I have used the Honeywell SS345PT. It looks very similar
to the one Grumpy_Mike suggested.
I have an example sketch at
http://wiblocks.luciani.org/docs/app-notes/pico1trc-hall-effect.htmlWhenever the magnetic field changes a pin change interrupt
routine is called. When the main program detects a change
the time is read from the real-time clock and is output to the screen.
(* jcl *)
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