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.