GB2225470A - Alarm tamper detection device - Google Patents

Abstract

The device detects intrusion by a foreign body into the interior of an alarm housing and comprises a sensing member 12 which in use of the device is positioned for example adjacent apertures in the housing so as to be movable in response to entry into the housing of a foreign body. Movement of the sensing member is detected by a magnetic reed switch 36 connected to activate the alarm. The sensing member is preferably held in its rest position by a magnet 28 attached to the sensing member and a fixed metallic block 30. The magnet also acts to operate on the reed switch. <IMAGE>

Description

Protection Device This invention relates to a protection device, more particularly a protection device for alerting against attempts to tamper With an alarm system.

Many alarm systems employ an audible alarm signal to indicate an alarm situation such as a burglar@ or a fire, The alarm signal is produced by a bell. siren or other sound generator.

which is usually located in the vicinity of the area protected b the alarm system.

An alarm sound generator is commonly placed in an open.

unconcealed position so as to be audible to all nearby persons. However, this positioning may render the sound generator liable to tampering by burglars or other person@ intent on silencing the alarm to carry out unauthorised acts.

In view of the risk of tampering, attempts are usually made to prevent unauthorised access to the sound generator, As a first step. the sound generator can be covered by a protective alarm housing. This maw te sufficient to deter casual villains but, to deal with more determined attacks, it has been found necessary to provide means which actively warn of tampering, For instances electrical switches are commonl employed to activate an alarm signal in response to removal of the housing cover.

Although alarm housings are designed to enclose the ond generator within as fully as possible, it may be necessary to provide apertures in the housing 50 as to al low a satisfactory sound output. Unfortunately, these apertures may also allow a vil!sin to insert a foreign bod" such as a crowbar into the housing in an attempt to disable the sound generator.Of course, even if no suitable aperture exists, a determined villain could penetrate the housing wlth a dill or the like so as to gain access to the sound generator The v@nerability of alarm systems to attack via apertures in the alarm housi ng has been heightened In recent @ears, because villains have taken to disabling the sound generator by in@ecting foaming agents through the apertures. Foaming agents are widely available for the legitimate purpose of filling cavities or gaps in structures, and are commonly sold in compact, aer@sol-type cartridges.The agent is released from the cartridge as a flow of semi-liquid. expanding foam which is directed through a nozzle into the housing, where it quickly solidifies into a substantially rigid. cellular foam mass. Once solidified5 the foam muffles the sound generator or simply prevents it from operating, and thus provides h simple, quick and effective means for disabling the alarm.

The alarm industr@ has responded to this threat by developing new housings which incorporate means for detecting and alerting against the presence of foam or other foreign bodes. Such means ma@ include optical sensor devices which detect the presence of foreign bodies by interruption of a lit beam. However, these devices tend to be expensive. and n any event they do little to protect the numerous existing (and future) al arm installations which do not have means +or detecting the pre=-ence of foreign bodies.

An obiect of this invention is to provide an improved protection device which overcomes or mitigates the disadvantages of existing devices.

In a broad sense, this invention provides a protection device for protecting the interior of an alarm housing against intrusion by a foreign body such as a mass of foamy the device including a sensing member for location within the alarm housing which is movable upon contact with the foreign bod, and detecting means for detecting movement of the member whereby presence of the foreign body within the alarm housing can be detected.

In preferred embodiments of tis invent ion, the protection device includes a support member for f i xed positioni rig within the alarm housing5 to which the sensing member is movabl attached. The sensing member may be attached to the support member by hinge means or may be attached by other means. such as sprigs, which afford substantial freedom of movement.

The sensing member is preferably a plate, which may be perforated The detecting means ma include a switch which is responsive to movement of the sensing member; advantageousl@.

the switch is a f ixed magnetic reed switch which is responsive to movement of a magnet mounted upon the sensing member Retaining means may be provided to weak 1 retain the sensing member in a fixed position until disturbed by contact with a foreign bode. onvenientl' > 14 the retaining means includes the aforementioned magnet. which retains the sensing member t magnetic attraction with a fixed metallic block.

A specific aspect of this invention provides an alarm system having an alarm housing sbrouding an alarm component such as a sound generator, wherein the alarm housing contains a sensing member which is movable upon contact with a foreign body such as a mass of foam, and also contains detecting means for detecting movement of the member whereby presence of the foreign body within the alarm housing can be detected.

This invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a perspective view of a preferred embodiment of the invention; Figure 2 is a sectional view taken along line II-II in Figure 1.

Referring to the drawings, in a preferred embodiment of this invention a protection device 10 comprises a sensing member in the form of a @ectangular plate 12, and a support member 14. Plate 12 has to protruding hinge pins lp situated adjacent, and extending parallel to, an edge 18 thereof. The support member 14 is substantially flat but has two upstanding, parallel flaps 20. Each flap 20 has a circular hole within which a hinge pin 16 is received! whereby the plate 12 is hingably attached to the support member. 14.

In use, the protection device 10 is placed within an alarm housing and may be fixed in place by any suitable means. In the embodiment illustrated, the protection device 10 is attached to an appropriate surface 22 by means of an adhesive pad 24 1 tufted on the underside of the support member 14.

The adhesive pad 24 is suitably a strip of double-sided adhesive material In other embodiments, attachment means such as screws, rivets or clips could be employed, or the support member 14 could be an integral part of the alarm hous i rig.

The protection device 10 senses the presence of a foreign body, such as a stick or a mass of foam, b@ detecting movement of the plate 12 as the foreign body comes into contact with the plate. To this end, it is preferred that the protection device 10 is situated directly behind any aperture through which a foreign bod may be inserted, so that an alarm may be sounded as soon as tampering begins.The plate 12 ma be provided with perforations 26 as shown so as to minimise attenuation of the alarm signal; the size. shape and number of the perforations may te selected to ensure that +oam cannot pass through the plate without causing movement thereof.

The plate 12 carries a permanent magnet 28 at one edge 30 which magnetically attracts a block 32 of @ron. steel or the like attached to the support member 14, Plate 12 is normally retained in the rest position shown in solid 1 ines in Figure 1 @.e. substantially parallel to support member 14, by means of the magnetic attraction between the magnet 28 and the block 32. This attraction is strong enough to prevent the plate 12 +rom causing a false alarm b; moving in response to wind pressure. insects. gravity, or normal external vibration, but is weak enough to allow the plate to move if lampering takes place.

Movement o@ plate 12 can be detected b." any suitable means such as a microswitch or other switch arranged to make or break a circuit as the plate moves. In the preferred embodiment a magnetic reed switch 34 is employed, The reed switch 54 is shrouded by a cylindrical cover 30 attached to the support member 14 and is situated adjacent the magnet 20 on plate 12, Conveniently, in this embodiment the magnet 20 performs a double function because, in addition to retaining the plate 12. the magnet 26 serves to activate the reed switch 34 as the plate 12 moves.

Plate 12 and support member 14 may be made of any suitable metal or plastic material and may be conveniently made by die @asting, injection moulding. or fabrication.

As will be clear to those skilled in the art. man@ variations re e possible without departing from the scope of the present invention. For instance, means other than a magnet could be used to retain the plate 1:; it is envisaged that the plate could be retained by engagement of interlocking means arranged to disengage under a given pressure. Other embodiments could employ spring means to retain the plate the spring means advantageously having an 'over-centre' action.

In further embodiments ot this invention. the hinge means attaching plate 12 to the support member 14 may be replaced by springs, ball-and-socket joints or other means which afford the plate greater freedom of movement. It is envisaged that embodiments having this provision would be capable 3f detecting attack +rom any of several directions, as may be the case where a villain eschews use of any standard apertures but instead makes an access opening in another part of the alarm housing. These embodiments are therefore particularly suited to the many appi ications where the direction of attack is unpredictable.

This invention, and particularly the preferred embodiment described above, provides a protection device which should be very cheap to make and install. It is envisaged that the protection device could be witted to a standard alarm housing to provide a protected alarm installation which is cheaper than the protected devices currently in use. moreover existing unprotected alarm installations could be upgraded by fitting the protection device of this invention.

Claims (6)

1. A device for protecting the interior of an alarm housing against intrusion by a foreign body, the device including a sensing member for location within the alarm housing which is movable upon contact with the foreign body and detecting means for detecting movement of the member

2. A protection device according to claim 1 comprising a support member adapted for fixed positioning within the alarm housing and to which the sensing member is movably attached

3. A protection device according to claim 1 and claim 2 wherein the detecting means comprises a switch which is responsive to movement of the sensing member

4. A protection device according to any one of claims 1 to 3 wherein the detecting means comprises a magnetic reed switch responsive to movement of a magnet mounted on the sensing member.

5. A device for protecting the interior of an alarm housing against intrusion, the device being constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

6. An alarm system having a housing shrouding on alarm component wherein the alarm housing contains a protection device according to any one of the preceding claims, the detecting means being connected to activate the alarm when movement of the sensing member is detected