close proximity to coil 103 as the wheel rotates due to the change in magnetic field as the lobes pass the coil a periodic voltage is generated in coil 103 whose frequency is representative of the wheel 39 s rotational speed a simple magnetic reed switch may also be used to sense rotation in fig2 multi pole permanent magnet 202 is attached to the wheel in close proximity to magnetic reed switch 202 switch 202 will periodically open and close as the magnets pass by thus alternately enabling and interrupting the current flow in connection 203 the rate of switch closure may be used to measure wheel rotation an alternate and very common sensor may also be constructed using a hall effect device in fig3 the hall effect device 304 is shown as a four terminal semiconductor device that will generate a signal on connection 303 that is proportional to the strength of the magnetic field 302 generated by permanent magnet 302 an implementation of a hall effect sensor to detect wheel rotation is shown in fig4 permanent magnet 402 may be attached to or is part of the wheel hall effect sensor 401 is in close proximity 403 from the rotating magnet 402 and will generate a signal.