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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

About LEAP#336 OpAmpOscillators
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LEAP is my personal collection of electronics projects, 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 (IMHO!).

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