Wireless detector for an alarm system

Abstract

A wireless movement detector 10 is provided for use with a building intruder alarm system. The detector 10 may be used to protect for example a vehicle or an item of farm machinery by attachment there

Description

The present invention relates to a wireless detector for an alarm system, in particular a movement detector which can be mounted to a range of objects, including vehicles and machinery. BACKGROUND TO THE INVENTION Intruder alarm systems are well-known for protecting property. A wide range of different types of detector are available, and an effective intruder alarm system will typically utilise a number of them. For example, reed switches can be used to detect when doors are opened, passive infra-red (PIR) detectors can be used to detect movement inside a protected premises, and so on. The object is generally to detect actions which might indicate that an intrusion is happening (e.g. a door being opened or a window being broken) and to detect the presence of intruders in protected places (i.e. detecting a person inside a room which is supposed to be locked and alarmed). It is known to place detectors on particularly high-value and / or vulnerable items which need to be specifically protected from theft. Typically, a movement detector (which may contain for example an accelerometer and / or a tilt switch) can be attached in some way to the item being protected. The item may be for example an expensive painting or antique which is kept indoors, but it is also useful to protect items which might be kept outdoors, and / or outside of the alarmed premises. For example, farm machinery is often a target for thieves. Cars can also be protected in this way, and although all modern cars have their own alarms, a detector linked to an alarm in a building is particularly advantageous since the owner of the car may well be at home when an intruder is detected. The earliest possible alert to an intrusion provides the best opportunity to prevent theft or damage, and if not then at least an increased chance of tracing and apprehending the criminal. However, intruder alarm systems for buildings are generally designed on the premise that all detectors which are part of the system will be present at all times (or if not, then there is a fault). This is all very well for example to protect a painting or antique which will usually stay in a building, but items such as farm machinery for example may well be taken out of range of the system in normal use. Expensive machinery may be rented or borrowed and detectors may need to be attached on a temporary basis. A car or other vehicle may be replaced every few years and so it is inconvenient to permanently attach a detector to a vehicle so that it will need tools and / or effort to remove and attach to another vehicle. Also, particularly with a vehicle, it may well be desirable to ensure that the detector is mounted in a prominent position, where it will deter thieves. It is known to strap a detector to the steering wheel, where it can easily be seen by a would-be thief. However, it is desirable to be able to remove the detector quickly and easily so that the vehicle can be driven without anything attached to the steering wheel. It is also desirable to ensure that movement, including small vibrations, is detectable by the sensor in the detector even when temporarily attached to (for example) a steering wheel of a vehicle. Vibrations need to be transmitted from the vehicle (or other article) to the sensor in a predictable way. If the detector is not held tightly against the article (for example, if it is just left on a car seat) then vibrations of the vehicle may be damped by the time they reach the sensor. Likewise, if a detector is “dangling” from the steering wheel on a strap, that may serve to attenuate vibrations. The response to an impact can also be affected in other ways by such an attachment. For example if an impact causes a “dangling” detector to start swinging on its strap, movement of the detector may continue for some time after movement of the actual vehicle has stopped. This can affect the reliability of detection, causing false positive detections (false alarms) or false negatives (i.e. failing to detect an intruder). It is an object of the present invention to provide a detector for an alarm system which can be mounted in a wide variety of ways, temporarily or permanently, to protect different articles. STATEMENT OF INVENTION According to the present invention, there is provided a wireless detector for use with an intruder alarm system in a building, the detector comprising: a housing, the housing including a contact surface for placing against an object to be protected in use; a strap extending from a first position on the housing; a fastening at a second position on the housing for releasably holding the strap, the second position being spaced from the first position and the contact surface being disposed to extend between the first position and the second position; and a movement sensor in the housing, the movement sensor being positioned and adapted to detect movement of the detector and transmit an alarm condition to the intruder alarm system when movement indicative of an intrusion is detected. Advantageously, the strap can be used to releasably hold the detector in place on an object. For example, the housing can be attached to the steering wheel of a car. The strap holds the contact surface of the housing against the object being protected, and that ensures that vibrations are transmitted to the movement sensor so that they can be detected. Preferably, the strap is elastic and so can force the contact surface of the housing against the object, for a wide range of sizes of object. Preferably, the fastening can releasably hold the strap in a selected position along the length of the strap so that the detector can be fastened by the strap around a range of different objects. For example, the strap can be used to attach the detector to the steering wheel of a car. This mounts it in a prominent position where it is a deterrent for thieves. It can be used on almost any car, including for example on classic cars which have much thinner steering wheel rims than modern cars. When mounted to the steering wheel, the contact surface of the housing is urged against the wheel, so that any vibrations caused by interference with the car will be detected by the movement sensor and can trigger an alarm. The fastening may be for example a buckle, a hook, or any other fastening which can grip the strap, preferably in any position, or at least in a selected one of several alternative positions. The first position and the second position may be at either side of the housing, with the contact surface being for example a back surface of the housing extending between the two sides. However, the strap may extend from and be attachable to any two positions which are spaced from each other, so that a contact surface can be urged against the object to be protected, in between the first and second positions. In one embodiment, the first position, from where the strap extends, and the second position, where the fastening is provided, are movable. In particular the contact surface, and the strap attachment points on either side of the contact surface, may be rotatable with respect to the rest of the housing. The housing is preferably rectangular and has a length longer than its width. Accordingly the contact surface and first and second positions may be rotatable so that a line between the first and second positions is either across the width of the housing, or along its length. In one embodiment, the strap, fastening, and contact surface are provided on a part which is detachable from, and when attached is rotatable with respect to the housing. Alternatively the part may be detachable from the housing and re-attachable in a different (rotated) configuration. Having a housing which is rotatable relative to the contact surface and strap assists in allowing the device to be used to protect a variety of different articles. The strap can be positioned relative to the elongate extent of the detector in order to accommodate the detector, for example on a car steering wheel where it is prominently visible but will be removed during use of the car, or on a bicycle on the post underneath the seat (where the strap I housing might be rotated so that the length of the detector is substantially vertical, in line with the seat post). On equipment such as quad bikes, trailers, and farm machinery there will typically be parts to which the strap can be attached, and having the strap rotatable with respect to the housing increases the flexibility by allowing the detector to be accommodated in more places. The movement sensor preferably includes an accelerometer but alternatively embodiments could use for example a piezo vibration sensor, a tilt switch, or a combination of sensors. Output from the sensor is preferably processed, for example in a microprocessor, and an alarm condition is detected and transmitted based on criteria which is found to be indicative of an intrusion. Some techniques for processing accelerometer data and triggering an alarm condition are disclosed in GB2615892. The housing may be provided in two (or more) parts, which are releasable from each other. For example, in one embodiment, the strap is attached to an outer housing, and the movement sensor is provided in an inner housing. The inner housing fits into the outer housing and the inner housing is releasably held in the outer housing. For example, the inner housing may be held in the outer housing by clips. In one embodiment, the outer housing is in the form of a resiliently deformable outer, made for example from a rubber-like material such as polyurethane or silicone. The outer housing may substantially surround the inner housing by covering the back, sides, and ends of the inner housing. The outer housing may be open at the front for insertion and removal of the inner housing. The outer housing may have a flange or rim extending inwardly to cover an edge of the front surface of the inner housing and hold the inner housing in place. The outer housing may be deformable to move the flange I rim out of the way to allow removal and insertion of the inner housing, but may naturally spring back into a position which grips the inner housing and retains it within the outer housing. Preferably, a deformable material may be provided on the contact surface. When the detector is held tightly against an article to be protected, the deformable material may conform to the shape of the article against which the contact surface is held. Because the detector is mounted to the article by a strap, in many cases the surface of the article against which the contact surface of the detector is held will not be flat. By providing a deformable material on the contact surface, the surface area over which the detector is in contact with the article to be protected is increased. This ensures that vibrations from the article are transmitted through the housing to the movement sensor in a predictable way. The detector of the invention may be used with an intruder alarm system associated with a building. In other words, it may form part of a wireless intruder alarm system. The intruder alarm system will typically include a range of detectors, for example door sensors and PIR detectors, which are designed to detect intrusion into the building and trigger an alarm condition accordingly. However, the detector of the invention may be used to protect an object (e.g. a vehicle) which is outside of the protected premises. In other words, walking up to the vehicle may not be something which would (or should) cause the intruder alarm to be triggered. A vehicle can be protected when it is, for example, parked outside a house on a public road. The detector of the invention communicates wirelessly with the intruder alarm system in the house. When the movement sensor detects movement indicative of an intrusion (according to predetermined processing criteria) an alarm condition is transmitted to the intruder alarm system. The intruder alarm system will typically be configured so that the detector of the invention which protects, e.g. a vehicle, is “armed” separately from the zones protecting the building from intrusion. Therefore, the alarm may sound if an intruder attempts to interfere with a vehicle parked outside the protected building on a public road. This may ensure a fast response from the householder and owner of the vehicle, which hopefully would deter the intruder before any damage was done. According to a second aspect of the invention, there is provided a wireless detector for use with an intruder alarm system in a building, the detector comprising: an inner housing and an outer housing, each of the inner housing and the outer housing including a base for placing against and fixing to an object to be protected in use, and the inner housing being removably insertable within the outer housing, an outer surface of the base of the inner housing facing or contacting an inner surface of the base of the outer housing when the inner housing is inserted within the outer housing, the inner housing further comprising a rear section which includes the base, and a front cover section, the front cover section being detachable from the rear section for allowing access for screws to be driven through the base to fix the rear section to an object to be protected, and the front cover section being replaceable on the rear section for covering the screws; and a movement sensor in the inner housing, the movement sensor being positioned and adapted to detect movement of the detector and transmit an alarm condition to the intruder alarm system when movement indicative of an intrusion is detected. In use, the front cover section can be detached from the rear section, the rear section screwed onto an object to be protected (for example, a trailer or a piece of farm machinery), and then the front cover section replaced over the rear section. Preferably, the front cover section is completely detachable from the rear section, but in some embodiments it is only partially detachable, and may remain partially attached, for example on a hinge. The purpose of detaching the front section from the rear section is to allow access for screws to be driven through the base to fix the rear section to an object to be protected, and then the front section covers up the screws once driven in. Preferably, the front cover section is attached to the rear section by, for example, resilient plastic clips. Advantageously, the inner housing may be inserted into the outer housing, and then screws driven through both the base of the inner housing and the base of the outer housing. However, as an alternative, screws may be driven through only the base of the outer housing, and then the inner housing may be inserted into the outer housing. Preferably, a space is provided for receiving a screw head between the inner surface of the base of the outer housing and the outer surface of the base of the inner housing. The space may be for example a countersink surrounding an aperture in the base of the outer housing. The outer housing may be screwed to an object to be protected, and the inner housing will still be able to be inserted within the outer housing. Preferably, the outer housing releasably grips the inner housing. For example, the outer housing may be made from a resiliently deformable material, for example a rubber-like material such as polyurethane or silicone. The base of the outer housing may be rigid, but resiliently deformable side wall(s) may extend substantially perpendicularly from the base of the outer housing for surrounding the inner housing in use. A rim or flange may extend inwardly from the top of the side wall(s) for gripping over the front cover of the inner housing when the inner housing is inserted into the outer housing. The outer housing may be deformed for inserting or removing the inner housing, but springs back into its natural state in which it will grip and retain the inner housing. Preferably, a tamper switch is provided, and the detector is configured to transmit a tamper condition whenever the front cover section is detached from the rear section of the inner housing. Preferably, the tamper switch (or another tamper switch) may be configured to detect when part of the base of the inner housing is broken. A weakened (thinned) region may be provided around a screw which fixes the base of the inner section to the protected object. If the detector is forced off the object, then the base section will break at the weakened region leaving a small part of the base section attached to the object by the screw. This part of the base section breaking away will activate the tamper switch, and cause the detector to transmit a tamper condition to the alarm system. In a preferred embodiment, a weakened region of the base of the inner housing is provided in a position corresponding to an aperture in the base of the outer housing. The aperture may be a countersunk aperture providing a space for receiving a screw head. In this way, the outer housing may be screwed to the protected object, and the inner housing may then be removably insertable into the outer housing. The screw head sits in the space (countersink) and allows the inner housing to be inserted into the outer housing with the screw in place. Advantageously, in this mode of use, the outer housing may be left attached to an object (which might be for example a rented or borrowed piece of machinery) and the inner housing may be easily used to protect the object when it is in proximity to the building associated with the alarm. In this mode of use, removing the inner housing from the outer housing will not activate the tamper. However, such motion should cause an alarm condition as long as the system is “armed” at the time. However, in another mode of use it is also possible to remove the front cover from the inner housing, and drive a screw through both the base of the inner housing and the base of the outer housing. As described above, the screw can be driven through an area of the base of the inner housing which is surrounding by a weakened region. The area inside the weakened region corresponds with the position of a pre-drilled countersunk aperture in the base of the outer housing. Hence all parts of the device can be securely attached to the object to be protected. This may provide for a more permanent and secure fixing for equipment on which the detector is supposed to be permanently installed, and has the additional advantage that the tamper switch will activate if an attempt is made to remove the detector, even when the alarm system is not “armed”, i.e. when an alarm condition signalled by the detector will not cause an alarm to be sounded by the alarm system. The detector of the invention therefore provides for a flexible solution which can be conveniently used, for example, to protect both owned (permanent) and rented (temporary) machinery. Preferably, the outer housing provides a watertight seal which prevents ingress of water into the join between the front cover section and the rear section of the inner housing. The watertight seal may be provided in the form of a sealing bead which extends inwardly from the sidewalls of the outer housing, and seals against an outer surface of sidewalls of the inner housing. Preferably, two sealing beads are provided, one sealing bead being provided in a position between the front of the inner housing and the join between the front cover section and the rear section of the inner housing, and the other sealing bead being provided in a position between the base of the inner housing and the join between the front cover section and the rear section of the inner housing. In other words, the sealing beads protect the join line from water ingress both from the front (from the open front of the outer housing) and from the back (bearing in mind that screws may have been driven through the base of the outer housing, compromising watertight integrity from the back side). BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which: Figure 1 is a perspective view from in front of a wireless detector according to the invention; Figure 2 is perspective view from behind of the wireless detector of Figure 1, with the strap and contact surface having been rotated by 90 degrees compared with Figure 1; Figure 3 is a perspective view from behind of an outer housing, forming part of the wireless detector of Figure 1, and a strap and strap attachment for use by attaching to the outer housing; Figure 4 is a perspective view of the inner housing and outer housing, shown separated from each other; Figure 5 is a perspective view from in front of a rear section of the inner housing, inserted into the outer housing; and Figure 6 is a cross section through the detector. DESCRIPTION OF PREFERRED EMBODIMENTS Referring firstly to Figure 1, a wireless movement detector for use with an intruder alarm system is indicated generally at 10. The wireless movement detector 10 includes an inner housing 12, which contains a movement sensor (in this embodiment an accelerometer), and electronics which cause an alarm signal to be transmitted to the associated intruder alarm system when movement is detected. The inner housing 12 is substantially surrounded at the back, sides, and ends by an outer housing 14. The sidewalls of the outer housing 14 are made from a flexible and waterproof silicone, and a rim 16 extends inwardly from the front of the sidewalls, to overlie a periphery of a front surface of the inner housing 12. In this way, the inner housing 12 is held within the outer housing 14, but can be removed or reinserted by deforming the outer housing 14. A strap 18, which in this embodiment is also made from a flexible and elastic silicone, extends from the back of the outer housing 14. In the configuration shown in Figure 1, the strap extends from one side of the back of the outer housing, and can be attached on to a fastening 20 on the other side of the back of the outer housing. The fastening 20 is in the form of a hook, and any selected one of three apertures 22 can be fastened around the hook 20 in order to secure the strap around items of different thicknesses. The strap is ideal for securing a detector to a car steering wheel, including on both modern and classic cars which have very different widths of steering wheel rim. Referring now to Figure 2, the arrangement of the strap 18 and fastening 20 can be seen more clearly. In this configuration, the position where the strap 18 has been mounted to the outer housing 14, and the position of the fastening 20, have been rotated through 90 degrees so that the strap runs in line with the length of the outer housing, rather than in line with the width as in Figure 1. It can be seen that the strap extends from a first position 24, spaced from a second position in which the fastening 20 is disposed. In between the first position 24 and the fastening 20, a contact surface 26 is provided. A deformable material (which in this embodiment is the same silicone material used for the strap) is provided on the contact surface. The silicone is ridged in this area so that it can deform as it is pressed against a surface. Referring now to Figure 3, the strap 18 is shown detached from the main body of the outer housing 14. The outer housing 14 can be used without the strap, by fixing it on to an object to be protected using screws, and the strap can also be removed and repositioned into either the “across the width” configuration shown in Figure 1, or the “along the length” configuration shown in Figure 2. The strap attachment position 24, as well as the fastening 20 and the contact surface (26, underside in Figure 3) are all provided on a single component, which also includes an attachment formation 28. The attachment formation has four resilient arms which can be inserted into slots 30 and rotated to engage in front of parts of the base of the outer housing. The attachment formation is rotationally symmetrical, as is the arrangement of slots on the base of the outer housing. Figure 4 shows the inner housing 12 separately from the outer housing 14. The inner housing 12 contains the movement sensor and all the electronics, and can be used without the outer housing 14, for example in indoor applications. However, the outer housing provides weatherproofing, and also a flexible means for attaching the detector to an object to be protected. The outer housing can be used with the strap as has already been described. However, it can also be used without the strap, by screwing the outer housing to an object to be protected. Screws can be driven through apertures 32 in the rigid base of the housing. A countersink or counterbore around each aperture 32 allows the head of the screw to sit flush with an inner surface of the base of the outer housing, so that it does not obstruct the insertion of the inner housing. The inner housing is formed of a rear section 34 and a front section 36. The front section 36 can be removed from the rear section 34 for servicing and installation, for example to change the batteries (the batteries should last at least 12 months so this is not a frequent operation) and potentially to configure the unit to pair it with a wireless intruder alarm system and to set options. Figure 5 shows the rear section 34 of the inner housing, inserted within the outer housing 14. Countersinks 38 for receiving screw threads are provided in positions corresponding with the apertures (32, Figure 4) in the outer housing. Note that no apertures are actually provided in the base of the inner housing, since it may be that it will be used without screws (for example, with the strap as described above). However, screws can easily be driven through the base and will form their own apertures. When a screw is driven through the base, silicone sealant for example can be used to ensure watertight integrity is maintained in the base. A weakened path 40 surrounds one of the countersinks 38. The base of the inner section is designed to break around the weakened path 40 (which is just a thinner plastic than the rest of the base) if the inner housing is forced away from the object to which it has been screwed. A tamper switch (not visible in the figures, but attached to a circuit board with the other electronics) will be activated either if the front section 36 is removed from the rear section 34, or if the detector is forced from a surface to which it is screwed, breaking the base around the weakened path 40. Referring to Figure 6, the front section 36 and the rear section 34 of the inner housing 12 are joined together, and the inner housing is inserted into the outer housing 14. In the cross section, the extent of the rigid base 38 can be seen. The rigid base is made from a rigid plastic, and rigid sidewalls 40 which are part of the same component as the base extend upwardly from the base, to about half the height of the outer housing. The rigid sidewalls 40 are overmoulded by a flexible silicone material 42, to form some flexibility in the sidewalls of the outer housing, allowing them to deform to allow insertion and removal of the inner housing. The inwardly extending rim 16 overlies the front of the inner housing. Sealing beads 44 extend inwardly from the inside of the sidewalls of the outer housing. The sealing beads compress against the outer sidewalls of the inner housing, when the inner housing is inserted into the outer housing, to prevent ingress of water. The sealing beads 44 are disposed either side of the join between the front section 36 and the rear section 34 of the inner housing. The detector of the invention can be fitted to a wide variety of objects. It can be strapped on to, for example, a car steering wheel or a bicycle seat tube. The adjustability of the strap by rotating the points where the strap is attached to the housing allows a for wide range of mounting options. The detector can be used without the outer housing, or can be screwed permanently onto an object together with the outer housing (and will transmit a tamper condition if forced off), or the outer housing can be mounted permanently to the object while allowing the inner housing to be easily removed. This may be convenient, for example on rented equipment. The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A wireless detector for use with an intruder alarm system in a building, the detector comprising:a housing, the housing including a contact surface for placing against an object to be protected in use;a strap extending from a first position on the housing;a fastening at a second position on the housing for releasably holding the strap, the second position being spaced from the first position and the contact surface being disposed between the first position and the second position; anda movement sensor in the housing, the movement sensor being positioned and adapted to detect movement of the detector and transmit an alarm condition to the intruder alarm system when movement indicative of an intrusion is detected.

2. A wireless detector as claimed in claim 1, in which the strap is elastic.

3. A wireless detector as claimed in claim 1 or claim 2, in which the fastening releasably holds the strap in a selected position along the length of the strap.

4. A wireless detector as claimed in any of the preceding claims, in which the first position and the second position are movable with respect to the rest of the housing.

5. A wireless detector as claimed in claim 4, in which the first position and the second position are movable by rotation about an axis between the first and second positions.

6. A wireless detector as claimed in claim 4 or claim 5, in which the first position, the second position, and the contact surface are provided as an attachment, the attachment being rotationally movable with respect to the housing.

7. A wireless detector as claimed in any of claims 4 to 6, in which the first and second positions are movable between a first configuration in which a line between the first and second positions is along a length of the housing, and asecond configuration in which a line between the first and second positions is across a width of the housing.

8. A wireless detector as claimed in any of the preceding claims, in which the first position, the second position, and the contact surface are provided on a component which is detachable from the housing.

9. A wireless detector as claimed in any of the preceding claims, in which the movement sensor includes an accelerometer and / or a tilt sensor.

10. A wireless detector as claimed in any of the preceding claims, in which the housing comprises an outer housing, and an inner housing which is insertable within the outer housing, the strap and fastening being provided on the outer housing, and the movement sensor being provided in the inner housing.

11. A wireless detector as claimed in claim 10, in which the outer housing is resiliently deformable for insertion and removal of the inner housing.

12. A wireless detector as claimed in any of the preceding claims, in which a deformable material is provided on the contact surface.

13. A wireless detector for use with an intruder alarm system in a building, the detector comprising:an inner housing and an outer housing, each of the inner housing and the outer housing including a base for placing against and fixing to an object to be protected in use, and the inner housing being removably insertable within the outer housing, an outer surface of the base of the inner housing facing or contacting an inner surface of the base of the outer housing when the inner housing is inserted within the outer housing, the inner housing further comprising a rear section which includes the base, and a front cover section, the front cover section being detachable from the rear section for allowing access for screws to be driven through the base to fix the rear section to an object to be protected, and the front cover section being replaceable on the rear section for covering the screws; anda movement sensor in the inner housing, the movement sensor being positioned and adapted to detect movement of the detector and transmitan alarm condition to the intruder alarm system when movement indicative of an intrusion is detected.

14. A wireless detector as claimed in claim 13, in which the front cover section is attached to the rear section by resilient clips.

15. A wireless detector as claimed in claim 13 or claim 14, in which a space is provided for receiving a screw head between the inner surface of the base of the outer housing and the outer surface of the base of the inner housing.

16. A wireless detector as claimed in claim 15, in which the space is a countersink or counterbore in the base of the inner housing.

17. A wireless detector as claimed in any of claims 13 to 15, in which the outer housing releasably grips the inner housing.

18. A wireless detector as claimed in claim 17, in which the outer housing is made at least in part from a resiliently deformable material.

19. A wireless detector as claimed in claim 18, in which the base of the outer housing is substantially rigid, and in which resiliently deformable sidewalls extend from the base to surround sides of the inner housing.

20. A wireless detector as claimed in claim 18 or claim 19, in which a rim extends inwardly from the top of sidewalls for gripping over the front of the inner housing.

21. A wireless detector as claimed in any of claims 13 to 20, in which a tamper switch is provided, and the detector is configured to transmit a tamper condition whenever the front cover section is detached from the rear section of the inner housing.

22. A wireless detector as claimed in any of claims 13 to 21, in which a tamper switch is provided, and the detector is configured to activate the tamper switch and transmit a tamper condition when part of the base of the inner housing is broken.

23. A wireless detector as claimed in claim 22, in which a weakened region is provided around a fixing point for a screw on the base of the inner housing.

24. A wireless detector as claimed in claim 23, in which the weakened region is provided surrounding a position corresponding to an aperture in the base of the outer housing.

25. A wireless detector as claimed in any of claims 13 to 24, in which a sealing 5 bead is provided, the sealing bead extending inwardly from the sidewalls of theouter housing for sealing against an outer surface of sidewalls of the inner housing.

26. A wireless detector as claimed in claim 25, in which two sealing beads are provided, each sealing bead extending inwardly from the sidewalls of the outer 10 housing for sealing against an outer surface of sidewalls of the inner housing,a first sealing bead being provided in a position for sealing against sidewalls of the rear section of the inner housing and a second sealing bead being provided in a position for sealing against sidewalls of the front cover section of the inner housing.15 27. An intruder alarm system including the detector of any of the preceding claims.17

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