What is the simplest possible sine wave generator circuit?

Question

Let's say I have a variety of simple resistors, capacitors, transistors, et cetera -- but no advanced equipment. No integrated circuits, piezo-crystals, or any of that. I want to construct the simplest possible oscillator that can generate a basic sine wave with as few of these components as possible. What would the circuit consist of?

Answer 1

A twe-transistor, two capacitor, four resistor astable multivibrator circuit driving an incandescent bulb which in turn illuminates an LDR; the inherent hysteresis of the bulbs filament will smooth the astable's square wave into an acceptable sine wave as the output from the LDR...

Answer 2

If you do not want to use an inductor and you do want a sine wave, then a phase shift oscillator is the way to go. You can do this with a single transistor if the hfe is high enough, otherwise use a darlington pair.

http://www.visionics.ee/curriculum/Exper...

If you have no oscilloscope, you can hear it run using a crystal earpiece.

Answer 3

Hmmmm that is a very good question.  I am interested to see what other answers crop up but I'm going to have a stab.

If you want a basic (reliable) designed oscillator then a hartley and/or colpitts oscillator would be it.  This consists of a transistor, inductors,capacitors and sometimes a resistor.  You can look those up to get schematics, they are very well known circuits.

Another simpler circuit is the relaxation oscillator which consists of a inverting schmitt trigger, a capacitor and resistor.  You will need a low pass filter to knock off the higher order harmonics (convert from a square wave to a sine wave) which will consist of capacitors and inductors.

I have seen simpler circuits oscillate but they were by accident (through paracitic capacitances etc) so wouldn't count those.

When getting up in frequency you can substitute the caps and inductors with PCB traces, stubs etc.  See a dielectric resonator oscillator, they operate with only a gain device as a non-monolithic component.

Cavities can resonate too, so a milo can with a gain device could technically provide you with a sine wave.  

Hope that helps!