Patent ID: 8669787

Claim:
An I/O module circuit for use in an industrial control system comprising: a housing; terminals supported by the housing for receipt of electrical power to be controlled by the I/O module circuit; and a circuit contained in the housing providing: a transistor switch element having an input connected to a terminal for receiving an AC waveform therefrom and having an output providing a zero-crossing signal indicating a voltage of the AC waveform within a first and second predetermined voltage threshold near zero voltage of the AC waveform; an optical isolator including a light emitting diode (LED) connected to the transistor switch element to switch on and off according to the zero-crossing signal and having a photosensitive solid-state switch receiving light from the LED to provide an optically isolated zero-crossing signal; wherein the zero-crossing signal from the transistor switch element provides conduction of the LED only during times when the AC waveform is between the first and second voltage threshold; wherein the transistor switch element provides a duty cycle of less than five percent for AC waveforms in a range of 120 volts AC to 240 volts AC; wherein the transistor switch element includes a first transistor that conducts when the AC waveform is above a first threshold and a second transistor that conducts when the AC waveform is below the second threshold and a third transistor communicating with the first and second transistors to conduct only when neither the first or second transistor is conducting.