Magnetic displacement detector for burglar alarm systems - comprises three switches, with principal magnet and keeping magnet

摘要

A magnetic detector is described which senses the relative displacement of two objects and which is particularly applicable in burglar alarm systems as a detector of opening doors or windows. A magnet (17) is fitted to one point of the door or window while the detector (16) is fitted to the other part. It comprises a first switch (13) which is normally open but which is closed under the effect of the magnetic field created by the principal magnet, and a second switch (14) which has a keeping magnet (18). The two switches are (13, 18) are in parallel in a circuit and the magnetic fields created by the principal magnet (17) and the keeping magnet (18) are opposed so that the circuit is momentarily opened when the principal magnet (17) moves away from the detector (16). A third switch (15) is fitted in another part of the circuit which is normally open but which is kept closed under the action of the principal magnet (17).