The Wander Alarm Motion Detector senses movement of a person from a bed, chair, room, or doorway and will either cause an audible alert from the sensor unit or send a signal to a wireless receiver located up to 100 feet away (depending on building construction). You can select if you prefer an alert from the sensor unit or from the wireless receiver. Both will not emit an alert at the same time. Perfect for caregivers to monitor individuals who tend to wander (such as those with dementia) or those who go into areas of the house where they need supervision (such as persons with Prader-Willi going into the kitchen). Works in the dark as long as ambient light allows motion to be detected. (Can detect the movement of animals and children.) Requires 2 9-volt batteries, not included.