US Patrol Watchdog Door AlarmSimply hang on the inside of any door knob; no special wiring required. Super-sensitive alarm works even if an intruder comes within inches of the door knob. Scares off unwanted intruders, keeping you well protected. Requires one 9V battery, not included. ABS plastic, 7 3/4 x 1 3/4 x 1″.Features:System works even if the intruder comes within inches of the knob Inexpensive way to keep your home safe and secure Requires no special wiring Simply hang on the inside door knob Ideal for any door knob at home, office, dorm, hotel room anywhere