A mains voltage zero-crossing detector has a constant voltage forced on an external node by driving transistor devices with appropriate control signals provided by a feedback loop around a highly power efficient class B configuration comprising an operational amplifier having single ended dual outputs and a class B control circuit. Mains power zero crossings may then be detected by monitoring the drive current of devices driven by this amplifier. Wherein Class B control and current mode detection provide accurate detection of the driven signal without depending on any voltage threshold that may depend on temperature, process fabrication and/or supply voltage.