A controller provides amplitude modulated power line coding of an output voltage waveform. A transformer has a first winding connected in series with the output. An input mains voltage supply and/or the output voltage are monitored, and the second winding of the transformer is configured in dependence on the monitored mains voltage supply, the monitored output voltage and a target output voltage. This target comprises a voltage which encodes a binary bit of digital information. This provides a feedback approach which enables the transformer to be used to provide an accurate modulation of the binary bit to be encoded.