intruder alarm - a combination of infra-red and electronics technologies

P Theunissen The alarm uses a minimum of two printed circuits: one for the control circuits, and the other for the interfaces. The latter may be repeated a number of times if required. The interface board is designed to connect either two infra-red sensors or one infra-red sensor and an anti-tamper circuit to the control board. Other than infra-red sensors may also be used. To this end, each interface offers two operating conditions: (a) NO, in which the switching contact is normally open and closes when the alarm is triggered, and (b) NC, in which the contact is normally closed and opens when the alarm is triggered. A delay has been incorporated which allows time between setting the alarm and vacating the property and between enter- ing the property and disabling the alarm. In the latter case, a pre-alarm buzzer sounds and an LED lights to indicate that the alarm has been triggered. This arrangement is also very useful for testing the alarm. The LED remains on when the alarm has ...