For this project i ordered a Raspberry Pi model A. The main difference between model A and model B is that model A doesen't have an RJ45 port and 2 usb, also the ram have been lowered to 256 mb. Becase of this downgrades it is more cheaper, but for an automation project it is perfect.
The idea is to set some ports as output, connect those ports to some optocouplers that acts as a switch for the 12V adpter that power the 12V relays that turn on/off the lights.
The andorid tablet is set to automatically connect to the raspberry via wifi, the light of the display set to minimum and always on.
The interface that control the relays is called Webiopi which allow you to use HTML codes to create a nice interface for the output ports.
Raspberry pi • Wireless router • Android tablet
In this image you can see the web interface on the tablet mounted in the wall.
In this picture you can see the optocouplers and the relays soldered on an old pc power supply circuit board.