Messy top-posting fixed. I *think* I got the attributions right.

tachyon_1@email.com wrote:
> Alan B. Pearce wrote:
> > > I think you'd be way better off epoxying a small magnet to your
> > > harmonic balancer and mounting a Hall Effect sensor to read it.
> > 
> > Hmm, only give one pulse per revolution. I would look at something
> > that would sense the starter ring gear teeth - lots of pulses per rev.
>
> Well, this is exactly what you want, no conversion. However, if it makes
> you feel better. Glue two tiny magnets (or 4) and divide appropriately.
> These Hall sensors are pretty reliable when setup properly.

Real engine (and ABS) sensors use the variable reluctance principle.
A U-shaped magnet (really a small magnet embedded in some steel pole
pieces) is placed next to the starter gear so that the pole points
line up with two of the gear teeth at once. A coil of wire wrapped
around one of the poles develops an AC signal as the magnetic flux
varies with the spacing of the poles to the metal of the starter gear.
You get a pulse per tooth *without* modifying any moving parts in any
way, and only one magnet is required.

Some ASCII art:

                 ____________
        pole    /            \
        piece  /  __________  \
             _|  |_         |  |
            (______)        |  |
            +------+      +------+
            |   N  |      |      |==============---
            |      |      |      |
            |      |      |      |
            |magnet|      | coil |                signal
            |      |      |      |
            |      |      |      |
            |   S  |      |      |==============---
            +------+      +------+
            (_    _)        |  |
        pole  |  |          |  |
        piece \  |          |  /
               \_|          |_/
                __    __    __    __
             __/  \__/  \__/  \__/  \_

               ============>
            starter gear teeth motion

-- Dave Tweed
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