Ever had to drag yourself out of bed and go downstairs to make sure the garage door was closed? With the Garage Door Monitor, you'll know with just a glance. Simply install the wireless sensor on the door, using hook-and-loop tape (included) and plug the indoor receiver into any AC outlet. When the receiver's LED is red, the door is open. If it's green, the door is closed. Works with any garage door. Monitor up to 4 doors using additional sensors (see below). Batteries included.