GB1563064A

GB1563064A - Alarm system

Info

Publication number: GB1563064A
Application number: GB376277A
Authority: GB
Original assignee: R AND D INSTRUMENTS Ltd
Current assignee: R AND D INSTRUMENTS Ltd
Priority date: 1977-12-23
Filing date: 1977-12-23
Publication date: 1980-03-19

Description

(54) ALARM SYSTEM (71) We, R. & D. INSTRUMENTS LIMITED, a British Company of Spring Valley Industrial Estate, unit 16, Braddon, Douglas, Isle of Man, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to an alarm system for use with an electrical circuit to detect unauthorised operation of such circuit and initiate alarm action in response thereto.

It is visualised that the invention will find particular application in relation to the detection of unauthorised entry into and/or use of motor vehicle. It is however to be understood that the invention is not intended to be restricted to this field of application and may be used in any other suitable context.

According to the invention, there is provided an alarm system for use with an electrical circuit, comprisng connection means for direct conductive connection to a d.c. power supply for said circuit, detection means connected to said connection means so as to be capable of detecting only the occurrence of a change in an electrical parameter of the supply as applied to said connection means and to produce an output in response thereto irrespective of the direction of such change, and alarm means connected to said detection means so as to be actuated thereby in response to production of said output, the detection means comprising an electronic circuit which is arranged to receive operating power in use from said d.c. power supply via said connection means.

With this arrangement, in the event that there is unauthorised operation of the circuit such as to cause a change in the said supply parameter, the said output is produced by the detection means and this actuates the alarm means which may be arranged to produce a nearby or remote visual and/or audible warning and if desired also to actuate switch means which disconnects the power supply and/or takes other protective action.

In the case where the system is to be used with a motor vehicle, the system may be connected to the ignition circuit and/or other circuits so as to cause the said output to be produced when the ignition circuit is switched on and/or when other electrical devices are operated such as door courtesy lights, headlights, electrical accessories and the like. Most preferably, the alarm system is connected to the power supply at or close to the vehicle battery whereby the said output can be produced in response to unauthorised operation of all electrical circuits powered by the battery.

The said parameter change to which the detection means responds may be a change in the voltage of the supply and/or a change in current flow therefrom.

Most preferably, the said detection means is a pulse detection means responsive only to a sudden change in said parameter. In particular, in the case where the d.c. power supply is a battery conveniently the alarm system may be connected across same so as to respond to voltage fluctuations. In this case, the said connection means may be connected to the aforesaid pulse detection means via a blocking circuit element, for example, including a capacitance and/or a transformer, through which there is only appreciable current flow when an electricl pulse is applied thereto.

A latching device is preferably connected to the detection means to produce a sustained output when actuated by the first said output, such latchinng device comprising a relay, thyristor or the like. An appropriate switch preferably a security switch such as a key switch may be provided for re-setting such latching device.

If desired, and particularly in the case of a motor vehicle, the system of the invention may have an auxiliary input adapted to be connected to one or more switches associated, for example, with motor vehicle accessories such as a radio, such switches being arranged to be operated on use of and/or tampering with the accessory such as to cause an output to be produced which is indicative of such switch operation.

The invention will now be described further by way of example only and with reference to the accompanying drawings, Figures 1 and 2, which are circuit diagrams of two forms of alarm system according to the present invention.

The detection device of Figure 1 is for use with a motor car and comprises an encased electronic circuit which can be mounted at an appropriately concealed position in the vehicle and has leads or terminals for connecting the circuit to electrical circuits of the motor car in a manner to be described.

The circuit includes a circuit element comprising a capacitor C1 in series with the primary winding of a transformer T1 which circuit element is connected directly across the usual motor car battery (not shown).

The transformer T1 has a secondary winding which is connected to a transistor amplifier circuit 1 powered from the bat terv. The output of this circuit 1 is fed to the control gate of a thyristor THY1 which is connected in series with an audible alarm device 2 such as a siren and with a switch 3 operated by a key which switch 3 is situated at a convenient position on or adjacent the encased circuit or remote thereto.

With the arrangement so far described, with the switch 3 set in a closed position, any sudden fluctuation in battery voltage due for example to operation of the car ignition circuit or opening of a car door fitted with a courtesy light, gives rise to the application of an impulse to the circuit element including the capacitor C1 and the transformer primary winding, irrespective of the direction of said fluctuation. Such impulse produces a current flow in the transformer secondary winding which is amplified by the circuit 1 and acts to fire the thyristor THY 1.

The alarm device 2 is thereby actuated and held actuated even if no further impulse is applied to the said circuit element and until power supply to the thyristor THY1 is cut off by opening the key switch 3.

Transient noise signals and the like tend not to give rise to actuation of the alarm device possibly due to the capacitive effect of the car battery. Further, the screening effect of the car bodv reduced noise pick-up.

With the arrangement so far described it will be appreciated that warning can be given of unauthorised tapering with the motor car. Further, it will be noted that the alarm system is of simple and inexpensive yet reliable construction and the system can be fitted easily and quickly particularly in so far as specal sensor switches are not required. The system is simply connected across the car battery. If desired however, one or more additional switches 4 may be provided associated for example with a particular region of the car and/or with a particular accessory such as a car radio. As shown in the drawings such a switch 4 may be a normally closed switch, for example of the pressure mat kind, which is connected in the base circuit of an amplifier transistor Q2 the emitter/collector circuit of such transistor being connected to the output of the amplifier circuit 1 whereby the thyristor THY1 is fired whenever the switch 4 opens.

The system of Figure 2 is similar to the system of Figure 1 in so far as it has the same input circuit element, comprising capacitor C1 and transformer T1, and the same output circuit comprising the thyristor THY1, the siren 2 and the switch 3, but the secondary of the transformer T1 is connected to the control gate of the thyristor THY1, via a diode D2 only, there being no intermediate amplifying stage. Also, an additional switch 5 is provided, corresponding to the switch 4 of Figure 1. and this is connected to a potential divider R6. R7, R8 connected to the control gate of the thyristor THY1 again without the intermediary of an amplifying stage.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiment which are described by way of example only.

WHAT WE CLAIM IS: 1. An alarm system for use with an electrical circuit, comprising connection means for direct conductive connection to a d.c. power supply for said circuit, detection means connected to said connection means so as to be capable of detecting only the occurrence of a change in an electrical parameter of the supply as applied to said connection means and to produce an output in response thereto irrespective of the direction of such change. and alarm means connected to said detection means so as to be actuated thereby in response to production of said output, the detection means comprising an electronic circuit which is arranged to receive operating power in use from said d.c power supply via said connection means.

2. An alarm system for use with an electrical circuit comprising connection means for direct conductive connection to a d.c. power supply for said circuit, pulse detection means connected to said connection means so as to be capable of detecting only the occurrence of a sudden change in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. such latching device. If desired, and particularly in the case of a motor vehicle, the system of the invention may have an auxiliary input adapted to be connected to one or more switches associated, for example, with motor vehicle accessories such as a radio, such switches being arranged to be operated on use of and/or tampering with the accessory such as to cause an output to be produced which is indicative of such switch operation. The invention will now be described further by way of example only and with reference to the accompanying drawings, Figures 1 and 2, which are circuit diagrams of two forms of alarm system according to the present invention. The detection device of Figure 1 is for use with a motor car and comprises an encased electronic circuit which can be mounted at an appropriately concealed position in the vehicle and has leads or terminals for connecting the circuit to electrical circuits of the motor car in a manner to be described. The circuit includes a circuit element comprising a capacitor C1 in series with the primary winding of a transformer T1 which circuit element is connected directly across the usual motor car battery (not shown). The transformer T1 has a secondary winding which is connected to a transistor amplifier circuit 1 powered from the bat terv. The output of this circuit 1 is fed to the control gate of a thyristor THY1 which is connected in series with an audible alarm device 2 such as a siren and with a switch 3 operated by a key which switch 3 is situated at a convenient position on or adjacent the encased circuit or remote thereto. With the arrangement so far described, with the switch 3 set in a closed position, any sudden fluctuation in battery voltage due for example to operation of the car ignition circuit or opening of a car door fitted with a courtesy light, gives rise to the application of an impulse to the circuit element including the capacitor C1 and the transformer primary winding, irrespective of the direction of said fluctuation. Such impulse produces a current flow in the transformer secondary winding which is amplified by the circuit 1 and acts to fire the thyristor THY 1. The alarm device 2 is thereby actuated and held actuated even if no further impulse is applied to the said circuit element and until power supply to the thyristor THY1 is cut off by opening the key switch 3. Transient noise signals and the like tend not to give rise to actuation of the alarm device possibly due to the capacitive effect of the car battery. Further, the screening effect of the car bodv reduced noise pick-up. With the arrangement so far described it will be appreciated that warning can be given of unauthorised tapering with the motor car. Further, it will be noted that the alarm system is of simple and inexpensive yet reliable construction and the system can be fitted easily and quickly particularly in so far as specal sensor switches are not required. The system is simply connected across the car battery. If desired however, one or more additional switches 4 may be provided associated for example with a particular region of the car and/or with a particular accessory such as a car radio. As shown in the drawings such a switch 4 may be a normally closed switch, for example of the pressure mat kind, which is connected in the base circuit of an amplifier transistor Q2 the emitter/collector circuit of such transistor being connected to the output of the amplifier circuit 1 whereby the thyristor THY1 is fired whenever the switch 4 opens. The system of Figure 2 is similar to the system of Figure 1 in so far as it has the same input circuit element, comprising capacitor C1 and transformer T1, and the same output circuit comprising the thyristor THY1, the siren 2 and the switch 3, but the secondary of the transformer T1 is connected to the control gate of the thyristor THY1, via a diode D2 only, there being no intermediate amplifying stage. Also, an additional switch 5 is provided, corresponding to the switch 4 of Figure 1. and this is connected to a potential divider R6. R7, R8 connected to the control gate of the thyristor THY1 again without the intermediary of an amplifying stage. It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiment which are described by way of example only. WHAT WE CLAIM IS:

1. An alarm system for use with an electrical circuit, comprising connection means for direct conductive connection to a d.c. power supply for said circuit, detection means connected to said connection means so as to be capable of detecting only the occurrence of a change in an electrical parameter of the supply as applied to said connection means and to produce an output in response thereto irrespective of the direction of such change. and alarm means connected to said detection means so as to be actuated thereby in response to production of said output, the detection means comprising an electronic circuit which is arranged to receive operating power in use from said d.c power supply via said connection means.

an electrical parameter of the supply as applied to said connection means and to produce an output in response thereto irrespective of the direction of such change, and alarm means connected to said detection means so as to be actuated thereby in response to production of said output, the detection means comprising an electronic circuit which is arranged to receive operating power in use from said d.c. power supply via said connection means.

3. An alarm system according to claim 2, wherein said electrical parameter constitutes the voltage of said power supply.

4. An alarm system according to claim 2 or 3, wherein said electrical parameter constitutes a current flow from said power supply.

5. An alarm system according to any one of claims 2 to 4, wherein said connection means is connected to said pulse detection means via a blocking circuit element arranged to permit passage only of electrical pulses.

6. An alarm system according to claim 5, wherein said circuit element comprises a capacitor and/or a transformer.

7. An alarm system according to any one of claims 2 to 6, wherein said detection means comprises a switch device.

8. An alarm system according to claim 7, wherein said detection means further includes an amplifying stage interposed between said connection means and said switch device.

9. An alarm system according to claim 7 or claim 8, wherein said switch device is a latching switch device.

10. An alarm system according to claim 9, wherein latching switch device comprises a thyristor.

11. An alarm system according to claim 9 or 10. wherein said latching switch device has a release switch connected thereto for releasing latching of the switch device.

12. An alarm system according to claim 11, wherein said release switch is a kevoperated switch.

13. An alarm system according to anv one of claims 2 to 12, wherein said detection means has an auxiliary input and said detection means is arranged to produce said output when an auxiliary alarm signal is applied to said auxiliary input.

14. An alarm system according to claim 13. wherein said auxiliary input is connected to a normally closed switch whereby said auxiliary alarm signal is produced when said normally closed switch is opened.

15. An alarm system according to claim 13 or 14, wherein said auxiliary input is connected to said detection means via an amplification stage.

16. An alarm system according to any one of claim 2 to 15. wherein said alarm means comprises a visual and/or audible warning device.

17. An alarm system according to any one of claims 2 to 16, further including a switch means arranged to disconnect the power supply from the electrical circuit on actuation of said alarm means.

18. An alarm system substantially as hereinbefore described with reference to and as illustrated in Figure 1 or Figue 2 of the accompanying drawings.

19. An alarm system according to any one of claims 2 to 18, with said connection means connected across said d.c. power supply which power supply constitutes a battery.

20. An alarm system according to any one of claims 2 to 18, with said connection means connected across said d.c. power supply which power supply constitutes a battery in a motor vehicle.