One of the essential parts of any home security system is a door detector that lets you know when a door opens. Over on Adafruit, they show you how to build your own that hooks into IFTTT so you can be alerted any way you like when the door opens.
The system here uses a cheap Arduino board (known as Genuino here in Australia), a dew sensors, and a magnetic contact switch. It's then hooked into the Adafruit IO and connected to IFTTT, so you can set up alerts in any way you like. If you're building your own home security system, this is a pretty clever place to start.