The piercing siren of this door alarm will scare off unwanted intruders. It's an inexpensive way to keep your home safe and secure, and it requires no special wiring. Just hang it on an inside door knob, and the system works even if the intruder comes within inches of the knob. Portable so you can use it at home, the office, in a dorm or while staying at a hotel. Uses one 9-volt battery; not included. One-year limited warranty.