Alternative to HALL sensor.
http://www.eetindia.co.in/ARTICLES/2002DEC/A/2002DEC11_AMD_AN08.PDF?SOURCES=DOWNLOADThe advantage of this magnetoresistive sensor is that is not affected by near metal parts,
and work well with ferrite magnets.
Cons - cost more than HALL sensor, since need a amplifier, build a PCB...
Sokol1