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. In the chime mode, retail stores in businesses can use it to announce customers are alert them when someone enters an area that is "off limits". You can choose to have the chime sound only one time (when a customer enters a store for example) or the chime can sound continuously until you turn it off (when a customer enters an area that is "off limits" or at home when a child leaves a safe area for example). Distance can be sent to high or low- maximum distance 60 ft. In the chime mode, volume can be set to low, medium, or high. Two AC adapters are included. Three AA back up batteries are not included.