Abstract: A smoke detector having a light source for generating a beam of light is disclosed. The smoke detector is arranged to transmit the beam along an optical axis and includes a sensor for receiving light scattered from the beam off the optical axis by smoke; and a light trap comprising one or more walls that define a volume for receiving light that passes unscattered from the light source past the sensor along the optical axis. The volume includes a first end closest on the optical axis to the light source; and a second end furthest on the optical axis from the light source. The volume is open at said first end but is otherwise closed, and at least one of the walls is sloped from the first end to the second end, such that the light trap progressively narrows from the first end axially towards the second end.

Abstract: A wired data communications network for use in a building for the detection of fire or intrusion, the network including a control unit connected by cable to a series of uniquely addressable transducers for the transmission and reception of data in a polling signal. Each transducer has a memory for control data and for its own unique address and responds to a predetermined start voltage pulse in the polling signal to clear its memory and then responds to control data and address data in the polling signal to store the control data when the address matches its own address and transmit data to the control unit representative of its own status. At least one of the transducers determines whether the control data is extended data and, if so, stores the extended control data and the further control data, and performs the said transmission of data to the control unit and any other process dictated by the extended control data.

Abstract: A signal for use on a wired network interconnecting electronic devices, comprising: a base voltage for conveying power to the electronic devices; a pulsed voltage signal bearing coded information for transfer between the electronic devices; and a digital signal superimposed onto selected portions of the pulsed voltage signal, wherein the digital signal comprises a carrier signal modulated by a data signal for transfer between the electronic devices. This is used in fire alarm networks, for example, to convey multimedia or other data such as control signals, whilst maintaining compatibility with existing protocols using the pulsed voltage signal.

Abstract: A hazard detector has an electronic circuit with a start-up program for causing emission of a local indicator signal, such as a flashing signal from a LED, if power and ground terminals of the detector are connected with proper orientation, i.e. polarity, to power and ground lines of a power supply. Through this means, a person installing the hazard detector can tell immediately after connection if the detector has been connected with proper orientation, and avoids the need for introducing a hazard such as heat or smoke to test the operation of the detector. A variation uses a more sophisticated program that disables, during a test mode, complex filtering algorithms that are used by detectors to block false alarm signals; if such filtering is not disabled, impedes normal testing of the detectors.

Abstract: A mounting base 3, for example, a bezel assembly, which can be fixed, by means of screws 4, directly to a wall or ceiling, or to a conduit box 2, has cover members 5 movable between open position and closed positions. The open positions provide access to the screws so that the mounting base can be fitted or removed. The cover members are retained by the mounting base so that they can be moved between the open and closed positions without becoming detached. Each cover member is preferably hinged to the mounting base so that it can be pivoted away from the screw into the open position without becoming detached. The hinge preferably comprises a shell 8 and a pin 9, the shell being open to enable it to be fitted to the pin. A resilient tab 6 can be provided to snap fit the cover member to the base and secure it in the closed position, prior to fitting a removable head 1 to the base. The invention may be used in the field of fire detection to conceal screws to prevent vandalism or theft of a fire detector.

Abstract: A signalling device applies a pulsating signal to supply lines when a head is removed from a base. A monitoring system includes a plurality of such signalling devices and is responsive to the pulsating signal to indicate the absence of a head from its base. When a pulsating signal is already on the supply lines, its waveform is preserved independently of the number of heads removed from respective bases. A timer is triggered either to ensure synchronisation of pulsating signals generated by two or more bases, or to prevent any second base from generating a pulsating signal, if a first base is already generating one. The timer can be an oscillator where a capacitor is prematurely discharged by a pulsating signal already present on the supply lines. The base can include additional means for inhibiting the pulsating signal when a voltage applied to the signalling device either exceeds a predetermined threshold, or changes in polarity, or both.

Abstract: A fire detection and alarm system has fire detecting devices of different types (T1,T2) and alarm means connected across supply lines. Both types of device normally apply a high impedance across the supply lines in a standby condition, and a low impedance in an alarm condition. The fire detecting devices are operable by a first voltage. This is lower than a second voltage applied, by a CCU, to the lines in an alarm condition for operating the alarm means. The alarm means respond to operating current supplied under different conditions to give respectively different kinds of fire warnings. Each first and second type of fire detecting device has current limiting means for differently limiting the respective current drain on the supply lines in the alarm condition.

Abstract: A fire alarm system has detecting devices and alarm devices connected in parallel across the same supply lines. A control unit (CCU) supplies a first voltage to operate the detecting devices and a second higher voltage to operate the alarm devices. When a fire is detected, signalling means, in the detecting device, produce a change of state signal which causes impedance switching means to switch from a high to a low impedance state. This causes a current drain across the supply lines which is recognized by the CCU as the fire detection signal, which then applies the second voltage to the lines. Voltage responsive means respond to the second voltage to cause the impedance switching means to switch to the high line impedance state. The current drain is thereby reduced to conserve battery power. This avoids large current drains when several detecting devices respond to the fire (e.g. when smoke detectors are triggered by spreading smoke). The voltage responsive means may include threshold voltage means.

Abstract: A fire detector assembly comprises a base (1) and a sensor (5) adapted to fit the base. The base (1) is provided with passive means, such as pins (6) or contacts arranged in a pattern to represent an address of the detector. When the sensor (5) is fitted to the base (1), reading means in the sensor reads the address code and thereby conditions the sensor to receive appropriate signals from a control unit.

Abstract: Circuitry is described for isolating a section of a loop circuit (L) from a bi-directional supply unit (20) in the event of a short circuit in the section. One or more circuit elements (21), such as fire detectors, are connected in respective portions of the loop circuit and between respective pairs of bi-directional isolators (1,2,3,4). Each isolator includes switching means (TR1,TR3) providing low and high impedance paths for respectively supplying current to said circuit elements and isolating said circuit elements from the supply unit (20). Each isolator also includes sensing means (TR2, D1, D7) for sensing a short circuit in any one of the circuit elements (21) whereby the sensing means causes a change in state of the switching means from the low to the high impedance path thereby isolating the circuit elements in the respective section of the loop from the supply unit.

Abstract: A fire detector assembly comprises a base (1) and a sensor (5) adapted to fit the base. The base (1) is provided with passive means, such as pins (6) or contacts arranged in a pattern to represent an address of the detector. When the sensor (5) is fitted to the base (1), reading means in the sensor reads the address code and thereby conditions the sensor to receive appropriate signals from a control unit.

Abstract: Circuitry is described for isolating a section of a loop circuit (L) from a bi-directional supply unit (20) in the event of a short circuit in the section. One or more circuit elements (21), such as fire detectors, are connected in respective portions of the loop circuit and between respective pairs of bi-directional isolators (1,2,3,4). Each isolator includes switching means (TR1,TR3) providing low and high impedance paths for respectively supplying current to said circuit elements and isolating said circuit elements from the supply unit (20). Each isolator also includes sensing means (TR2, D1, DS7) for sensing a short circuit in any one of the circuit elements (21) whereby the sensing means causes a change in state of the switching means from the low to the high impedance path thereby isolating the circuit elements in the respective section of the loop from the supply unit.