some fan circuit I pulled out of a computer fan. Linear Hall effect sensor is a 3pin U18 something, looks like open collector output. represented in the simulation by the clock and Q1. coil is bifilar wound and connected directly to +12V in opposing polarity. Q2/Q3 are some TO92 that I can't read. Q2 is biased on and gets pulled low by the HE. Q3 is driven from the output of Q2 directly. diode just looks like reverse polarity protection? simulation without the diode shows the coils want to pull current back up through the base junctions, so it's probably protecting against back EMF as well.
