Pure resistor and non-polarity output. "off" at the high resistance. Simple circuit configuration suitable for DC and AC use. An optocoupler, also known as photocoupler or opto-isolator, is a device which can transfer an electrical signal across two galvanically-isolated circuits by way of optical coupling. Current flowing through the LED results in a proportional current flowing in the photo detector. The current transfer ratio. The allowable maximum alternating current voltage that can be applied between the input pins and output pins is expressed as a root mean square (rms) value. On the input side an infrared light emitting diode is used with all optocoupler types.