Guess What This Circuit Does?

before you see the answer (below) what do you think this circuit does?...

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What it does...

This circuit is the simplest FM circuit you can get. It has no microphone but the coil is so MICROPHONIC that it will pick up noises in the room via vibrations on a table. The circuit does not have any section that determines the frequency. In the next circuit and all those that follow, the section that determines the frequency of operation is called the TUNED CIRCUIT or TANK CIRCUIT and consists of a coil and capacitor. This circuit does not have this feature. The transistor turns on via the 47k and this puts a pulse through the 15 turn winding. The magnetic flux from this winding passes through the 6 turn winding and into the base of the transistor via the 22n capacitor. This pulse is amplified by the transistor and the circuit is kept active. The frequency is determined by the 6 turn coil. By moving the turns together, the frequency will decrease. The circuit transmits at 90MHz. It has a very poor range and consumes 16mA. The coil is wound on a 3mm drill and uses 0.5mm wire.

I can't imagine this being that great, the drift concerns me too much to even build this.

The circuit does not have any section that determines the frequency.

That is not true, it is the coil and stray capacitance of the transistor that determine the frequency. I once modulated a transmitter by the microphone effect from the valves ( tubes )

My favourite example of microphony and modulation. Note also its famous inventor.

The coil isn't what is microphonic, the ceramic capacitor is.

Class 2 and 3 ceramics are made of barium titanate (the same material used in record-player needle cartridges) and is piezoelectric. Run audio frequencies through it, they will sing. Vibrate them, they generate electricity.

From looking at the picture, I would think neither the coil is microphonic and picking up vibrations [per se], nor that the pickup is due to the ceramic cap, but rather that the actual modulation is due to the method of construction.

IE, that cap is hanging in space, and has a mechanical transducer attached to it [specifically a piece of wire - the antenna - cantilevered in space], and they are both attached to a non-rigid coil of wire.

This is the perfect prescription for the cap/wire cantilever to pickup mechanical vibrations, and minutely change the spacing between the coil turns.

The funny thing is that even a "traditional" microphone-based transmitter, with a proper oscillator, is quite often challenging to build and troubleshoot by a beginner. Honestly I don't expect this to work as a "voice/noise transmitter" in the real world.