This 2 IN 1 PERSONAL/DOOR ALARM can be used as either a personal alarm or as a burglar alarm. When you hang this unit on any door knob it will sound an alarm when the doorknob is touched. This only works on non-metallic doors. Or, when used as a personal alarm, you can just pull on the metal strap to trip the alarm and sound the 120db alarm to deter an attacker. This unit requires one nine volt batter (not included).