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Project Notes

#336 Comparator-based Relaxation Oscillator

Test a classic comparator-based relaxation oscillator, modified for single-supply LM358 OpAmp.



The comparator-based relaxation oscillator is similar to the astable opamp oscillator - see LEAP#039 - but is a more straight-forward and symmetrical circuit, one that is easy to intuit.


Rather than use dual (+/-) supply, I’m using a voltage divider and buffer to provide a half-supply reference.

The theoretical frequency f = 1/(2ln(3)RC)

I tried a few combinations, and performance was remarkably close to the theory:

R C f (actual) f (theoretical)
100kΩ 10µF 437mHz 455mHz
100kΩ 100nF 44.8Hz 45.51Hz

Here’s the 100kΩ/100nF combo on the scope. Channels connected per the schematic, clearly showing the hysteresis of the capacitor charging curve as it battles the positive feedback.






Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

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.