Dave Dudley
At 12:10 PM 16/01/98 EST, you wrote:

>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.
--------snip-----------

A raw hall sensor won't but try using one of the auto spec devices with the
magnet built in. The not produces a disturbance in the field which .....

Alternatively, you could use a reluctor sensor from an old Volvo or the
rear wheel speed sensor from a BMW K100 motorbike. This only requires a peg
in the flywheel (use 2 to keep balance) to trigger. These units have
inbuilt magnets, respond at low revs (eg, starting) and are super reliable
(can't kill it with dirt or oil). Watch the bi-polar pulse at revs. It bites!

Bill