Window breakage detector for intruder alarm - has piezoelectric element and evaluation circuit combined in flat housing

摘要

The detector comprises a housing containing a piezoelectric element (16) which is adhered to the window pane. The alarm signal is given by an LED (17) controlled by the piezoelectric element (16) and an evaluation circuit comprising an oscillator (11) and a threshold amplifier (12) with an alarm signal store (13). The LED is (17) connected in series with a coupling transistor having its base (T3) coupled to the output of the alarm signal store (13) via a resistor. Pref. a hybrid evaluation circuit is used, its substrate lying in a plane immediately above the plane of the housing wall adhered to the surface of the pane. This housing is pref. made of sheet aluminium and measures 17 x 17 x 9 mm. The detector is compact and flat to give it an aesthetic appearance and without sufficient weight to impair the window pane.