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.