Enable i2c
If you want to use i2c bus, you have to enable it first:
Open /etc/modules on Rasbian:
sudo nano /etc/modules
or /etc/modules-load.d/raspberrypi.conf on Arch Linux:
sudo nano /etc/modules-load.d/raspberrypi.conf
and add these two lines to the end
i2c-bcm2708
i2c-dev
Check if file /etc/modprobe.d/raspi-blacklist.conf exist and open it:
sudo nano /etc/modprobe.d/raspi-blacklist.conf
Put # in front of i2c line:
blacklist spi-bcm2708
#blacklist i2c-bcm2708
Reboot your system.