Smoke alarm systems

Abstract: A smoke alarm system utilises a conventional smoke detector (1) for producing audible alarm signals when the presence of smoke is detected. These signals are received by a mainsborne communication means (5) which may then produce further alarm signals or transmit electrical signals over a mainsborne communication system to activate remote alarm units.

Description: A SMOKE ALARM SYSTEM This invention relates to a smoke alarm system including; at least one detector unit arranged to produce an alarm signal in response to the presence of smoke; mainsborne communication means responsive to the alarm signal to produce electrical signals for transmission over a mainsborne system; at least one remote alarm unit responsive to the electrical signals to provide an indication that an alarm signal has been produced; and a first control circuit for inhibiting the production of the electrical signals until an alarm signal of predetermined amplitude and duration is detected by the mainsborne communication means, the or each remote alarm unit being incorporated in a further mainsborne communication means and being effective to produce a further alarm signal. A smoke alarm system of this general type is disclosed in US-A-4,365,238. Such a system suffers the disadvantage however that the range over which the alarm signal may be heard is relatively limited. It would thus be desirable to produce a smoke alarm system in which the production of one alarm leads to several other alarms being sounded in different places thereby increasing the range over which the alarm may be heard. It is an object of the present invention to provide a smoke alarm system wherein this problem is at least alleviated. According to the present invention a smoke alarm system as defined in the first paragraph above is characterized in that each mainsborne communication means and the or each further mainsborne communication means incorporates a piezoelectric device and electronic switching means for causing the piezoelectric device to act as either an alarm for producing the further alarm signal, or a microphone selectively responsive to the frequencies of the alarm signals. Preferably each detector unit is remote from the associated mainsborne communication means. Each mainsborne communication means may then incorporate a piezoelectric device, and electronic switching means for causing the piezoelectric device to act as either an alarm for producing the further alarm signal, or a microphone selectively responsive to the frequencies of the alarm. The system suitably includes a first control means for inhibiting the production of the electrical signals until an alarm signal of predetermined amplitude and duration is detected by one of the mainsborne communication means. The system also suitably includes a second control means for causing each further alarm signal to be produced for a first predetermined length of time, and inhibiting the production of subsequent further alarm signals for a second predetermined length of time. One smoke alarm system in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of part of the system; Figure 2 is a schematic circuit diagram of the alarm and microphone circuit incorporated in the system of Figure 1; Figure 3 is a side view of the electro-acoustic transducer incorporated in the circuit of Figure 2; and Figure 4 is a plan view of the transducer of Figure 3. The alarm system to be described is a smoke alarm system for a domestic premises. Referring firstly to Figure 1 the smoke alarm system incorporates a number of smoke detector units 1 distributed around a building, only two such units being shown in the figure. Each unit 1 includes a smoke detector of conventional form (not shown), for example an ionisation chamber, and an alarm 3 which is arranged to produce an alarm signal, such as an audible alarm signal, when smoke is detected by the smoke detector. In respect of each unit 1, located at a mains wiring socket there is provided a unit 5, in the form of a mainsborne communicator, which in use of the system is connected into the mains wiring 7 of the building. Referring now also to Figure 2 each unit 5 incorporates an alarm/microphone circuit indicated generally as 8, each circuit 8 incorporating an electro-acoustic transducer 9 which will be described in more detail hereafter. A transistor switch 11 operating under the control of a microprocessor 13 is effective to control the mode of the transducer 9 such that it operates as either an alarm indicator or a tuned microphone. This is achieved by means of a series arrangement of two inverters 15, 17 effective to apply alternating voltages across electrodes M and G of the transducer 9 when it is required to operate as an alarm sounder, a feedback electrode F being effective to cause resonance in the electrical circuit 8. An amplifier 19 is effective to amplify the electrical signals produced by the transducer 9 when it is operating as a microphone. Referring now particularly to Figure 1 again the output of the amplifier 19 is connected through a rectifying and RC smoothing circuit 21 to a control circuit in the form of a signal discrimination circuit 23. The output of circuit 23 is connected via the microprocessor 13 to a modem 25 which is inductively connected to the mains wiring 7. Referring now particularly to Figures 3 and 4 the transducer 9 comprises a metal resonating disc 27 resiliently mounted in a plastic case 29, the case 29 being formed so as to define an acoustic resonant cavity 31. Carried on the metal disc 27 is a piezo-ceramic layer 33. It will be seen that by the use of such a dual purpose electro-acoustic transducer, the frequency to which the microphone is most sensitive should automatically match the resonant frequency of an alarm in a different unit 5, if all the units 5 are of similar construction. In order for the peak frequency of the transducer 9 in the microphone mode to match the resonant frequency in the alarm indicator mode, the impedance between the electrodes M and G in the microphone mode is minimised. The alarms 3 incorporated in the units 1 will of course be of the same construction as that of the transducer 9. In use of the system an alarm is deemed to have occurred if an alarm signal of sufficient amplitude is received by a transducer 9 operating in the microphone mode in at least one of the units 5 for about 5 to 10 seconds. The signal discrimination circuit 23 and microprocessor 13 will reject alarm signals which do not conform to these criteria. If during the 5 to 10 second time period the alarm signal ceases the timing cycle for the unit 5 is reset to zero by the microprocessor 13. An alarm message is then sent via the associated mains modem 25 and through the mains wiring 7 using an appropriate mainsborne communication protocol, the message also identifying the originating unit 5. The message is transmitted along the mains wiring 7 to all the other units 5 connected to the mains wiring. If more than one unit 5 senses the alarm, whichever unit manages to transmit a message over the mains wiring 7 first prevents the other units 5 from transmitting. Collision messages are resolved by random backoff. The received message causes all the other units 5 to operate their transducers 9 in the alarm indicator mode for a suitable period of time, for example 20 seconds. Thus there is produced an enhanced audio indication that smoke has been detected. The message may be repeatedly transmitted, for example eight times, to increase the probability of reception of the message by the other units 5. After sending the message, the transmitting unit 5 sounds its own alarm for 20 seconds minus the time taken to transmit the messages. Thus each unit 5 may be made aware that smoke has been detected either by the noise of the alarm from another unit 5 or by receipt of a mainsborne message. If the message does not reach some of the units 5 these units may be able to sense the alarms of the units 5 which did receive the message. It is therefore possible that these units 5 will continue to sense each other and sound their alarms indefinitely even though the initial stimulus is no longer present. To avoid this it may be arranged that the units 5 are not responsive to alarm noise for a further time period, for example, 20 seconds after the silencing of their alarms. One of the main problems in sensing the sound produced by the alarms 3 or transducers 9 is the variability of sound pressure levels within a room due to interference effects. This may be reduced, if necessary by use of a technique known as diversity. In one particular diversity technique, known as spaced diversity, two or more units 5 each incorporating a transducer 9 are used, the transducers being separated by at least a quarter of a wavelength of the sound generated, typically about 0.03 metres. The signals produced by these transducers 9 are then processed in different ways to optimise the overall signal from them. This may be done, for example, by periodically switching between the transducers 9 or by selecting the transducer 9 and amplifier 19 which gives the output of greatest amplitude. Alternatively frequency diversity may be used. In this method the sound produced by the alarms 3 or transducers 9 is warbled i.e. there is more than one frequency in the sound burst causing the interference pattern to change. The gain of the amplifier 19 is arranged to be set high enough to cope with the least expected signal level but low enough to ensure that the signal discrimination circuit 23 and microprocessor 13 do not react to the worst case ambient noise. Such a worst case noise might comprise a series of bursts of high level sound at the alarm frequency, as could for example emanate from a hi-fi system. It will be appreciated that it may be useful to use the mainsborne communication system to control other devices, for example light sensors which operate light switches, temperature sensors etc. In such an event a central application controller (not shown) may be useful, this being effective for example to cause certain lights of the building to switch on, or electrical equipment to switch off, in the event that one of the smoke detectors triggers the system. Furthermore either the noise from the alarms 3 or the consequent mainsborne signals may be used to initiate de-energisation of an electromagnet which, when energised, is effective to hold open doors, thereby to limit the spread of smoke through the premises. The signals may also be used to operate a vibrator under a pillow so as to alert people with impaired hearing. It will also be appreciated that it is particularly convenient for the smoke detector units 1 to be separate from the units 5. The smoke detector units 1 may then be battery powered, and thus mountable at any suitable position without the need for wires draped across, for example a ceiling. In some circumstances however the smoke detector units may be mounted in the units 5. This will obviate the need for a separate alarm in the unit 1. It will be appreciated that whilst in the example herebefore, audible alarm signals have been used, the system is equally well adaptable to use with inaudible alarms.

Claims: A smoke alarm system including; at least one detector unit arranged to produce an alarm signal in response to the presence of smoke; mainsborne communication means (5) responsive to the alarm signal to produce electrical signals for transmission over a mainsborne system (7); at least one remote alarm unit responsive to the electrical signals to provide an indication that an alarm signal has been produced; and a first control circuit (23, 13) for inhibiting the production of the electrical signals until an alarm signal of predetermined amplitude and duration is detected by the mainsborne communication means, the or each remote alarm unit being incorporated in a further mainsborne communication means (5) and being effective to produce a further alarm signal, characterised in that each mainsborne communication means and the or each further mainsborne communication means incorporates a piezoelectric device (4) and electronic switching means (8) for causing the piezoelectric device to act as either an alarm for producing the further alarm signal, or a microphone selectively responsive to the frequencies of the alarm signals. A smoke alarm system according to claim 1 wherein the or each detector unit is remote from the associated mainsborne communication means. A smoke alarm system according to claim 1 or claim 2 wherein the first control circuit comprises a discrimination circuit (23) and a microprocessor (13). A smoke alarm system according to claim 1 or claim 2 wherein the system further includes a second control circuit for causing each further alarm signal to be produced for a first predetermined length of time, and for inhibiting the production of subsequent further alarm signals for a second predetermined length of time. A smoke alarm system according to any preceding claim wherein the alarm signals comprise audible alarm signals. A smoke alarm system according to any preceding claim wherein the further alarm signals comprise audible alarm signals.

Inventors: ALLAN ROBERT EDWARD, OLIVER COLIN CHARLES, ALLAN,ROBERT EDWARD, OLIVER, COLIN CHARLES
Applicants: EMI PLC THORN, THORN EMI PLC
Application year: 1990
Main classification: G08B17
IPC codes: G08B17, G08B1