GB2263352A

GB2263352A - Alarm sensor

Info

Publication number: GB2263352A
Application number: GB9200871A
Authority: GB
Inventors: David Scott; Timothy John Martin-Wright; Victor Lindley Buckler; Colin Trafford Marsh
Original assignee: A1 Security & Electrical Ltd
Current assignee: A1 Security & Electrical Ltd
Priority date: 1992-01-16
Filing date: 1992-01-16
Publication date: 1993-07-21
Anticipated expiration: 2012-01-16
Also published as: EP0621972A1; WO1993014479A1; GB2263352B; AU3358993A; JPH07507167A; GB9200871D0

Abstract

The sensor (10) comprises a case housing a smoke detector (16) and an infra-red detector (14). The smoke detector (16) is mounted in a region (36) of the case open to the ambient atmosphere by slots (38, 40) in the wall of the case (12). The infra-red detector (14) is mounted in a substantially sealed chamber (34) partitioned from the open region (36) to prevent false triggering of the infra-red detector (14). A front wall (28) of the chamber (34) comprises a window section (28) on the case which is transparent to infra-red radiation. The detectors (14, 16) are mounted on a common circuit board (20) which also forms a wall of the chamber (34). <IMAGE>

Description

ALARM SENSOR This invention relates to a sensor for use in a burglar alarm and/or a smoke alarm system.

In a first aspect, the invention provides a sensor comprising a case, a smoke detector within the case, the smoke detector communicating, in use, with the ambient atmosphere outside the case to allow smoke to be detected, and an infra-red detector within the case, the infra-red detector, in use, being able to detect infra-red radiation outside the case.

Sensors which include either an infra-red detector, or a smoke detector, are known in the art, but hitherto it has not been known to incorporate both such detectors into a single sensor as in the present invention.

In the past, in order to obtain protection from a passive infra-red burglar detector and a smoke detector, it has been necessary to mount two such respective sensors in each room to be protected. Such an arrangement is relatively expensive because each sensor has to be provided with its own local alarm circuitry and a case. It is also inconvenient and sometimes unsightly to mount two such sensors in each room.

With the present invention the two detectors can be monitored by local alarm circuitry, at least portions of which are common to both of the detectors, thereby reducing the cost and complexity of the alarm system. Furthermore, only a single sensor needs to be mounted in each room to give both smoke alarm and infra-red burglar alarm protection.

One aspect which is believed to have led designs of sensors away from that of the present invention is that the installation requirements for a smoke detector generally conflict with the requirements for an infra-red detector.

For example, a smoke detector is designed to be in direct communication with the ambient atmosphere of the room so that it can easily detect the presence of smoke. It is preferred that the air from the atmosphere around the detector be allowed to flow into or through the case in which the smoke detector is housed. On the other hand, an infra-red detector is designed to be housed in a sealed case, because insects might otherwise be able to walk over the detector which might set the alarm off falsely.

One application in which the above requirements can lead to problems is when the infra-red detector is a passive detector intended to detect the body heat of a burglar or other intruder. If an insect crawls across the surface of the detector, its body heat at such close distance may be sufficient to trigger the alarm falsely.

Preferably, in the present invention, the interior of the case comprises a region open to external ambient atmosphere, the smoke detector being arranged in the open region, and the sensor further comprising means for partitioning infra-red detector from the open region.

Such an arrangement ensures that the smoke detector is in communication with the ambient atmosphere and, at the same time, that the infra-red detector is partitioned away from the open region to protect it from insects which may enter the open region of the case.

Preferably, the means for partitioning the infra-red detector comprises chamber means within the case, at least a portion of a wall of the chamber means being substantially transparent to infra-red radiation to allow infra-red radiation to enter the chamber means, the chamber means enclosing the infra-red detector. The chamber means may be substantially sealed from the open region.

The transparent portion of the chamber wall preferably comprises a portion of the external wall of the case, and may be formed as a window in the wall of the case.

In accordance with conventional infra-red sensor design, the infra-red detector is preferably spaced a distance from the infra-red transparent portion of the wall, so that the alarm will not be triggered falsely by insects crawling over the transparent portion.

The infra-red detector may be mounted on a circuit board, which circuit board forms a-wall of the chamber means. The smoke detector may be mounted on the same circuit board as the infra-red detector.

The case may be formed as front and rear case halves which fit together to form the case. The chamber means may comprise a shroud attached to the front case half.

The case may include openings in its walls to allow the smoke detector to communicate with the external atmosphere. The openings may be formed in one or more of the walls, but preferably the openings allow smoke to flow through the open region of the case.

In a closely related aspect, the invention provides a case for a sensor as defined above, the case comprising a region for containing a smoke detector, the region, in use, being open to the external ambient atmosphere surrounding the case, the case further comprising a chamber for containing an infra-red detector, the chamber, in use, being partitioned from the region for containing the smoke detector, at least a portion of a wall of the chamber being substantially transparent to infra-red radiation, in use, to allow infra-red radiation to enter the chamber.

Such a case enables a particularly preferred form of the sensor to be formed, as explained above.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a sensor in accordance with the invention; and Fig. 2 is a sectional view from the side showing the sensor mounted on a wall.

Referring to the drawings, a sensor 10 comprises a case 12 housing a passive infra-red detector 14 and a smoke detector 16. The case 12 comprises a generally arcuately shaped front case half 12a and a rear case half 12b having end walls 18 which are shaped to complement the arcuate shape of the front case half 12a.

The infra-red detector 14 and the smoke detector 16 are mounted on a circuit board 20 which is secured to the rear case half 12b by retaining clips 22. The circuit board 20 also carries a connector block 24 and other circuit components (not shown) which form local alarm circuits for the detectors, some of which circuits can be common for both the infra-red detector 14 and the smoke detector.

The front case half 12a is formed with a rectangular opening 26 behind which a sheet of infra-red window material is secured to form a window 28 transparent to infra-red radiation. Behind the window 28 a funnel-shaped shroud 30 is secured. The window 28 and the shroud 30 are secured to the front case half 12a by glue so that they form a substantially airtight join with the inner face of the front case half 12a.

As shown in Fig. 2, the arrangement is such that when the case halves 12a and 12b are assembled, the infra-red detector 14 is received in the narrow end of the shroud 30, with the infra-red sensitive face of the detector 14 facing the window 28. The rear edge 32 of the shroud 30 makes a close fit against the surface of the circuit board 20 to form an effective seal. The window 28, the shroud 30 and the portion of the circuit board 20 surrounding the infrared detector 14 thereby together define a chamber 34 which encloses the infra-red detector 14 and partitions it away from the smoke detector 16.

The remainder of the interior of the case forms a region 36 which is open to the external atmosphere by openings 38 in the end walls 18 of the case, and openings 40 in the side walls. (For clarity, only the openings 40 which are near the end walls are shown in the drawings, although it is to be understood that the openings 40 are provided along the whole length of the side walls.) The openings 36 and 40 are each about 2mm in width, to allow gas and smoke from the atmosphere around the sensor 10 to flow through the case 12 and in particular past or through the smoke detector 16. This size of the opening is sufficiently large to allow spiders, insects and other bugs to crawl through the openings and into the.interior of the case.However, the close fitting of the shroud 30 around the infra-red detector 14 ensures that any such insect intruder cannot enter the chamber 34 in which the infradetector 14 is housed, and set off the alarm falsely.

In accordance with conventional infra-red sensor design, the infra-red detector 14 is spaced a distance from the window 28, so that even if an insect crawls across the front face of the window 28, it will not be close enough to the detector 14 to trigger the alarm falsely.

The present embodiment is designed to be installed vertically on a wall of a room, fairly near the ceiling.

This gives the detector a clear infra-red "view" of the room, free of furniture obstructions. The window 28 is adapted for such an installation by being formed as a Fresnel lens, arranged to focus infra-red radiation incident from below the sensor 10 (as depicted by the arrow 42) onto the sensitive face of the infra-red detector. It will be appreciated that in such an elevated position, the case 12 is likely to become home for insects or spiders, and so the partitioning chamber 34 is particularly advantageous in ensuring the sensor 10 continues to operate reliably and effectively, without giving false alarms caused by insects on the infra-red detector.

The openings 38 in the end walls 18 of the case 12 ensure that when the case 12 is mounted vertically, smoke is free to enter the case 12 from below (as depicted by the arrow 44), and to flow through the case 12. However, the openings 40 are provided in the side walls to allow for the case 12 being mounted at an angle to the vertical, or even horizontally, for example, on a ceiling.

A light indicator may be provided on or in the case 12 to indicate that the sensor 10 is active and "armed".

For example, the indicator may be mounted with the infrared detector 14 within the chamber 34 so that the indicator illuminates the window 28. Alternatively, for example, the indicator may be mounted on the circuit board 20 outside the chamber 34 and a sealed light-pipe 46 (shown phantom in Fig.

2) provided as part of the shroud 30 for directing light from the indicator on to the surface of the window 28.

Although, in the present embodiment, the close fitting of the shroud 30 against the circuit board 20 is sufficient to ensure that spiders, insects and bugs cannot enter the chamber 34, in other embodiments, it may be desirable to incorporate a sealing compound or device to ensure that a reliable seal is achieved. For example, a sealing member might be provided to form a positive seal between the shroud 30 and the surface of the circuit board 20, and thereby ensure that chamber 34 is substantially sealed from the open region 36.

It will be appreciated that the present invention, and particularly the preferred embodiment as described above, provide a sensor which can overcome the disadvantages of the prior art described above.

It will of course be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Claims

CLAIMs

1. A sensor comprising a case, a smoke detector within the case, the smoke detector, in use, communicating with the ambient atmosphere outside the case to allow smoke to be detected, and an infra-red detector within the case, the infra-red detector, in use, being able to detect infra-red radiation outside the case.

2. A sensor according to claim 1 wherein the interior of the case comprises a region open to the external ambient atmosphere, the smoke detector being arranged in the open region, the sensor further comprising means for partitioning the infra-red detector from the open region.

3. A sensor according to Claim 2 wherein the means for partitioning the infra-red detector comprises substantially sealed chamber means within the case, at least a portion of a wall of the chamber means being substantially transparent to infra-red radiation to allow infra-red radiation to enter the chamber means, the chamber means enclosing the infrared detector.

4. A sensor according to claim 3, wherein the transparent portion of the chamber wall comprises a portion of an external wall of the case.

5. A sensor according to claim 4 wherein the transparent portion of the chamber wall comprises an infra-red transparent window in an external wall of the case.

6. A sensor according to claim 5, further comprising means for enabling an indicator light within the case to be shone through the transparent window, to indicate that the sensor is armed.

7. A sensor according to claims 3, 4, 5, or 6 wherein the infra-red detector is mounted on a circuit board, the circuit board forming a wall of the chamber means.

8. A sensor accordingly to claim 7 wherein the smoke detector is mounted on the same circuit board as the infrared detector.

9. A sensor accordingly to any of claims 3 to 8 wherein the case comprises front and rear case haves which fit together to form the case, the chamber means comprising a shroud attached to the front case half.

10. A sensor according to any of claims 3 to 9, wherein the chamber is substantially sealed from the open region.

11. A sensor according to any of claims 2 to 10 wherein the open region of the case communicates with the external ambient atmosphere through openings in the walls of the case.

12. A case for a sensor according to any preceding claim, the case comprising a region for containing a smoke detector, the region, in use, being open to the external ambient atmosphere surrounding the case, the case further comprising a chamber for containing an infra-red detector, the chamber, in use, being partitioned from the region for containing the smoke detector, at least a portion of a wall of the chamber being substantially transparent to infra-red radiation, in use, to allow infra-red radiation to enter the chamber.