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When the door or window is opened, one magnet is removed from the surrounding area of the other and the sensor registers
that the door has been opened. More advanced home alarm systems may also contain vibration sensors
to detect when glass has been broken by measuring the vibrations caused by the breakage. Also, many systems use motion sensors that compute the proximity of a room’s walls by producing infrared beams and measuring the time required for them to bounce back; if a beam comes back sooner than expected, it likely bounced off of a person and the sensor registers motion. The control center keeps track of the status of the sensors and reports activity through LCD displays and other output devices. Moreover, when activity takes places, such as the opening of a window, the sensors report the activity to the control center and the center registers that a disturbance has occurred. In managed alarm systems, the control panel also checks the sensors to verify that they are still operational; if a sensor fails to reply, the center alerts the home owner of an out of order sensor.