> > sensors that count gear teeth
>
> Eh, most gear teeth counters I know of are differential

A device like the UGN3059 has two quadratic Hall sensors,
differential low level amp, BP filter and a Schmitt trigger o/p.
You need to set up a field with a small magnet, which the
ferrous object perturbs and the chip detects. As the chip
responds to changing differential fields from 0Hz upwards,
it doesn't matter if there is just one "tooth" (the screw). The
chip doesn't know that's all there is

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