Best Buy has a good deal on the Samsung SmartThings ADT Wireless Smart Door/Windows Sensor for $5!
A bit about this item:
Help protect your home and family with an ADT Door and Window Detector. Initiate optional ADT professional monitoring services when an intrusion is detected in your home. Receive alerts if doors or windows are opened unexpectedly. Set lights and other connected devices to turn on and off as people come and go.