Wireless Temperature Sensor : La Crosse TX29-IT+ and TX29D-IT+ - 2011 June 2nd - lang=fr


I was offered a weather station for X-mas :


It measures indoor hygrometry and temperature and thanks to a wireless sensor it also measures outdoor temperature.

You can buy it from "Nature et Decouvertes". It is a special edition of a "La Crosse Technology" weather Station.

Reference at the back : WS-7014-IT

Wireless Temperature Sensor

The station comes with a wireless sensor for outdoor temperature. It is powered by two AA 1.5V batteries, reference : TX29-IT

The TX29-IT sensor comes without LCD screen whereas the TX29D-IT has one :
  

The sensor and the station feature the "Instant Transmission Plus" technology from "La Crosse Technology" in the 868MHz band.

But...

What the station misses is a serial port at its back so that we can read the information gathered from its internal sensors and from the wireless sensor(s). Indeed what's the point of measuring a physical entity then digitizing it, then wirelessly transmitting it, then displaying it on a LCD, if in the end you don't have a mean to store this digital information or to send it to a PC ?

Objective : Home automation

The sensor is sold as spare parts for about 13-15 EUR. It would be perfect for my home automation project. I would just have to dispatch a bunch of such sensors in different areas of the house (and outdoor) and monitor them from a PC which would control the HVAC, the heating, etc.

Let's hack this WS and find a way to retrieve the information from a PC.


At first I wanted to use the station as a receiver and read the indoor temperature/hygrometry and the outdoor (wireless) temperature from any available signal, but after investigations I obtained something better.

Part One : Hardware Reverse Engineering


Part Two : Software Reverse Engineering


Part Three : CRC Reverse Engineering


Conclusion


The following plot has been obtained after 2 days of temperature monitoring :


  • green : indoor temperature, TX29 sensor
  • red : outdoor temperature, TX29D sensor



There are still some wrong measurements but they are quite easy to cope with because there are few chances that the ambiant temperature goes from +20 °C to -20°C in less that 4 seconds...

So we can say that our goal is achieved :-)

Next steps


The use of the soundcard to retrieve the receiver demodulated signal makes things easy but it is quite sub-optimal, the next steps would consist in :


  • interfacing with the PC through a RS232 serial adapter (use of DCD signal as input)
  • powering the WS from the PC through a serial to 3.0V convert or through an USB to 3.0V adapter.
  • extracting the receiver module and integrating it with an arduino interfaced with a GPIO


The last one would be a great thing in terms of powersaving because the Arduino would run the message decoding and CRC computation, which would minimize the PC wakes-up. Or it could even run "offline" and store the measurements so that a constantly powered on PC would not be necessary.

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