This IR reflective sensor utilizes a TCRT5000 to detect color and distance. It emits IR and then detects if it receives the echo. This sensor is often used in line following robots, auto data logging on utility meters because this module can sense if a surface is white or black.
The measuring distance range from 1mm to 8mm and the central point is about 2.5mm. There is also an on-board potentiometer to adjust the sensitivity.
The infrared diode will be emitting the infrared continually when the module is connected to the power when the emitted infrared light has not been reflected or the strength is not big enough, the triode will in the off state, at this time, D0 output logic LOW and the signal indicate LED off.