break-beam sensor to detect low paper supply
Put the breakbeam modules on each side.  The module with the white wire will generate the signal to control the LED. 

In fact there is a sample circuit showing exactly how to set it up in the datasheet. The resistor values are 5.1k and 1k.  

The 5.1k pullup resistor is needed because the output is open-collector. 

The 1k resistor sets the current through the LED (brightness).  The current will be $$ \frac{5V- V_{LED}}{R_{1k}}$$



If you want to invert the sensors output, you can accomplish that with a PMOS.  Make sure the turn on voltage of the PMOS is not too high.  ~2V should be fine. When the output is high, the PMOS turns off and prevents the LED from getting any current.  When the output is low, the PMOS turns on and so does the LED.