Reply To: Patio heaters on 433 MHz sockets: switching them at sunset from a rule through a Pi

The GPIO4 trick uses the Pi’s clock generator: it can put out a square wave at tens of MHz, and the software wiggles the frequency to make FM. A square wave is full of harmonics, so you’re also transmitting on frequencies you have no right to use. Fine for a lab demo, I wouldn’t put it on a hotel terrace.