Detector removal signalling device

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. Timing means 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 timing means 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. An end of line device can be used to connect a load across the lines if a head is removed and to disconnect the load from the lines if the voltage rises above a threshold. A signalling device is also described which signals a fault if a head of one type is inadvertently fitted to a base which requires a different type. The invention can be used in fire detection systems.