More on observing the 433MHz band, Nexus IW004

One of the Christmas gifts in the family this year was a weather station, "Nexus IW004 / 36-5136". Some 'googling' of the brand got me to one of the local gadget stores in the region, Clas Ohlson. It is capable of measuring indoor and outdoor temperature and humidity, air pressure and the date. It also predicts the upcoming weather based on these data. The outdoor unit includes a sensor with mounting possibilities attached to the unit with a 1 meter wire.

Analysing the data with a 433 MHz receiver unit and a logic analyser showed a burst of twelve identical signals, 3.9 ms apart, for each reading of outdoor status. For channel 1 these bursts were located 56.95 seconds apart. For channel 2, 67.05 seconds and for channel 3 84.97 seconds apart.

Each pulse is 0.47 ms wide, with 1.98 ms separating 'ones' and 0.99 ms separating 'zeros'. The last pulse in a 'burst' is narrower, 0.25 ms wide.

Using this information and by varying the environmental parameters the following table could be compiled:

bit  1 - 10  Sensor ID and battery status(?)
bit 11 - 12  Channel ID
bit 13 - 24  Temperature * 10 in two's complement notation
bit 25 - 28  Always 1
bit 29 - 36  Humidity

Something also happens in bit 3 - 7 when the channel changes on the outdoor sensor.