Abstract: A lens segment used in passive infrared motion detectors is disclosed. A preferred embodiment has semi-circular rings separated by grooves. The semi-circular rings are offset about a separation axis passing through their centers or optical axes. The separation axis effectively divides the lens segment into two subsegments causing projection of two cross-sectional areas of coverage, one offset from the other. This increases the size of the projected cross-sectional area of coverage preventing a rodent near the lens from substantially filling the projected cross-sectional area of coverage, thus avoiding detection of the rodent.

Abstract: An apparatus for switching an alarm-signalling device connectable to a protective circuit so as to signal an alarm when a door or the like is displaced from its closed position in a jamb or the like by an intruder, in which the switching apparatus is mounted in the door and jamb or the like. The switching apparatus includes a switch connectable to the protective circuit, mounted in a housing installable so as to be recessed in the jamb, and a metal plate installable so as to be recessed in the side of the door and disposed opposite the switch when the door is closed in the jamb.

Abstract: An apparatus for switching an alarm-signalling device which is connectable to a protective circuit so as to signal an alarm when a door or the like is displaced from its closed position in a jamb or the like by an intruder, in which the switching apparatus is mounted in the door and jamb or the like. The switching apparatus includes a switch which is connectable to the protective circuit, which is mounted in a housing which is installable so as to be recessed in the jamb, and a magnet which is magnetized such that opposite faces thereof define poles of opposite polarity, which is mounted in a housing which is installable so as to be recessed in the side of the door and disposed opposite the switch when the door is closed in the jamb.