Alternative to HALL sensor.

http://www.eetindia.co.in/ARTICLES/2002DEC/A/2002DEC11_AMD_AN08.PDF?SOURCES=DOWNLOAD

The 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