Used in Object Detection, Line Tracking, Material Detect, Limit Switch, etc.
The TCRT5000 and TCRT5000L are reflective sensors which include an infrared emitter and phototransistor in a leaded package which blocks visible light. The package includes two mounting clips. TCRT5000L is the long lead version. The TCRT5000(L) has a compact construction where the emitting-light source and the detector are arranged in the same direction to sense the presence of an object by using the reflective IR beam from the object. The detector consists of a phototransistor.
Making sensor out of TCRT5000
Here is one example of multiple sensor on one board making a line following robot sensor
Another line following sensor example