An I/O module circuit for use in an industrial control system comprising: terminals for receipt of electrical power to be controlled by the I/O module circuit; and a circuit providing: a transistor switch element (82, 90, 92) having an input connected to a terminal for receiving an AC waveform therefrom and an output, the transistor switch element (82, 90, 92) being adapted to provide 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 at the output; an optical isolator including a light emitting diode -LED- that is so connected to the transistor switch element (82, 90, 92) that it switches on and off according to the zero-crossing signal and has 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 (82, 90, 92) provides conduction of the LED only during times when the AC waveform is between the first and second voltage threshold; characterized by further comprising a housing; and in that said terminals are supported by the housing; said circuit is contained in the housing; and the transistor switch element (82, 90, 92) 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 (90, 92) to conduct only when neither the first or second transistor (90, 92) is conducting. terminals for receipt of electrical power to be controlled by the I/O module circuit; and a circuit providing: a transistor switch element (82, 90, 92) having an input connected to a terminal for receiving an AC waveform therefrom and an output, the transistor switch element (82, 90, 92) being adapted to provide 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 at the output; an optical isolator including a light emitting diode -LED- that is so connected to the transistor switch element (82, 90, 92) that it switches on and off according to the zero-crossing signal and has 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 (82, 90, 92) provides conduction of the LED only during times when the AC waveform is between the first and second voltage threshold; characterized by further comprising a housing; and in that said terminals are supported by the housing; said circuit is contained in the housing; and the transistor switch element (82, 90, 92) 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 (90, 92) to conduct only when neither the first or second transistor (90, 92) is conducting. a transistor switch element (82, 90, 92) having an input connected to a terminal for receiving an AC waveform therefrom and an output, the transistor switch element (82, 90, 92) being adapted to provide 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 at the output; an optical isolator including a light emitting diode -LED- that is so connected to the transistor switch element (82, 90, 92) that it switches on and off according to the zero-crossing signal and has 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 (82, 90, 92) provides conduction of the LED only during times when the AC waveform is between the first and second voltage threshold; characterized by further comprising a housing; and in that said terminals are supported by the housing; said circuit is contained in the housing; and the transistor switch element (82, 90, 92) 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 (90, 92) to conduct only when neither the first or second transistor (90, 92) is conducting.