The FC-51 module works on the principle of .
The blue potentiometer on the FC-51 adjusts the reference voltage for the LM393 comparator. This directly changes the detection distance. Fc 51 Ir Sensor Datasheet
If the reflected intensity is high enough, the OUT pin goes LOW and the onboard "Status LED" lights up. Infrared Sensors Specs, Operation, Types and Applications The FC-51 module works on the principle of
if (sensorValue == HIGH) { // Object detected Fc 51 Ir Sensor Datasheet