How to test an optical sensor board from printer
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A much more likely arrangement.

Usually these are wired as shown in Figure 1.


The LED is permanently on.
The photo-transistor is wired in "common-emitter" mode.
The collector has a pull-up resistor. This may be on the board you linked or it may be on another board connected to NODE2.
With no obstruction the LED turns on Q1 and the voltage at NODE2 is pulled down close to 0 V.
With an obstruction Q1 turns off and R2 pulls the voltage up to V+.