Above and below are the diagrams of the electronics that will interface the alarm system sensors with the Raspberry Pi. The first diagram is a conventional electronics schematic and the second is how you can build the circuit prototype on a bread board.
The Magpi Magazine (www.themagpi.com) is an excellent resource for learning electronics on the Raspberry Pi. Refer issues 2 & 3 for deeper explanation behind the electronics shown on this page. The diagrams below are adaptions of the circuits in Magpi Issue 3.
For more detail: Alarm Electronics