Dave,

There is a common type of Hall sensor that *will* detect gaps in an iron or
steel rotor, e.g. a gear. The sensor assembly includes a permanent magnet to
bias the Hall device.

Mike Hardwick, for Decade Engineering
In Oregon, Land of Magnificent Stumps
decade@worldnet.att.net ~ www.decadenet.com

>A hall effect sensor won't detect a disk with a notch cut out of it.  An
>optical interupter sensor will.  Hall effect sensors detect magnetic fields.
>You could simply strap a good rare earth magnet to the crank shaft and detect
>that with a Hall sensor.  They do that for auto trip computers.  I designed
>one about five years ago.  We had a magnet held to the drive shaft by two
>cable ties.  Worked OK.