That should fit the bill. Radio Shack also used to sell a cheap IR beam break door alarm system - they're all over Amazon now (as was posted elsewhere.)
I installed one of these, and a remote light switch as well that sends alerts to my phone if the door is open for longer than x minutes.
http://www.amazon.com/Chamberlain-CI...hamberlain+myq
They also make models for older doors, and even have a setup that can be retrofitted to even older non-compatible stuff to remotely sense if the door is open or closed.