Cheap weather stations send their outdoor readings over 433 MHz radio. Pogodynka (“weather station” in Polish) listens to an Auriol sensor with an RFM83C receiver and an Arduino, and decodes the packets itself.
The protocol is pulse-distance coded: a long sync gap, then bits whose value depends on the gap length (about 4.5 ms for a one and 2.5 ms for a zero, with a tolerance window). An interrupt captures the timing of every pulse into a ring buffer, and a small state machine finds the sync, collects the bits into nibbles and assembles a packet.
The repository keeps the documents it is based on: the receiver datasheet and a community description of the Auriol RF protocol.