An I/O circuit for use with an industrial controller provides a zero-crossing detector circuit with low power dissipation through the use of a zero-crossing circuit that activates a light emitting diode of a photo coupler only for a very brief period of time at the zero-crossing (as opposed to at all times other than the zero-crossing). The circuit is coupled with a power supply circuit that uses a reactive element for voltage dropping as opposed to a resistive voltage drop element further reducing power consumption possible with the low power consumption of the photo coupler.