Cracking Weather Station Checksum

[BaronVonSchnowzer] is spinning up some home automation and settled on an inexpensive ambient temperature sensor which is sold to augment the data a home weather station collects. He found that the RF protocol had been reverse engineered and will use this information to harvest data from a sensor in each room. In true hacker fashion, he rolled his own advances out to the Internet so that others may benefit. Specifically, he reverse engineered the checksum used by the Ambient F007TH.

He got onto this track after trying out the Arduino sketch written to receive the sensor’s RF communications. One peculiar part of the code turned out to be a filter for corrupt messages as the protocol’s checksum hadn’t yet been worked out. Figuring out how the checksum byte owrks wasn’t an easy process. The adventure led him to dump 13k samples into a spreadsheet to see if sorting similar sets of 5-byte message and 1-byte checksum would shed some light on the situation. The rest of the story is some impressive pattern matching that led to the final algorithm. Now [BaronVonSchnowzer] and anyone else using these modules can filter out corrupt data in the most efficient way possible.