Hall Effect Sensors - quasics/quasics-frc-sw-2015 GitHub Wiki
A Hall-effect sensor is a device that measures the strength of a magnetic field, and output a voltage that is directly proportional to the field's strength. They are typically used for proximity sensing, positioning (e.g., height of an elevator), speed detection (e.g., as part of an encoder), or current sensing. They are often combined with threshold detection (i.e., the field strength exceeds X%), so that it can work like a switch, and another use for them in FRC is as a limit switch, allowing the robot to detect when some mechanism has moved as far as is feasible/safe.