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I have implemented an i2c temperature+humidity sensor on my ArduPilotMega. This could be useful for anybody, so I will try to explain here how I did it.

What is needed:

the sensor i used is the SHT21 from Sensirion, but it should also work for the SHT25. On the manufacturer website you can find technical data about the sensors, and they also sell their sensors themselves:

I received the sensor from Sensirion mounted on a breakout board.

a connector to easily change the sensor breakout borad.I used one from samtec(ref SMS-104-01-G-S).

An I2C/SMBus Voltage Traslator (I2C Level Shifter) With adapter because the sensor uses 3.3V i2c when the APM outputs 5V

An EM-406/uBlox/MTK Adapter Cable 10 cm , or whatever length.

(only 4 wires are required. So you can remove the red one and the black one standing next to it)

Code:

I did include all the files I modified here.

Code features:

-I did not implement mavlink code for data transmission. It seems it requires changes in Mission Planner, which I cannot do.

-In the Command Line Interpreter (CLI), in the test "menu", the "meteo" command output temperature and humidity, until "enter" is pressed.

-It can log temperature and humidity about every second (I have to check if the logging interval is really 1 sec). The logged data can be retrieved in Mission Planner. Logging can be enabled or disabled in the "logs" menu.

So far it seems to work fine. I will test more in the next days.