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Project Notes

#563 Dual Variable Mode Oscillator

Testing a 555 timer astable oscillator circuit that can be switched between variable duty cycle and variable frequency.



This is a very effective circuit that I found as an example in the Multisim circuit simulator samples. I’ve made a few modifications to the circuit, primarily to reduce the clock to a very low (visible) frequency.

The circuit can be manually switched between variable duty cycle and variable frequency, and it performs impressively:

  • variable duty cycle mode gives almost full range control (4-96% in this build) with no significant impact on a fixed frequency
  • in variable frequency mode, the duty cycle remains stable at around 50% for the entire frequency sweep


I selected VR1 = 50kΩ, and C1 = 10µF for the build in order to keep down to very low frequencies so that the output LED indicator can be followed by eye.




With VR1 = 50kΩ, and C1 = 10µF, frequency can be adjusted between 1.2Hz and 32 Hz, or duty between 4% and 96% at a fixed frequency of ~2 Hz. Here are the output scope traces for the minimum and maximum VR1 setting for variable duty (upper) and variable frequency (lower)


Some ranges I’ve measured with various VR1 component values:

VR1 f(min) f(max) duty(min) duty(max) f(fixed duty)
10kΩ 5Hz 35Hz 14% 86% ~6Hz
20kΩ 3Hz 34Hz 8% 92% ~5Hz
50kΩ 1.2Hz 32Hz 4% 96% ~2Hz
100kΩ 2Hz 34Hz 2% 98% ~1Hz

NB: I should have switched C1 for a higher frequency range in order to capture more accurate measurements. Perhaps later?


I only had a brief look at Multisim Live, but it seems quite capable, and is very intuitive.

Here are the results and a glimpse of the user interface for the simulation of the circuit similar to the one shown here:


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.