GB2306218A

GB2306218A - Combination fire and smoke alarm

Info

Publication number: GB2306218A
Application number: GB9520749A
Authority: GB
Inventors: Stephen Henry Ellwood; David Appleby
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-10-11
Filing date: 1995-10-11
Publication date: 1997-04-30
Anticipated expiration: 2015-10-11
Also published as: GB2306218B; GB9520749D0

Abstract

A combined detector and sounder device for a fire alarm system comprises a base 10 which houses a PCB 12 and a sounder device 16, and a removable dome-shaped head 11 which houses an optical smoke sensing chamber 33. A temperature sensor 15 mounted to the PCB 12 extends into an airway 32 between the sensing chamber 33 and the body 10. Electrical smoke sensing components e.g. LED 17 and a phototransistor extend from the PCB 12 through the airway 32 into the smoke sensing chamber 33. The arrangement of all of the electrical components on the base means that the sensing chamber 33 can be opened for cleaning the optical components without the need disconnect electrical connections between the removable head 11 and the base 10.

Description

Combined detector & sounder device for a fire alarm system This invention relates to a combined detector and sounder device for a fire alarm.

Small self-contained domestic smoke alarms having a combined detector and sounder are well-known. A disadvantage of such alarms is that they comprise an ionisation-type smoke sensor, which is not very sensitive to smoke from smouldering fires. Optical-type smoke sensors are much more sensitive to this type of fire, and thus commercial smoke alarm detectors generally use optical-type smoke sensors.

It is known that the sensitivity of detectors incorporating optical-type smoke sensors can be improved by incorporating an additional temperature or gas sensor, which senses the heat or smoke emitted from a free burning fire.

Typically, such additional sensors are mounted on the lower end of the detector housing below the optical-type sensor, so that they are exposed to a free flow of air inside the room. A disadvantage of this arrangement is that opticaltype sensors require regular cleaning in order to prevent the build up of dust inside their optical sensing chamber. For this reason the optical-type sensor needs to be removable for cleaning, however this can cause problems since the electrical connections to the additional sensor pass through the opticaltype sensor.

A disadvantage of combined detectors and sounders for fire alarms incorporating optical-type smoke sensors is that government regulations specify that the sounder has to remain functional whilst the sensor is removed for cleaning.

Accordingly, it has hitherto been necessary to mount the sounder and detector side-by-side in a housing.

A disadvantage of this side-by-side arrangement is that it resembles a domestic smoke alarm, and accordingly such arrangements are poorly received in the commercial alarm market. Another disadvantage is that the arrangement is relatively large when viewed from the front, and it will be appreciated that it is desirable to keep the product as small as possible to reduce its visual impact.

I have now devised a combined detector and sounder device for a fire alarm system, which alleviates the abovementioned problems.

In accordance with this invention there is provided a combined detector and sounder device for a fire alarm system, the device comprising a base for mounting to a fixed structure, an optical smoke sensor having an optical smoke sensing chamber and optical smoke sensing means, a removable head incorporating the optical smoke sensing chamber, a printed circuit board mounted in the base, a temperature or gas sensor mounted to the printed circuit board and arranged in an airway which extends between the optical smoke sensing chamber and the base, and a sounder device mounted to the base and arranged to emit sound into said airway.

In use, the head can be removed for cleaning leaving the temperature or gas sensor and sounder mounted to the base.

The arrangement of the temperature or gas sensor in an airway between the optical sensing chamber and the base means that the device is stacked away from the base, such that it does not have a great visual impact. The airway allows air in the room to flow around the sensor inside the device.

Preferably the optical smoke sensing means comprises a light emitting device and a light detecting device which are mounted to said printed circuit board, and which project into said smoke sensing chamber when the head is fitted. Thus, when the head is removed for cleaning, the light emitting and detecting devices remain attached to the base thereby alleviating the need for complicated electrical connections between the base and head.

Preferably the temperature or gas sensor is mounted centrally of the base, the airway extending radially outwardly from the sensor, substantially at all angles thereto. Thus, although the temperature or gas sensor is effectively mounted inside the device, the airway allows heat from fire at any angle to flow around the sensor.

Preferably the light emitting and detecting devices extend through said airway into the sensing chamber when the head is fitted.

Preferably the airway opens into an opening which extends around the periphery of the device.

Preferably the smoke sensing chamber comprises an inlet which allows smoke to enter the chamber, said inlet also opening into said opening which extends around the periphery of the device. The provision of a single opening for smoke and heat to enter the device and for sound to radiate from the device substantially increases the overall aesthetic appearance of the device.

Preferably the smoke sensing chamber is mounted inside a cover which serves to shield the inlet thereof from ambient light.

Preferably the gas sensor comprises a carbon-monoxide sensor.

An embodiment of this invention will now be described by way of example only, and with reference to the accompanying drawings, in which: FIGURE 1 is a perspective view of a combined fire alarm detector and sounder device in accordance with this invention; FIGURE 2 is an exploded view of the device of Figure 1; and FIGURE 3 is a longitudinal sectional view through the device of Figure 1.

Referring to Figure 1 of the drawings, there is shown a combined fire alarm detector and sounder device for fitting to a ceiling. The device is connected to an alarm control panel by wires (not shown). A plurality of such devices are preferably connected in parallel across the wires, such that when one of the devices senses a fire it sends signals along the wires to the control panel, so as to trigger an alarm condition. The control panel then sends signals back along the wires, to activate the sounder in each device.

The device comprises a cylindrical base 10 for attaching to the ceiling and a removable dome-shaped head 11.

The base 10 and head 11 are formed from moulded plastics material.

Preferring to Figures 2 and 3 of the drawings, a connection block (not shown) having terminals for connecting to the wires of the alarm circuit is mounted in the base. A circular printed circuit board (PCB) 12, mounted coaxially inside the base 10, has terminals connected to respective terminals of the connection block. The PCB 12 is held in place by a removable bottom cover 13 of the base. A large opening 14 is formed in the centre of the bottom cover 13 of the base, and this opening is covered by a circular cage 19 which depends from the bottom cover of the base.

A temperature sensor 15, such as a bead thermistor having a negative temperature coefficient, extends downwardly from the PCB 12 into the space inside the cage 19, the latter giving mechanical protection to the temperature sensor. A sounder device 16, such as a speaker, is mounted to the PCB 12, and an aperture is formed in the bottom cover 13 of the base over the sounder, so that sound can emerge from the base. In use, the bottom cover 13 of the base also acts as a resonator to enhance the sound generated by the sounder 16.

The head 11 comprises a one-piece optical chamber moulding 20, which is formed from black plastics material. The moulding 20 comprises a circular portion 21, which is supported co-axially below a tubular portion 23 by radially extending webs 22.

When the device is assembled, the tubular portion 23 of the mounting 20 engages in a peripheral slot formed around the bottom cover 13 of the base, such that the circular portion 21 of the moulding is mounted about lcm below the bottom cover 13 of the base. The space between the base 10 and the circular portion 21 of the moulding forms an airway 30, which extends radially outwardly at all angles from the temperature sensor 15.

The circular portion 21 of the moulding forms an upper wall of an optical smoke chamber 33, the lower wall thereof being formed by a black plastics dish-shaped disc 29 that engages with the moulding 20. An annular peripheral wall of the chamber 33 if formed by a tubular steel mesh 26, which allows smoke to enter the chamber 33, but prevents insects etc.

from entering.

A light emitting diode (LED) 17 and phototransistor 18 extend from the PCB 12 through the bottom cover 13 of the base, through the airway 32 and plate 21 and into the chamber 33, where they are received in respective housing 27,28. A plurality of light baffles 25 are arranged on the plate 21 to help shield the phototransistor 18 from ambient light.

Lenses (not shown) are preferably mounted to the housing for increasing the optical gain of the sensor by concentrating the beams into and out of the phototransistor 18 and LED 17 respectively. A dome-shaped cover 30 fits onto the underside of the moulding 20, and its peripheral edge seats on the lower peripheral edge of the webs 22. The disc 29 and cover 30 are locked into engagement with the moulding 20 by a cap 31. The rim of the cover 30 extends around the tubular steel mesh 26, and thus acts as a baffle against light entering the chamber 33.

When there is fire, smoke flows into the airway 32 in the device and enters the optical chamber 33 through the mesh 26. Under normal circumstances, the phototransistor 18 does not detect any light emitted by the LED 17, however when smoke enters the chamber 33 light emitted by the LED 17 is scattered by the smoke particles, such that some light becomes incident upon the phototransistor 18. This change in light intensity on the phototransistor 17 is sensed by a sensing circuit on the PCB 12, which sends alarm signals to the control panel.

In free burning fires, heat enters the airway 32, and the change in temperature is sensed by the temperature sensor 15. A microprocessor may be mounted on the PCB 12 to monitor and process the pattern of signals from the temperature sensor 15 and optical smoke detector, in order to determine whether there is a fire.

When the alarm is triggered, the control panel activates the sounder 16, and the generated sound radiates from the device through the airway 32. This secondary use of the airway to radiate sound, saves having to incorporate an airway for sound in addition to an airway for smoke and heat. The overall aesthetic appearance of the device is thus improved.

The head 11 can be removed from the base 10 in order to clean the inside of the optical chamber 33. However, when the head is removed all of the electronic components, such as the temperature sensor 15, sounder 16, LED 17 and phototransistor 18 remain attached to the base. Thus, if an alarm is triggered by another device, whilst the head is removed, then the sounder 16 will still operate. The fact that the LED 17 and phototransistor 18 remain attached to the base, saves having to provide a complicated electrical connection between the head and base.

Claims

1) A combined detector and sounder device for a fire alarm system, the device comprising a base for mounting to a fixed structure, an optical smoke sensor having an optical smoke sensing chamber and optical smoke sensing means, a removable head incorporating the optical smoke sensing chamber, a printed circuit board mounted in the base, a temperature or gas sensor mounted to the printed circuit board and arranged in an airway which extends between the optical smoke sensing chamber and the base, and a sounder device mounted to the base and arranged to emit sound into said airway.

2) A combined detector and sounder device for a fire alarm system as claimed in claim 1, comprising a light emitting device and a light detecting device which are mounted to said printed circuit board, and which project into said smoke sensing chamber when the head is fitted.

3) A combined detector and sounder device for a fire alarm system as claimed in claim 2, in which the light emitting and detecting devices extend through said airway into the sensing chamber when the head is fitted.

4) A combined detector and sounder device for a fire alarm system as claimed in any preceding claim, in which the temperature or gas sensor is mounted centrally of the base, the airway extending radially outwardly from the sensor, substantially at all angles thereto.

5) A combined detector and sounder device for a fire alarm system as claimed in claim 4, in which the airway opens into an opening which extends around the periphery of the device.

6) A combined detector and sounder device for a fire alarm system as claimed in claim 5, in which the smoke sensing chamber comprises an inlet which allows smoke to enter the chamber, said inlet also opening into said opening which extends around the periphery of the device.

7) A combined detector and sounder device for a fire alarm system as claimed in any preceding claim, in which the smoke sensing chamber is mounted inside a cover which serves to shield its inlet from ambient light.

8) A combined detector and sounder device for a fire alarm system as claimed in any preceding claim, in which the gas sensor comprises a carbon monoxide sensor.

9) A combined detector and sounder device for a fire alarm system substantially as herein described with reference to the accompanying drawings.