GB2298727A

GB2298727A - Theft alarm device

Info

Publication number: GB2298727A
Application number: GB9519084A
Authority: GB
Inventors: Halim Alain Ahmad
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-03-08
Filing date: 1995-09-19
Publication date: 1996-09-11
Also published as: GB9504623D0; GB9519084D0

Description

ALARM DEVICE It is desirable to provide valuable electrical apparatus with an alarm device to warn off any tampering when the apparatus is tampered with in certain circumstances. An example of an alarm device of this type which is already known is a car alarm which is activated when the electrical apparatus of the car is de-energised by removal of the ignition key and which is provided with a vibration switch which energises a self-powered alarm circuit when the car is subjected to vibration over a given threshold.

The present invention provides an alarm device of this type, with various improvements, which may be provided independently or in combination.

According to the invention there is provided an alarm circuit as set out in claim 1. Optional features of the invention are set out in the subsidiary claims.

According to another aspect of the invention there is provided an alarm circuit as set out in claim 1 with the exception that the disabling switch is not necessarily mounted in a blanking plate, but the enabling switch comprises an infrared radiation sensitive switch.

When the electrical apparatus is an electrical computer provided with a power supply for receiving mains electricity and producing DC supplies to other parts of the apparatus including the motherboard, the alarm device is preferably provided with means connected directly to the power supply for sensing whether the power supply is energised and for de-energising the alarm circuit when the power supply is energised. This direct connection to the power supply avoids the use of any of the expansion outlets of the computer device which are connected to the motherboard.

An example of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic circuit diagram of an alarm circuit; Figure 2 shows an exploded view of the switch 2 mounted in an expansion port blanking plate, and Figure 3 shows the mounting of the switch 2 in a blanking plate in the computer casing, but with the other components of the device inside the computer casing omitted.

In Figure 1 a self-powered alarm circuit is mounted within the casing of a computer and comprises a battery 10 connected through three switches in series to a loudspeaker 11. The first switch 12 is manually operated and is closed when it is desired to energise the alarm circuit. The second switch 13 is formed by the contacts of a first relay 14 whose coil 15 is directly connected to the power supply 16 of the computer to be protected, the second switch being opened when the power supply is energised and closed when it is not. The third switch 17 is formed by the contacts of a second relay 18 whose coil 19 is connected in parallel with the loudspeaker 11.The third switch is mounted in parallel with further activating switches, a first further switch 21 responsive to changes in orientation of the apparatus, commonly known as a tilt switch, and a second further switch 22 which is operated in response to visible light or infrared radiation falling upon a sensor. These further switches do not necessarily have to be mechanical switches with contacts, but could be solid state switches or other suitable components which will form a conducting path in response to certain stimuli.

When the manually operated switch 12 is closed and the power supply 16 of the computer to be protected is deenergised so that the contacts 13 of the first relay 14 are also closed, then the battery 10 will energise the loudspeaker 11 when any one of the triggering switches 21, 22 closes. It is possible that the triggering switches will only form a conducting path for a short time, and it is for this reason that the second relay 18 is provided so that its coil 19 in parallel with the loudspeaker 11 will be energised by the momentary conduction of one of the trigger switches 21, 22, causing the contacts 17 of the second relay to be closed to form a path in parallel with the triggering switches and to continue the conducting path through the other closed switches 12, 13 between the battery and the loudspeaker, causing a continuous signal to be emitted until the manually operated switch 12 is opened.

The casing 31 of the computer is provided with expansion slots 32, which are apertures closed by blanking plates 33 when not in use for carrying expansion cards. The majority of the alarm device is located within the casing of the computer, so that the direct connection between the power supply 16 and the coil of the first relay 14 can be made within the casing. The loudspeaker 11 could be a sound emitting device provided as part of the computer or it could be a subsidiary loudspeaker contained within the casing of the computer. Access from the exterior of the computer is required to operate the first switch which is mounted in a blanking plate 33 for one of the expansion slots. As will be seen from Figures 2 and 3, the plate 33 is provided with an aperture to receive the switch housing 34, which has a flange 35 which engages the front of the plate.The exterior of the switch housing is threaded and the switch is fixed in position by a nut 36 screwed up the threaded exterior of the housing to engage the back of the plate. When the nut 36 is wider than the expansion port 32, the flange 35 and nut 36 also serve to secure the blanking plate 33 over the port 32. In this embodiment, the switch is rotatable between its operative positions and can be operated by inserting a screwdriver into a slot 36 on the front (and hence accessible from the exterior of the computer casing) and then turning the screwdriver through 90 degrees. All of the alarmed device with the exception of the flange and front face of the switch 12 is contained within the casing 31.

The second switch 13 and the first relay 14 are optional components. If it is desired that the alarm should operate independently of the power supply of the computer, then these components are redundant. The switch 21 is responsive to vibration or other movement of the computer casing. The switch 22, if responsive to visible light, will detect if the computer casing is opened in a light room as might be the case if an authorised person opens a case to remove certain components. If the switch 22 is responsive to infrared radiation, it will close when infrared radiation from an unauthorised person falls upon it if the casing is opened even in darkness. The battery 10 can be conventional or rechargeable, or it may be replaced by a power supply derived from the device being protected.

When the first relay is provided, it is connected to the power supply of the computer which receives a mains input and provides various direct voltages for energising other components of the computer including the motherboard. The alarm circuit does not receive power from the motherboard, but receives it directly from the power supply of the computer. This avoids any disturbance to the operation of the components of the computer being disturbed when the alarm circuit is connected.

Claims

1. An alarmed device to give warning of tampering comprising a casing for the device formed with at least one aperture for receiving additional components and a blanking plate for closing the or each aperture, an alarm generator, a source of electrical power for the generator, an enabling switch for connecting the generator to the source to cause the generator to generate an alarm signal and a disabling switch to disable the generator independently of the state of the enabling switch, the disabling switch being mounted on a said blanking plate and the remainder of the device being located within the casing.

2. A device as claimed in claim 1 wherein the enabling switch comprises an infrared sensitive switch.

3. An alarmed device to give warning of tampering comprising a casing for the device, an alarm generator, a source of electrical power for the generator, an enabling switch for connecting the generator to the source to cause the generator to generate an alarm signal and a disabling switch to disable the generator independently of the state of the enabling switch, the enabling switch being operable in response to incident infrared radiation within the casing.

4. An alarmed device as claimed in any one of claims 1 to 3 wherein the enabling switch comprises a vibration sensitive switch which is responsive to vibrations experienced by the device over a given threshold.

5. A device as claimed in any one of claims 1 to 4 wherein the enabling switch comprises a light sensitive switch.

6. A device as claimed in any one of claims 1 to 5 comprising a latch circuit energised in response to the closing of the enabling switch to maintain the generator energised after closure of the enabling switch, subject to the disabling switch.

7. A device as claimed in any one of claims 1 to 6 wherein the remainder of the device apart from the disabling switch is mounted at locations within the casing other than on the blanking plate.

8. A device as claimed in any one of claims 1 to 7 which comprises a computer for which the or a said aperture comprises an expansion port.

9. A device as claimed in any one of claims 1 to 8 which comprises a computer having a power supply and a further disabling switch which overrides the enabling switch, the further disabling switch being opened to disable the generator when the power supply of the computer is energised.

10. A device as claimed in claim 9 wherein the computer comprises a motherboard connected to said power supply and the connection between the power supply and the further disabling switch is independent of the motherboard.

11. An alarmed device substantially as hereinbefore described with reference to the accompanying drawings.