#049 BJT RC Oscillator

Demonstrates a simple RC/transistor astable multivibrator

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

Notes

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.

Construction

Credits and References

• S9013 Datasheet
• RC time constant
• The Sparkfun transistors page - great tutorial and some nice simple circuit ideas
• Basic Astable Multivibrator Circuit - good write-up of the circuit
• Astable Multivibrator circuit - another example.