Alarm systems have the tendency to become expensive if you go for the better systems out there.  However, if you are not worried about having a complicated system, this noise maker would do the trick just as well.

This door stopper could slip in front of the door, then when someone opens the door it activates the alarm.  It has adjustable sensitivity and can be turned off entirely.  Which means during the day it won’t be an issue when you don’t want the alarm on.  To keep it powered you’ll need one 9V battery. 

Features:

• Wedge shaped desgin to fit to any hinged door
• 120db high pitched continuous siren when activated
• Vibration sensor 
• Adjustable sensitivity : H, M, L
• No installation required, no wiring is needed, operates with one 9Vbattery (not included)