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A smoke detector unit (20) comprising a housing containing a smoke detector (24) and at least two component mounting boards, such as printed circuit boards, in a stacked arrangement, the housing having at least one air gap (34), wherein one of the boards (36) is inverted with respect to the other an

A smoke detector unit (20) comprising a housing containing a smoke detector (24) and at least two component mounting boards, such as printed circuit boards, in a stacked arrangement, the housing having at least one air gap (34), wherein one of the boards (36) is inverted with respect to the other and the detector is positioned on the lower board whereby, in use, the detector extends upwardly between the boards. The housing may be in the form of a ceiling rose.

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The invention claimed is: 1. A smoke detector unit comprising a housing containing a smoke detector and at least two component mounting boards in stacked arrangement, the housing having at least one air gap, wherein one of the component mounting boards is inverted with respect to the other componen

The invention claimed is: 1. A smoke detector unit comprising a housing containing a smoke detector and at least two component mounting boards in stacked arrangement, the housing having at least one air gap, wherein one of the component mounting boards is inverted with respect to the other component mounting board with the components mutually opposing, and the detector is positioned on the lower component mounting board whereby, in use, the detector extends upwardly between the opposing component mounting boards. 2. A smoke detector unit as claimed in claim 1 wherein further functional accessories are included within the housing. 3. A smoke detector unit as claimed in claim 2 wherein the functional accessories include a sounder, control circuit and power supply. 4. A smoke detector unit as claimed in claim 3 wherein the smoke detector and sounder are provided on the opposing component mounting boards. 5. A smoke detector unit as claimed in claim 1 wherein the smoke detector is in the form of a smoke chamber having tapered sides. 6. A smoke detector unit as claimed in claim 1 wherein the components are mounted on the component mounting boards in such a manner as to allow air flow through the unit. 7. A smoke detector unit as claimed in claim 1 wherein the component mounting boards are printed circuit boards. 8. A smoke detector unit as claimed in claim 1 wherein the housing is attachable to a surface via a base plate. 9. A smoke detector unit as claimed in claim 8 wherein the housing is a truncated cylinder closed by a bridging member. 10. A smoke detector unit as claimed in claim 9 wherein one of the component mounting boards is attached to the base plate and the other component mounting board is attached to the bridging member of the housing whereby, upon fastening of the housing and base plate together, the component mounting boards are in parallel, stacked arrangement with the smoke detector located on the component mounting board attached to the bridging member. 11. A smoke detector unit as claimed in claim 8 wherein the base plate is a circular plate of similar diameter to the housing. 12. A smoke detector unit as claimed in claim 8 wherein the housing and base plate are attached to each other by means of a bayonet fixing. 13. A smoke detector unit as claimed in claim 12 wherein one of the parts is provided with a retaining barb that fits into a complimentary slot or recess provided in the other part, thereby enabling the two parts to be locked together. 14. A smoke detector unit as claimed in claim 8 wherein the air gap is provided between upper edges of the housing and the base plate to allow air to enter the interior of the housing. 15. A smoke detector unit as claimed in claim 8 wherein the housing comprises a cover and a mounting plate, the mounting plate being attachable to the base plate. 16. A smoke detector unit as claimed in claim 15 wherein the mounting plate supports the component mounting boards in a stacked arrangement, the component mounting boards having the required functional accessories thereon. 17. A smoke detector unit as claimed in claim 16 wherein the mounting plate has legs extending therefrom which allow the component mounting boards to be supported in a stacked arrangement, the smoke detector being located on the component mounting board furthest from the plate. 18. A smoke detector unit as claimed in claim 15 wherein the mounting plate is encased by the cover. 19. A smoke detector unit as claimed in claim 18 wherein an air gap is provided between the sides of the mounting plate and the upper edges of the cover. 20. A smoke detector unit as claimed in any one of the preceding claims wherein the housing is provided with a transparent region hereby an emergency light is contained within the unit. 21. A smoke detector unit as claimed in any one of the preceding claims wherein the housing includes one or more accessories selected from the group consisting of a carbon monoxide detector, a heat detector and an intruder alarm. 22. A smoke detector unit as claimed in any one of the preceding claims wherein the housing is in the form of a ceiling rose. 23. A smoke detector unit as claimed in claim 22 wherein a central hollow shaft is provided through the housing for passage of cable for a mains light that depends from the ceiling rose. 24. A smoke detector unit as claimed in claim 1, wherein the housing comprises a mounting plate, the plate being connectable to a base plate for mounting to a surface wherein electrical connections are provided in the housing and the base plate is provided with an aperture therethrough for passage of wiring to said electrical connections. 25. A smoke detector unit as claimed in claim 24 wherein the housing comprises a mounting plate and cover, the electrical connections being provided on the mounting plate and electrical wiring therefrom extends from the mounting plate into the cover and is retained therein. 26. A smoke detector unit as claimed in claim 25 wherein electrical pins or terminals extend from the mounting plate for connection to a complimentary socket provided at the ends of the wiring. 27. A combined ceiling rose and smoke detector unit comprising a housing in the form of a ceiling rose, the housing containing a smoke detector and at least two component mounting boards in stacked arrangement, the housing having at least one air gap, wherein one of the component mounting boards is inverted with respect to the other component mounting board with the components mutually opposing, and the detector is positioned on the lower component mounting board whereby, in use, the detector extends upwardly between the opposing component mounting boards. 28. A combined ceiling rose and smoke detector unit as claimed in claim 27 wherein appropriate electrical wiring for any functional accessories contained in the housing and/or for a mains light that depends from the housing is provided on or within a base plate that is attachable to the housing for connection to conventional mains wiring extending from a ceiling. 29. A combined ceiling rose and smoke detector unit as claimed in claim 27 wherein appropriate electrical wiring for any functional accessories contained in the housing and/or for a main light that depends from the housing is provided on or within the housing and the housing is attachable to the ceiling by a base plate. 30. A combined ceiling rose and smoke detector unit as claimed in claim 29 wherein a mounting plate is provided for connection to the base plate, the mounting plate supporting the component mounting boards and being encased in the housing, an intended exterior side of the mounting plate being provided with wiring connections. 31. A combined ceiling rose and smoke detector as claimed in claim 30 wherein the base plate is provided with apertures for passage of the wiring but is free from any wiring connections.