You can know when someone " crosses the line". This high tech device creates an invisible infrared beam up to 60 ft. long. When the beam is crossed a loud alarm or a pleasant chime sound will be emitted. Can be used as an invisible playpen to alert you to your child or pet leaving a safe area. The alarm can also be used at night to scare off would-be intruders.Two AC adapters are included. Three AA back up batteries are not included.