Apparatus is described for transmitting signals between electric terminals through photo diodes to electrically isolate circuits connected to the terminals, which is compact and rugged. The apparatus includes a metal shell, (24, FIG. 2) a pair of circuit boards (40, 42) mounted in the shell and axially spaced apart, pairs of terminals (12, 14) with one terminal of each pair mounted on a different one of the circuit boards, and pairs of photo diodes or other optoelectric transducers (16, 18). Each transducer of a pair is mounted to a different circuit board and connected to a corresponding terminal, and the two transducers are aligned and extend toward each other. A separator element (90) of opaque material occupies most of the space between the circuit boards and has holes (92) that receive the two transducers of each pair. The separator element prevents the leakage of light between different pairs of transducers, and also helps to assure alignment of each pair of transducers.