Vehicle intrusion alarm circuit
First Claim
1. A vehicle intrusion alarm circuit for use in combination with a vehicle having at least one compartment, an openable compartment closure therefore, at least one electrical load element for coupling to a battery, and switch means for energizing said load element when said compartment closure is opened, said intrusion alarm circuit comprising an operational amplifier having two inputs and an output, said operational amplifier being operable to produce an output signal which is proportional to the difference between its two input signals, means coupling both of said operational amplifier inputs to the ungrounded terminal of said battery, a capacitor coupled between one operational amplifier input and ground, the voltage on said capacitor and said one input being substantially equal to the voltage on said other input when the load element is deenergized and the voltage on said other input changing incidental to energization of said load element and coincidental change in said battery voltage to thereby produce a voltage difference between said inputs for operating said amplifier, alarm means coupled to the operational amplifier output, said alarm means being operable to produce an audible alarm in response to an output signal of said operational amplifier caused by the opening of said compartment closure, and alarm disabling means accessible from outside said vehicle for disabling said alarm circuit to permit opening of said compartment closure without activating said alarm.
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Accused Products
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.
15 Citations
16 Claims
- 1. A vehicle intrusion alarm circuit for use in combination with a vehicle having at least one compartment, an openable compartment closure therefore, at least one electrical load element for coupling to a battery, and switch means for energizing said load element when said compartment closure is opened, said intrusion alarm circuit comprising an operational amplifier having two inputs and an output, said operational amplifier being operable to produce an output signal which is proportional to the difference between its two input signals, means coupling both of said operational amplifier inputs to the ungrounded terminal of said battery, a capacitor coupled between one operational amplifier input and ground, the voltage on said capacitor and said one input being substantially equal to the voltage on said other input when the load element is deenergized and the voltage on said other input changing incidental to energization of said load element and coincidental change in said battery voltage to thereby produce a voltage difference between said inputs for operating said amplifier, alarm means coupled to the operational amplifier output, said alarm means being operable to produce an audible alarm in response to an output signal of said operational amplifier caused by the opening of said compartment closure, and alarm disabling means accessible from outside said vehicle for disabling said alarm circuit to permit opening of said compartment closure without activating said alarm.
- 7. A vehicle intrusion alarm circuit for use in combination with a vehicle having at least one compartment, an openable compartment closure therefor, a battery for supplying electric power in said vehicle, at least one electrical load element coupled to said battery, and switch means for energizing said load element when said compartment closure is opened, said intrusion alarm circuit comprising an operational amplifier having two inputs and an output, said operational amplifier being operable to produce an output signal which is proportional to the difference between its two input signals, a potentiometer having an arm terminal and two end terminals, each end terminal being coupled to a corresponding one of said operational amplifier inputs, and said arm terminal being coupled to the ungrounded terminal of said battery, a first resistor coupled between one potentiometer end terminal and the corresponding operational amplifier input, a second resistor coupled between the other potentiometer end terminal and the other operational amplifier input, a third resistor coupled between one operational input and ground, a fourth resistor coupled between the other operational input and ground, said first and second resistors being of the same type and being equal in resistance value and said third and fourth resistors being of the same type and being equal in resistance value to equalize temperature induced variations in the input of said operational amplifier, alarm means coupled to the operational amplifier output, said alarm means being operable to produce an audible alarm in response to an output signal of said operational amplifier caused by the opening of said compartment closure, and alarm disabling means accessible from outside said vehicle for disabling said alarm circuit to permit opening of said compartment closure without activating said alarm.
Specification