| 摘要 |
<p>A fire alarm system has detecting devices (D) and alarm devices (A) connected in parallel across the same supply lines (L1,L2). A control unit (CCU) supplies a first voltage to operate the detecting devices and a second higher voltage to operate the alarm devices (A). When a fire is detected, signalling means (1), in the detecting device, produce a change of state signal which causes impedance switching means (5) to switch from a high to a low impedance state. This causes a current drain across the supply lines which is recognised by the CCU as the fire detection signal, which then applies the second voltage to the lines. Voltage responsive means (6) respond to the second voltage to cause the impedance switching means (5) 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 (6) may include threshold voltage means (ZD3). Additional voltage responsive means (4) cause the impedance switching means (5) to switch to a high impedance state, whenever the line voltage falls below a predetermined level below the first voltage. Circuitry is also described for maintaining the current drain on the supply lines substantially constant, for latching an alarm state, for delaying operation of the impedance switching means (T9,R15) to enable the voltage on the supply lines to be switched rapidly between different levels without causing the impedance switching means (5) to be in its low impedance state, and for operating on different polarities. <IMAGE></p> |
