Abstract A magnetic door alarm for use in conjunction with normally unlocked emergency exit doors of a type in which an audible signal is produced when the door is opened by unauthorized persons. The device includes a housing normally mounted on an inner surface of the door which normally swings outwardly, the housing containing a first reed switch of normally open type responsive to a magnetic field to move to a closed position. A magnet is positioned within the housing adjacent the reed switch to effect this movement. Mounted on a door jamb or frame is a larger magnet, the polarity of which is opposite that of the first mentioned magnet, so that its magnetic field cancels that of the first magnet when the door is in closed position, whereby the reed switch remains open. Upon movement of the door, the field of the second mentioned magnet is removed, and the reed switch is operated by the magnetic field of the first magnet. If an attempt is made to place an additional magnet adjacent the magnet on the door jamb, the field of the door jamb magnet is either enhanced or cancelled, so that the equilibrium condition is destroyed, and the reed switch is operated by the first mentioned magnet. Additionally, a second reed switch is provided within the casing, electrically connected in parallel with the first switch, and shielded from the magnetic field of the first mentioned and second mentioned magnets, so that unauthorized use of a third magnet in the area of the second magnet will serve to close the second switch and operate the alarm irrespective of the condition of the first mentioned switch.
Inventors
👤 Bennett B Harman
Bibliographic Details
Application Number—
JurisdictionUS
Data Sourcegoogle_patents
Last Synchronized2026-11-04 06:00
CPC Classifications
G; G08; G08B; G08B13/00; G08B13/02; G08B13/08
Claims Analysis
Total Claims: 2
Claim 1
Independent Claim
235 words
Improved switching means for a magnetic emergency door alarm of a type including a casing mounted upon a surface of a door and a permanent magnet mounted upon an adjacent door jamb, comprising: a panel mounted within said casing having a planar surface substantially perpendicular to the plane of said door, a magnetically responsive normally open reed switch, magnet means disposed upon said panel adjacent said switch of field strength sufficient to maintain said switch in closed condition, said panel being so positioned within said casing as to lie in adjacent relation with respect to said permanent magnet when said door is in closed position, with said switch juxtaposed thereto; a second magnetically responsive normally open switch magnetically shielded from the field of said magnet means, and positioned parallel to said first mentioned switch, and connected in electrically parallel relation thereto; whereby said first mentioned magnet, the polarity of which is opposite to that of said magnet means serves to cancel the magnetic field of said magnet means when said door is in closed position, and upon movement of the door, the field of the magnet is removed, and the first reed switch is operated by the magnetic field of the magnet means, and said second magnetically responsive switch at all times remains sensitive to the unauthorized use of an external magnet to operate the alarm irrespective of the condition of the first mentioned switch.
Claim 2
Dependent Claim
235 words
Improved switching means for a magnetic emergency door alarm of a type including a casing mounted upon a surface of a door and a permanent magnet mounted upon an adjacent door jamb, comprising: a panel mounted within said casing having a planar surface substantially perpendicular to the plane of said door, a magnetically responsive normally open reed switch, magnet means disposed upon said panel adjacent said switch of field strength sufficient to maintain said switch in closed condition, said panel being so positioned within said casing as to lie in adjacent relation with respect to said permanent magnet when said door is in closed position, with said switch juxtaposed thereto; a second magnetically responsive normally open switch magnetically shielded from the field of said magnet means, and positioned parallel to said first mentioned switch, and connected in electrically parallel relation thereto; whereby said first mentioned magnet, the polarity of which is opposite to that of said magnet means serves to cancel the magnetic field of said magnet means when said door is in closed position, and upon movement of the door, the field of the magnet is removed, and the first reed switch is operated by the magnetic field of the magnet means, and said second magnetically responsive switch at all times remains sensitive to the unauthorized use of an external magnet to operate the alarm irrespective of the condition of the first mentioned switch.