Activating a circuit based on change of resistance
Usually comparator circuits are used with sensors like photoresistors, the photoresistor is put in series with another resistor to form a resistor divider. The comparator detects whether the voltage reaches a given level set by the potentiometer. 

This allows for a circuit that can detect light levels based on a threshold. 


Source: http://www.learningaboutelectronics.com/Articles/LM741-night-light-circuit.php