The sensor consists of an IR LED and a photo sensor (phototransistor). The light emitted by the IR LED gets reflected by any object placed in front of the sensor and this reflection is detected by the photo sensor (phototransistor). Any white (or lighter) colored surface reflects more than black (or darker) colored surface.
When the reflected light is detected, it produces Digital HIGH (or Binary 1) output on the Sig pin. The on-board LED indicator will also glow. If no reflection is detected or if the object is too far from the sensor, the output on the SIG pin stays at Digital LOW (Binary 0). The on-board LED indicator will be off as well.
The detectable range of this sensor is 4–16 mm. The module incorporates a Rail-to-Rail Operational Amplifier to amplify the output of phototransistor. There is a potentiometer which can be used to adjust the gain of the amplifier, that is, sensitivity of detection.