TCRT5000 Reflective Optical Sensor Infrared IR Switch infrared is an Infrared LED and an Infrared detector. It works by transmitting a beam of Infrared light downward towards a surface which is then reflected back and read by the detector.

When the infrared transmitter emits a beam of light to a piece of paper, if the rays shine on a white surface, they will be reflected and in turn be received by the Infrared detector , and pin S on the module will output a low level; If the beam of light encounters a black line, they will be absorbed, thus the Infrared detector gets nothing, and pin S will output a high level.

So this module basically acts like a simple switch when it gets close to a white / black object. You can adjust the sensitivity with the potentiometer on the module

You have to be about 2.5cm from white paper but some objects you may have to be slightly closer for other objects. This is the module that I used – there are a couple of minor variations to this one but generally they all work the same way.

Features:

Detector type: phototransistor
Operating range within > 20 % relative collector current: 0.2 mm to 15 mm
Daylight blocking filter
Emitter wavelength: 950 nm
Lead (Pb)-free soldering released

Specification:

Model	TCRT5000
Operating Voltage Range	3.6~5 VDC
Average Current Consumption (mA)	60
Distance Measuring Range	0.591″ (15mm);
Peak operating distance	2.5 mm
Output Type	Phototransistor
Working Temperature Range	-25~85ºC
Color	Black + blue
Material	Plastic + iron
Detecting method	Reflective
Transistor Properties	Collector-Emitter: Voltage: 70V; Collector Current: 100mA; Forward Current: 60mA.
Dimensions (L x W x H in mm)	10.2 x 5.8 x 7
Weight (gm)	8