Light-emitting diodes can double as sources and detectors of light. This capability makes possible free-space and optical fiber communication links with a single LED at each end of the link. Also possible is a bidirectional optoisolator made from two similar LEDs. To the best of my knowledge, no such devices have been made commercially available since I first described the idea in back 1979 (1). But that's not a problem, since you can easily make one yourself.