Between the ask and the answer my unit came, no magnet in sight. ...
The magnets do need to be aligned with a specific end toward the sensor. I cannot tell one end from the other, just changed them until it worked.
The sensor head (if it's like the crankshaft sensor on my car) is a coil of wire, a soft iron core, and a magnet.
If yours sticks to a nail, it has a magnet inside, all it needs to sense is a tooth (or gap) in soft iron. Yes,
a magnet (if it's aligned with the one inside the sensor) will work, but really all it needs is to see
a bit of iron pass next to the tip (which causes an electrical upward blip) or a gap/hole in the
rotating iron next to the tip (which causes an electrical downward blip).
Sense heads are available separately, of course <
http://www.banggood.com/DC-6V36V-In...h-p-925067.html?rmmds=detail-left-hotproducts> in case one wishes
to experiment. These are, as I understand it, passive sensors (don't need power supply), but you'd never
know that from the descriptions.... Beware, there are Hall effect variants, the 'inductive' type
is what I'm describing (Hall effect needs power, probably 3-wire connection, inductive type is usually two-wire connection).
This kind of sensor is disturbed by nearby iron; it might sense moving iron bits other than
at the 'tip' region, and could become less sensitive if surrounded by steel.
