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Project Notes

#049 BJT RC Oscillator

Demonstrates a simple RC/transistor multistable vibrator

The Build

Here’s a quick video of the circuit in action:



This oscillator is based on coupled RC circuits with associated switching transistors.

The frequency of oscillation is given by

f = 1/(ln(2) * ( R1 * C1 + R2 * C2))

Or in this case with 22kΩ and 22μF, that means about 1.5 Hz.

Note that for the oscillation to work correctly, the components need to be well matched. The first two transistors I stuck in the circuit produced a significantly uneven oscillation. Replacing the transistors with some newish S9013 NPN transistors fixed the problem, and to the human eye at least the circuit now runs completely balanced.

NB: I’m using S9013 but any small-signal NPN transistors will do.


The Breadboard

The Schematic

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.