There's also PiFM[1] which (iirc) modulates one of the GPIO pins fast enough on a Pi to produce an FM radio signal.

It's fun to play with, and is a great hack, but please don't try and use it for anything or any length of time; it puts out a tonne of horrible harmonics all over the radio spectrum.

[1] https://github.com/rm-hull/pifm

I have a „Hörmann“ garage door opener. For a long time, I want to use my phone instead of the original sender. Therefore, I thought to run a raspberry and send the signal from there.

I recorded the signal with HackRF One as .wav, the problem here is that HackRF records with a sampling rate much higher than 48kHz (by default it is 10MHz!). So one has to downsample the recorded file, which I did with Matlab (downsample(signal, „fs_of_record/fs_for_rpitx“)). The resulting .wav signal is now send at 433MHz (either my garage door listens their too, or 2nd harmonic works?!)

src: https://github.com/F5OEO/rpitx/issues/50