Our family room is actually an add-on and sits at the back of the house and seeing as I'm half deaf (damn that gunfire) I have a hard time hearing a knock or doorbell so I bought a little motion alarm made by Chamberlain and mounted it on the porch. Damn, that little sucker works great. If anybody walks up on the porch I get a beep from the base unit sitting right next to my Camouflaged Bass Pro Easy Chair.
I think I paid 60 bucks (on sale) for the base and one detector and 20 bucks apiece for the other sensors that are mounted in the back and side yards. Each sensor is gain sensitive and gives a different alarm so I can tell which one is signaling. Each base can handle 4 sensors.