To read a SHT21 temperature/humidity sensor from Raspbian Linux on the Raspberry Pi, first enable the I²C bus via raspi-config and reboot. Then, load the sht21 kernel module:
$ sudo su # modprobe sht21
(To make the change permanent, run echo "sht21" > /etc/modules-load.d/sht21.conf as root.)
If you want to verify that the sensor is wired up correctly, install i2c-tools and run i2cdetect -y 1 0x40 0x4f. If everything is fine, you should see it at the address 0x40:
# i2cdetect -y 1 0x40 0x4f
0 1 2 3 4 5 6 7 8 9 a b c d e f
00:
10:
20:
30:
40: 40 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50:
60:
70:
Since a computer can have multiple I²C busses, the kernel needs to know to which one the sensor is connected. We therefore register the sensor at address 0x40 with the I²C sysfs device:
# echo sht21 0x40 > /sys/bus/i2c/devices/i2c-1/new_device
Now the sensor values are exposed at /sys/class/hwmon/hwmon0:
# ls /sys/class/hwmon/hwmon0 device name subsystem uevent humidity1_input power temp1_input
To read the current temperature:
# cat /sys/class/hwmon/hwmon0/temp1_input 32279
...and the relative humidity:
# cat /sys/class/hwmon/hwmon0/humidity1_input 34512
If you have lm-sensors installed, you can also run the sensors command to get a sensor reading:
# sensors sht21-i2c-1-40 Adapter: 3f804000.i2c temp1: +30.8°C humidity1: 33.3 %RH
..and if you use collectd to log system metrics, load the sensors plugin like this:
echo "LoadPlugin sensors" >> /etc/collectd/collectd.conf