Abstract: An optical coupler having an optical transmitter and an optical receiver are disclosed. The optical transmitter receives a logic signal to be transmitted to a circuit connected to the optical receiver and a photo emitter for generating a light signal having an intensity related to the states of the logic signal. The optical receiver includes a photodetector assembly, a hold circuit, a threshold generating circuit, and a signal generator. The photodetector assembly receives the light signal and generates a photo signal having an amplitude related to that of the light signal. The hold circuit stores a potential related to the maximum amplitude of the photo signal during a preceding time period. The threshold generating circuit generates a high threshold signal related to the stored potential. The signal generator generates an output logic signal that has first and second states that change state when the photo signal crosses the high threshold.