control reference voltage.




Test Results

When adjusted to 52.3% duty cycle, the actual frequency is 14.4 Hz - very close to the predicted value. This is essentially the same as having the control pin bypassed/disconnected in the standard astable configuration.


The minimum duty cycle possible with this astable configuration is 34.4%, at a frequency of 19.92 Hz:


The minimum duty cycle possible with this astable configuration is 62.1%, at a frequency of 11.42 Hz:


Duty Frequency Note
34.4% 19.92 Hz Min duty cycle
52.3% 14.4 Hz  
62.1% 11.42 Hz Max duty cycle

Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.