Its a huge circuit.. Its hard to track with whom the device is connected. But any such transistor like thing with only 2 pins are diodes. Either a simple forward diode, a photo transistor, a temperature sensor, etc. For example, MV2101, a simple forward transistor, but in TO92 package. There's no option for it to be something else. If you have a multimeter, set it to continuity testing mode and put the probes in the pins. And also check the other way by changing the pins. If any one condition shows some continuation, and another shows complete discontinuation, then for sure, its a diode.