HiTechnic NXT Angle Sensor

“Measure axle rotation position and rotation speed with the HiTechnic Angle Sensor. The Angle Sensor enables you to measure three rotation properties:
Absolute Angle – the rotation position of a rotating axle from 0 – ۳۵۹ degrees with 1 degree accuracy
Accumulated angle – the accumulated number of degrees an axle has rotated
Rotation Speed – the speed of the axle rotation in RPM (revolutions per minute)
With its very low friction mechanism, the HiTechnic Angle Sensor is ideal for building models where accurate measurement of rotation properties of an axle is key. Perfect for building a distance measuring wheel or a weather station wind vane.”