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Last year, I posted this question about a very simple RF transmitter circuit: Does this simple RF transmitter circuit actually work?

I built it myself, and actually remade it after I posted an answer to my own question; I applied CA glue to the coil so that it would stay around the same frequency. Though I think that power and even temperature(or air pressure?) could alter the frequency, it remains stable for a long period of time.

Anyhow, I am curious as to how I can add a microphone or direct AUX input so that I can actually transmit sound instead of silence.

I have achieved this to some degree by connecting an audio source to a secondary coil placed next to the main coil in the circuit. The second coil acts like an electromagnet and seems to modulate the frequency of the main coil. This does work, though the volume of the audio is low yet it is still prone to getting distorted at a certain volume. I'm guessing this has something to do with the frequency being only modulated in one direction(my understanding is that an FM "channel" is the center frequency for the modulation), and maybe the difference between high and low volume needs to be less significant. I'm unsure how to word it better than that.

I want to stick to the spirit of the circuit and go with something as dirt simple as possible. Indeed, the coil itself can act as a microphone but I've yet to make this practical or not prone to frequency drift.

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What's the next logical step in applying input to this FM transmitter circuit?

You've taken that step it seems. The audio mag field will wobble the emitter about a bit and you'll get a mix of AM and FM effects. The AM effects might not be pushing in the same direction as the FM effects and your receiver may not be amplitude limiting stuff as good as it should and along comes the distortion - a nasty interaction of half-limited AM stuff and proper FM. If you want a better transmitter try googling "simple transistor microphone FM transmitter" to get some images: -

enter image description here

After you've tried your hand at a few of them go stereo: -

enter image description here

Here's the website for the design and don't forget that all these transmitter circuits are likely to be quite illegal in most areas of the world.

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Try adding a series RC network to the transistor base and feed the input with your audio. Start with values of 10k resistor and 10n capacitor and see if that does anything for you.

The idea is that the audio modulation shifts the DC operating point of the oscillator - this leads to changes in the output frequency.

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