Making your own security system isn't too hard, but if you'd like a super cheap and simple method, Instructables user deba168 shows how to make a wireless motion detector with an Arduino.

Real Time Home Security System using GSM Technology-An Anti-theft System! This project deals with the design & development of a theft control system for home, which is being used to prevent/control ...

He’s using an Arduino Uno and an Ethernet breakout board to hook it up to the network. This allows him to send text messages to himself when the alarm system is armed, disarmed, or triggered.

You'll need an Arduino along with a bunch of various sensors to get the project rolling, and once it's all set up you'll have a simple, but cheap system to monitor your home.

He figured that monitoring the alarm’s LEDs would make the most sense, so he used a bit of Ethernet cable and wired all of the system’s indicators to his Arduino board.

First, some background: Arduino boards, like the popular Raspberry Pi computers, are single-board systems. You can upload code to them, just as you would any electronics project, but Arduino ...