GB1491222A - Signalling apparatus - Google Patents

Abstract

1491222 Fire alarms C W MAXTED [TRADING AS PROTECTOR ALARMS (SALES)] 11 Oct 1974 44246/74 Heading G4N Signalling apparatus comprises a power supply circuit for supplying two output terminals and includes means for sensing a change in a circuit connected between said output terminals and a switching device controlled by said sensing means and operable for causing the power supply circuit to change from a first supply condition to a second supply condition (e.g. reversal of polarity) on such a change being sensed. A detection device is provided for connection between said two output terminals for producing the change in the circuit condition in response to an occurrence (e.g. fire) and including indicating means arranged to maintain its indication after resetting and an alarm signalling device for connection between said two output terminals to provide an alarm signal when the power supply circuit is in the second condition. As described, each detection device may be a smoke, flame or temperature detector or a FIRE switch of the "break-glass" type, connected in series with a diode poled so that it is responsive to the normal polarity of the power supply circuit. One or more audible or visible alarms is also connected across the two output terminals in series with a diode of opposite polarity. The two-wire circuit is also terminated in an asymmetrically conductive impedance so that the power supply circuit can give an indication of the continuity of the twowire line. In the normal condition of the power supply circuit (Fig. 7) relay 41 is not energized and connects terminals 3 and 4 respectively to positive and negative supplies via resistor 42. In the event of a fire, one of the sensors responds and the voltage developed across resistor 42 turns on transistor 44 which energizes relay 43 which, in turn, energizes relay 41 so as to reverse the polarity of output terminals 3, 4 and so cause operation of the alarm signalling devices connected via the two-wire line to terminals 3, 4. This action also holds the transistor conductive and produces a FIRE signal on line 49. A current limiting device 51 limits the current, and hence voltage, on the two-wire line connected to terminals 3, 4 during the change over condition. The circuit is reset by interrupting a negative line 55 so that the relays 41, 43 returns to their original state. In the event of a line fault, transistor 56 becomes turned on by the lack of current flowing in a Zener and conventional diode connected in series opposition at the remote end of the line and thus provides a fault signal at 59. Smoke detection.-Fig. 4 shows a smoke detector 15 connected across the two-wire line 5, 6. Under normal conditions 5 is positive with respect to 6, but when the sensor 15 responds, the power supply circuit (Fig. 7) reverses polarity. Diodes 28, 31 and transistor 32 now become conductive, providing the first polarity but of a lower voltage to the sensor 15. A local indicator lamp or LED 16, operated by a thyristor, is included.