MOTION DETECTOR WITH ALARM

摘要

A surveillance unit for monitoring an area such as a swimming pool, a car, the surroundings of a house, etc., includes a pair of closely spaced motion detectors, an alarm horn, and control circuit and switches to operate the surveillance unit. Each motion detector has a beam focused on a particular surveillance area coinciding with the area to be monitored (e.g. a pool and its immediate surroundings) by a focusing device. Each detector comprises a housing and a front window and therein a focusing device which is longitudinally slidable along guide ribs in the housing and which includes a lens for varying the transverse dimensions of the beam and a removable focusing plate which includes an outer opaque section and an inner transparent section which takes the form of the surveillance area to monitor such as the outline of the pool while taking into consideration the angle at which the detectors see the surveillance area. The focusing plate when its transparent section defines the proper focusing zone allows the beam to be directed towards the surveillance zone while the focusing device is positioned in the housing so as to obtain a proper size beam. The shape of the transparent section thus defines the outline of the surveillance zone whereas the relative position of the focusing device defines the transverse dimensions of the beam and thus of the zone being monitored. The transparent section can be customized so as to allow for the surveillance of zones of various shapes and sizes depending on the particular application of the detectors. False alarms are substantially eliminated by the use of two motion detectors.