GB2286667A

GB2286667A - Smoke detector

Info

Publication number: GB2286667A
Application number: GB9402871A
Authority: GB
Inventors: John A Sheehan
Original assignee: TRANSMOULD Ltd
Current assignee: TRANSMOULD Ltd
Priority date: 1994-02-15
Filing date: 1994-02-15
Publication date: 1995-08-23
Anticipated expiration: 2014-02-15
Also published as: GB2286667B; GB9402871D0

Abstract

A smoke detector comprises a light emitter (10) and a light receiver (11). Normally, light from the light emitter (10) is partially absorbed and partially deflected by parallel posts (2, 3) which lie on a straight line (21) on which the light emitter (10) and the light receiver (11) also lie, with the result that no signal is generated. When smoke enters a chamber (6) containing the posts (2, 3) it causes the light to be diffused and sufficient light reaches the light receiver to cause the generation of an electrical signal. The light emitter, posts, and light receiver may be mounted on a plate (1, Figure 2) which has a circular ring of baffle posts (4, Figure 2) which prevent outside light entering the smoke detector. <IMAGE>

Description

SMOKE DETECTOR This invention relates to a smoke detector.

It is known to make a smoke detector comprising a light emitter1 a light receiver, which creates an electrical signal when sufficient light falls upon it, a chamber and barrier means, between the light emitter and the light receiver, which ensure that in the absence of significant airborne particles in the chamber light emitted by the light emitter does not result in said signal being created, whereas a sufficient quantity of significant airborne particles, such as smoke, in the chamber can cause said signal to be created.

In the known device, the barrier means is a wall which is vertical when the device is mounted on a ceiling and there must be a space below that wall, referred to above as a chamber, to accommodate the smoke which diffuses the light so that it passes under the wall to the light receiver. Therefore, the overall height of the device is inconveniently great.

It is an object of the present invention to produce a smoke detector as defined at the beginning but of lesser overall height.

According to one aspect of the invention, a smoke detector as defined at the beginning is characterised in that the barrier means is in the chamber and comprises a first post and a second post substantially parallel to one another and so positioned that the light emitter, the first post, the second post and the light receiver all lie in that sequence on a straight line.

According to a second aspect of the present invention, there is provided a component for use in a smoke detector, characterised in that it comprises a plate from which project first and second posts and a part- ring of baffle posts which partly encircle the first and second posts, the plate also carrying means for locating a light emitter and means for locating a light receiver such that the light emitter, the first post, the second post and the light receiver all lie in that sequence on a straight line.

Preferably, at least one of the first and second posts has a length between 7 and 15 mm and a maximum width less than 5 mm.

An example in accordance with this invention is described below with reference to the accompanying drawings, in which: Figure 1 shows a perspective view of part of a smoke detector; Figures 2 and 3 show plan views of two components of the smoke detector; and Figure 4 shows an enlarged and diagrammatic view of part of what is shown in Figure 2.

The smoke detector shown in the drawings is described below as though it were in the orientation that it has when in use, attached to the ceiling of a room.

The smoke detector includes a moulding of black plastics material which comprises a plate 1 from which project first and second posts 2 and 3, which are substantially parallel to each other and of a length within the range 7 mm to 15 mm, for example, 11 mm.

These posts 2 and 3 are partly encircled by a part-ring of baffle posts 4. All the posts 2 to 4 have the same length and all project downwardly, substantially perpendicularly from the plate 1 and are free at their lower ends, which can be seen in Figure 2. Each baffle post 4 is of uniform hook-shaped cross-section over its entire length. This ensures that light entering from outside of the part-ring is reflected and thus absorbed by two surfaces of either of two adjacent baffle posts 4, thus preventing external light from influencing the smoke detector. The baffle posts 4 are further surrounded by a strip 5 of metal gauze which prevents insects or other large bodies entering a chamber 6 containing the posts 2 and 3.

The plate 1 has four holes 7 through which pass screws 55 which fasten the plate 1 to a ceiling; for example they fix the plate in a recess in the ceiling.

A circular wall 8 surrounds each screw head. There arealso formed on the plate 1 six depending pockets 9 each of which is open at the top and closed at the bottom and some or all of these pockets accommodate a circuit element (for example a capacitor) mounted on and depending from a printed circuit board (not shown) above the plate 1. The printed circuit board is separated from supporting bosses (not shown) for it, on the plate 1, by a gasket (not shown) of electrically insulating material. The printed circuit board also has depending from it three pairs of conductors (only one of which is shown, in Figure 1) leading to a light emitter 10, which emits infra red light, an infra red light receiver 11 and a second light emitter 12, which emits a red light which merely indicates whether the printed circuit board is powered-up, typically from a central indicator circuit.The conductors 14 leading to the light emitter 10 are passed through a hole 13 through the plate 1 and then bent through a right-angle so that when the smoke detector is in use the axis of the light emitter 10 is horizontal and its light is directed towards the light receiver 11. Adjacent the hole 13 there are locating means projecting downwardly from the plate 1 for locating the light emitter 10, this locating means providing a trough-shaped portion 15 and a narrower trough-shaped portion 16, each of which is open at the bottom and has a part-cylindrical upper surface. The conductors leading to the light receiver 11 extend through an entrance at the top of a housing 17 depending from the plate 1, the housing being closed at the bottom but formed with a window between the light receiver 11 and the post 3.The conductors leading from the printed circuit board to the light emitter 12 pass through a hole 18 in the plate 1, beside which is a crescent-shaped wall 19. The lower surface of the plate 1 within the part-ring of baffle posts 4, i.e. the upper side of the chamber 6, is serrated.

As shown in Figure 4, the light emitter 10, the post 2, the post 3 and the light receiver 11 all lie, in that sequence, on a straight line 21.

The post 2 is of substantially pentagonal cross-section being symmetrical about the line 21. It has two substantially planar faces 22 inclined to one another by about 900 and equally inclined to the line 21. The angle of inclination of the faces 22 to the line 21 causes light from the light emitter 10 to be deflected forward towards both sides of the light receiver 11. Thus smoke entering the chamber 6 will be most likely to diffuse light reflected from the faces 22 into the path of the light receiver 11. The faces 22 meet at a slightly rounded apex 23, formed by virtue of the injection moulding fabrication process, which is the part of the post 2 which is nearest the light emitter 10.Remote from the light emitter 10, the post 2 has a substantially planar face 24, to which the line 21 is perpendicular, and this is inclined by an obtuse angle to substantially planar faces 25, there being slight rounding where the faces 25 meet the faces 22 and 24.

The post 3 has a cross-section in the form of a regular trapezium. It has two substantially planar faces 28 and 29 which are both substantially perpendicular to the line 21, one of these faces, 28, being wider than the other and being the part of the post 3 which is nearest the light receiver 11. The post 3 has side faces 30 and rounding where they meet the faces 28 and 29. Each of the posts 2 and 3 has a protrusion 31 at its tip.

The thickness, measured vertically in Figure 4, of the post 2 is greater than that of the post 3 and so is the width, measured horizontally. The maximum width, measured horizontally, of each of the posts is less than 5 mm. Widths of about 4 mm and 3 mm for the posts 2 and 3 are believed to be satisfactory.

The reason for having a second post 3 is to reflect light which diffracts around the corners of respective faces 22 and 25 of the post 2 away from the light receiver 11.

The smoke detector also has a cover 35 which closes the chamber 6 at the bottom and on one face has a recess 36. The recess 36 ensures that a uniform cross-section not prone to shrinkage is formed during the injection moulding process. The cover has a central serrated area 37 on the opposite face within which are non-serrated flat areas 38 and 39, matching in size and shape the free ends of the posts 2 and 3, respectively, and containing recesses 40 which receive the protrusions 31 on the posts 2 and 3 to locate the free ends of these posts. The same result could be obtained by having recesses in the free ends of the posts and protrusions on the cover 35.

The serrated area 37 of the cover 35 and the serrated lower surface of the plate 1 are designed to allow dust to settle on the respective surfaces without causing a substantial rise in background signal level reaching the light receiver 11.

There project upwardly from the upper face of the cover 35 two ribs 41 which lie on opposite sides of the housing 17 and a latch 42 which engages a shoulder on the housing to hold the cover 35 on to the plate 1. Also projecting upwardly from the cover 35 is a structure 45 which includes recesses 46 which receive ribs 47 on the plate 1 and a tubular part 48 within which the light emitter 10 is fitted so that none of it lies in a trough-shaped part 49 of the structure 45, which is similar to the trough-shaped portion 16 shown in Figure 2. The trough-shaped portion 15 shown in Figure 2 receives the tubular part 48. Both the trough-shaped part 49 and the trough-shaped portion 16 have a vertical end wall 50 and these end walls ensure that the beam of light directed at the post 2 is very narrow.

Posts 51 and recesses in the ribs 47 hold the strip 5 of metal gauze.

The chamber 6 is defined by the plate 1, the cover 35 and the part-ring of baffle posts 4. Normally, in the absence of many significant particles in the chamber 6 and when the light emitter 2 emits its infra red light, the light is partially absorbed by the black post 2 and partly deflected by the faces 22 of that post. The deflected light is partly absorbed by the baffle posts 4 and the post 3 and partly deflected again by the post 3.

Either no light at all is received by the light receiver 11 or the latter receives light of such low intensity that no electrical signal is created by it. If smoke enters the chamber 6, through the strip 5 of gauze and between the baffle posts 4, the light is diffused by the particles in the smoke and the light receiver 11 receives sufficient light for it to create an electrical signal which is transmitted to the central indicator circuit which in turn emits an audible signal and/or a visible signal.

The light emitters 10 and 12 may be light-emissive semi-conductor diodes and the light receiver 13 may be a light-sensitive semi-conductor diode.

The angle between the faces 22 of the post 2 could be greater or less than 900 but preferably is within the range 700 to 1100.

Claims

CLAIMS:

1. A smoke detector comprising a light emitter, a light receiver, which creates an electrical signal when sufficient light falls upon it, a chamber and barrier means, between the light emitter and the light receiver, which ensure that in the absence of significant airborne particles in the chamber light emitted by the light emitter does not result in said signal being created, whereas a sufficient quantity of significant airborne particles, such as smoke, in the chamber can cause said signal to be created, characterised in that the barrier means is in the chamber and comprises a first post and a second post substantially parallel to one another and so positioned that the light emitter, the first post, the second post and the light receiver all lie in that sequence on a straight line.

2. A smoke detector according to claim 1, characterised in that at least one of the posts has a length between 7 and 15 mm and a maximum width less than 5 mm.

3. A smoke detector according to claim 1 or 2, characterised in that the part of the first post which is nearest to said light emitter is an apex between two substantially planar faces of the post, substantially equally inclined to said straight line and inclined to one another by an angle within the range 700 to 1100.

4. A smoke detector according to any preceding claim, characterised in that the first post has, remote from the light emitter, a substantially planar face to which said straight line is substantially perpendicular.

5. A smoke detector according to any preceding claim, characterised in that the part of the second post which is nearest to the light receiver is a substantially planar first face to which said straight line is substantially perpendicular.

6. A smoke detector according to claim 5, characterised in that the second post has, on its side remote from the light receiver, a substantially planar second face which is substantially narrower than said first face and to which said straight line is substantially perpendicular.

7. A smoke detector according to any preceding claim, characterised in that the first and second posts project substantially perpendicularly from a plate and are partly encircled by a part-ring of baffle posts which also project from the plate and are surrounded by a gauze strip, there being a cover which together with the plate and the baffle posts define said chamber.

8. A smoke detector according to claim 7, characterised in that that end of each of the first and second posts which is remote from the plate is formed with a protrusion or a recess which co-operates with a recess or a protrusion on the cover in order to locate the free end of the post.

9. A smoke detector according to claim 7 or 8, characterised in that the cover is formed with a tubular part which accommodates the light emitter.

10. A smoke detector substantially as hereinbefore described with reference to the accompanying drawings.

10. A component for use in a smoke detector, characterised in that it comprises a plate from which project first and second posts and a partial ring of baffle posts which partly encircle the first and second posts, the plate also carrying means for locating a light emitter and means for locating a light receiver such that the light emitter, the first post the second post and the light receiver all lie in that sequence on a straight line.

11. A smoke detector substantially as hereinbefore described with reference to the accompanying drawings.

12. A component for use in a smoke detector substantially as hereinbefore described with reference to Figures 2, 3 and 4 of the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS: 1. A smoke detector comprising a light emitter, a light receiver, which creates an electrical signal when sufficient light falls upon it, a chamber and barrier means within the chamber, between the light emitter and the light receiver, which ensure that in the absence of significant airborne particles in the chamber light emitted by the light emitter does not result in said signal being created, whereas a sufficient quantity of significant airborne particles, such as smoke, in the chamber can cause said signal to be created, the barrier means comprising a first post and a second post substantially parallel to one another and so positioned that the light emitter, the first post, the second post and the light receiver all lie in that sequence on a straight line, the posts projecting substantially perpendicularly from a plate and being partly encircled by a part-ring of baffle posts which also project from the plate.

2. A smoke detector according to claim 1, wherein at least one of the first and second posts has a length between 7 and 15 mm and a maximum width less than 5 mm.

3. A smoke detector according to claim 1 or 2, wherein the part of the first post which is nearest to said light emitter is an apex between two substantially planar faces of the post, substantially equally inclined to said straight line and inclined to one another by an angle within the range 700 to 110o.

4. A smoke detector according to any preceding claim, wherein the first post has, remote from the light emitter, a substantially planar face to which said straight line is substantially perpendicular.

5. A smoke detector according to any preceding claim, wherein the part of the second post which is nearest to the light receiver is a substantially planar first face to which said straight line is substantially perpendicular.

6. A smoke detector according to claim 5, wherein the second post has, on its side remote from the light receiver1 a substantially planar second face which is substantially narrower than said first face and to which said straight line is substantially perpendicular.

7. A smoke detector according to any preceding claim, wherein the part-ring of basle posts is surrounded by a gauze strip, there being a cover which together with the plate and the baffle posts define said chamber.

8. A smoke detector according to claim 7, wherein that end of each of the first and second posts which is remote from the plate is formed with a protrusion or a recess which co-operates with a recess or a protrusion on the cover in order to locate the free end of the post.

9. A smoke detector according to ciaim 7 or 8, wherein the cover is formed with a tubular part which accommodates the light emitter.