To add to this: https://www.sparkfun.com/products/13285 + https://www.sparkfun.com/products/13247 + https://www.sparkfun.com/products/8653 + https://www.sparkfun.com/products/13120

You now have 99% of the security offered by an alarm system, go buy an ADT yard sign from Amazon and you have 99.9%