The most likely method of forced entry into a home or office is via a door or window. That\’s why it\’s so important to have contact sensors attached to the windows and doors on your home – at least on the ground floor. These magnetic sensors can detect any time a window or door is opened. If the alarm is armed, it\’ll be triggered – setting off the siren and, in the case of the S02/GSM system will call a sequence of designated numbers until someone picks up. Each pair of sensors requires one 23A battery.