Communications: electrical – Selective – Having indication or alarm
Patent
1976-01-02
1977-07-26
Waring, Alvin H.
Communications: electrical
Selective
Having indication or alarm
340274R, 307 10AT, 307350, 180114, B60R 2510
Patent
active
040386350
ABSTRACT:
An audible alarm is set off by unauthorized opening of a vehicle's doors, trunk or hood. The alarm circuit includes an operational amplifier and means coupling both operational amplifier inputs to the ungrounded vehicle battery terminal through balanced input resistors. A capacitor is coupled between one operational amplifier input and ground. When the door or trunk is opened, a light is automatically switched on, thus causing the battery voltage to drop slightly and producing an output signal from the operational amplifier which energizes an audible alarm coupled thereto. A key switch which is accessible from the outside the vehicle can be turned to disable the alarm circuit to permit authorized entry of the vehicle. The differential input of the operational amplifier stabilizes the circuit's operating characteristics with respect to temperature and prevents malfunction at temperature extremes. The differential input also prevents false triggering due to lightning, electronic ignition on passing vehicles, radiation from nearby radio transmitters, etc. The alarm circuit includes a two-tone audio oscillator whose frequency is shifted periodically between fixed values by another oscillator.
REFERENCES:
patent: 3808466 (1974-04-01), Campbell
patent: 3956732 (1976-05-01), Teich
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