Item No. : KR07056
This is a tiny trackball those found on a number of Blackberry models (8100, 8300, 8320, 8800, 8830, etc). Each of the four spindles have a tiny circular magnet on each end. This picture that explain how blackberry trackball work with 4 hall effect sensors. Rubber roller at bottom trackball have small magnetic pole and detect by hall effect sensors. See Blackberry Information.
Theory of Operation
The Trackball Module has four tiny rotatable magnets attached to each corner of the component. Rotating the Trackball Module in the up, down, left, or right directions will result in the rotation of one magnet, while any diagonal movement will result in the rotation of two magnets. Whenever any of the magnets rotate, a tiny magneticfield is created, which can be detected by a Hall Effect sensor paired with each magnet. The Hall Effect sensor will then output a series of high/low signals which can be monitored by the Propeller microcontroller to determine the direction of movement of thetrackball. The speed of rotation is then proportional to the period of the pulse, where slow rotation will result in a longer period and faster rotation will result in a shorter period.
Sparkfun make this easy for you, they design the 4pcs hall effect sensors position (breakout board).
We also sell hall effect sensors in different packages => see KR08085