A signal generating circuit coupled to an AC supply, the circuit comprising at least one first switch device coupled to the AC supply, at least one triggerable switch device coupled to the first switch device, operation of the first switch device causing said triggerable switch device to trigger in response to the AC supply at a predetermined voltage, thereby providing at least a portion of a waveform of the AC supply as a control signal and wherein the control signal terminates within a predetermined period of time after operation of the first switch device terminates. A circuit for detecting and responding to the signals generated by the signal generator is also disclosed.