[list]http://www.ultrasecuredirect.com/acatalog/copy_of_Wireless-PIR-Magnetic-Door-Perimeter-Alarm-Alert-System-shed-garage.html
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I have one of these, the main unit sits in the house running on mains with a battery backup.It uses a door sensor and a PIR for detection. You can also get a extra loud alarm to put in the shed.
edited to make a little more sense.