Identifying mystery dual channel opto detector?
You've surmised that this is a phototransistor (detector). Since it is quadrature system, it may be a dual phototransistor (left), especially if the illuminating infra-red LED was a single emitter.
Which pin is which is unknown. You may have to try all combinations.
A test circuit is shown at right. Monitor the output while illuminating with an infra-red source. Unilluminated, the outputs should remain at +5V. A standard incandescent lightbulb should work as a source. The black plastic casing usually blocks visible light amazingly-well.







simulate this circuit &ndash; Schematic created using CircuitLab