A controller for inserting signaling transitions onto a line voltage, comprising: an auto-transformer having first and second end terminals, and configured to include a first winding portion connected between the first end terminal and a transformer tap, and a second winding portion connected between the transformer tap and the second end terminal, the first and second end terminals connected to the line voltage; a first switch having a first switch terminal connected to the first end terminal of the auto-transformer, and a second switch terminal connected to an output node of the controller; a second switch having a first switch terminal connected to the transformer tap of the auto-transformer, and a second switch terminal connected to the output node; and characterised in that said controller further comprising a switch controller configured to output first and second switch signals responsive to a control signal, to control the first and second switches to insert the signaling transitions onto the line voltage by selectively connecting a nominal line voltage at the first end terminal and a signaling voltage at the transformer tap to the output node, wherein the switch controller is further configured to output the first and second switch signals to control the first and second switches to insert: - a plurality of first transitions that indicate the start of a signaling mode, the plurality of first transitions having a predetermined first time interval between each other - a second signaling transition after the plurality of first signaling transitions, a time interval between the second signaling transition and a last of the first signaling transitions set responsive to a value of the control signal. an auto-transformer having first and second end terminals, and configured to include a first winding portion connected between the first end terminal and a transformer tap, and a second winding portion connected between the transformer tap and the second end terminal, the first and second end terminals connected to the line voltage; a first switch having a first switch terminal connected to the first end terminal of the auto-transformer, and a second switch terminal connected to an output node of the controller; a second switch having a first switch terminal connected to the transformer tap of the auto-transformer, and a second switch terminal connected to the output node; and characterised in that said controller further comprising a switch controller configured to output first and second switch signals responsive to a control signal, to control the first and second switches to insert the signaling transitions onto the line voltage by selectively connecting a nominal line voltage at the first end terminal and a signaling voltage at the transformer tap to the output node, wherein the switch controller is further configured to output the first and second switch signals to control the first and second switches to insert: - a plurality of first transitions that indicate the start of a signaling mode, the plurality of first transitions having a predetermined first time interval between each other - a second signaling transition after the plurality of first signaling transitions, a time interval between the second signaling transition and a last of the first signaling transitions set responsive to a value of the control signal. - a plurality of first transitions that indicate the start of a signaling mode, the plurality of first transitions having a predetermined first time interval between each other - a second signaling transition after the plurality of first signaling transitions, a time interval between the second signaling transition and a last of the first signaling transitions set responsive to a value of the control signal.