How the Sega opto receiver works
The same LED part is used as both emitter and receiver; as a receiver it works like a weak photodiode, so the receiver board amplifies it with a 2N5460 JFET and a 2N3906 transistor. With the MT5000UR the resistor to the JFET drain and 2N3906 base is 4.7k; boards built for the TLRH180P use 5.6k.
Systems
Related tips
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Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Three, Clay Harrell (pinrepair.com)