QTR sensor size comparison. Clockwise from top left: QTR-3RC, QTR-1RC, QTR-L-1RC, QTR-8RC.
The Pololu QTR reflectance sensors can be used to sense how much infrared light is reflected from a nearby surface. The QTR sensors are intended to be used as line sensors in line-following, maze-solving, and sumo robots, but they can be used as general purpose proximity or reflectance sensors.
This application note explains how the sensors work and what their output signals look like. There are two main sections for the two types of QTR sensors. Section 2 is about the QTR-xRC sensors, which can be read by a digital I/O line. Section 3 is about the QTR-xA sensors, which output an analog voltage.