The hama GM-300 garden monitor displays the temperature of the air and the humidity and temperature of the soil. Up to five sensors can be connected to the base station.

The sensors communicate via 433MHz OOK protocol with the base station. The transmission starts with a 6000 microseconds synchronization pulse, followed by 64 bits encoded like this: "1" bit = 500 microseconds high + 2000 microseconds low, "0" bit = 500 microseconds high + 1000 microseconds low. By using rc-switch's definition 'formula' this translates to { 500, { 1, 12 }, { 1, 2 }, { 1, 4 }, false }.

The received telegram (64 bits) can be decoded as follows:

HHHHHHHHHHHH-CCC-MB-HH-U-SSSSSSSSSSSS-AAAAAAAAAAAA-UUUUUUUUUUUU-RRRRRRRR
100001001111-100-10-01-0-100111001000-001111110000-000000000000-11110110

H = host id, changes after battery replacement, the upper four bits seem to be "1" fixed
C = channel, as set by the rotary dial on the back of the sensor
M = manual send, if you press the button on the back of the sensor
B = battery low indicator
H = humidity (0, 1, 2)
S = soil temperature, shifted by 10, e.g. 234 = 23.4 deg C
A = air temperature, shifted by 10
U = unknown / unused
R = CRC

Thanks to reveng, the CRC is calculated like this: width=8 poly=0x31 init=0xff refin=true refout=true xorout=0x00 check=0x0b residue=0x00.

hama-gm300.ino can be used to receive and decode telegrams:

Received 7BF402F077000000 / 64bit Protocol: 1
hostid:   4062
channel:  2
manual:   0
lowbat:   0
humidity: 0
soiltemp: 244
airtemp:  238

This projekt ist based on the rc-switch library, extended with 64bit support.