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Protocol of Clas Ohlson 36-6726 wireless thermometer/hygrometer

The Clas Ohlson product 36-6726 is an extra temperature/humidity wireless sensor for Clas Ohlson product 36-6725 and 36-6725-1.

The device sends on the usual frequency of 434 MHz. It can be set to one of three channels via a switch in the battery compartment. It transmits once every 57 seconds when on channel 1, once every ~70 seconds on channel 2, and once every ~80 seconds on channel 3.

Transmissions use OOK (on-off keying). Each transmission consists of 12 identical copies of the message, with 3.8 ms OFF between messages.

Each message consists of 36 bits, encoded in the following way:

  • 0 is transmitted as ~500 µs ON, ~850 µs OFF
  • 1 is transmitted as ~500 µs ON, ~1800 µs OFF

In other words: There are 37 ON pulses in each message. The information is encoded in the pauses between them.

The bits have the following meaning. ("Bit 1" is the bit transmitted first.)

  • Bits 1-10: This appears to always be 0101 1011 10.
  • Bits 11-12: The channel as set by the switch in the device. 00 is channel 1; 01 is channel 2; 10 is channel 3.
  • Bits 13-24: 10 times temperature in degree celsius, two’s complement (see below)
  • Bits 25-28: Seems to always be binary 1111
  • Bits 29-36: Humidity (% relative humidity)

The temperature is transmitted as a 12-bit value representing the temperature in degrees celsius multiplied by 10. For example, 17.9°C is transmitted as 0000 1011 0011 binary, 179 decimal. Negative temperatures are transmitted in normal two’s complement. That is, -1.3°C is transmitted as 1111 1111 0011 (12-bit two’s complement of 13).

There is possibly a “low battery” bit in the bits described above as being constant.