This is a very cute hack that does something which I thought was impossible: an implementation of an FM transmitter that has exactly two components: a battery and an ATtiny45 microcontroller. It’s brilliantly obtuse and cool:
Sprites mods – AVR-based FM-transmitter – Intro, theory
The basic idea is to trick the internal oscillator of the Atmel to run at 24Mhz. An internal PLL will run at 4x that frequency, which is a 96 Mhz signal (right in the middle of the FM broadcast band). By varying the frequency up and down, you can generate an FM modulated signal. Beyond clever. Very cool.
The only problem is that it’s very inefficient. It’s generating square waves, which contain plenty of harmonics (noise). Sprite’s post got me into playing with a PIC12F1840, which has an on-board mixer, and then running the output through a filter…it’s interesting, but I’m broadcasting at 8MHz.