A motion sensor constituted of: a housing; a rotating member secured to the housing and arranged to rotate about a rotation axis of the rotating member responsive to motion of the housing across a surface; at least one bipolar magnet secured to the rotating member so as to rotate about the rotation axis of the rotating member with the rotation of the rotating member; and a transducer secured to the housing and juxtaposed with the rotating member, the transducer arranged to vary its output voltage in response to changes in magnetic field caused by the rotation of the at least one bipolar magnet.
