magnetic field sensors - help requested

[Liana]

The setup:  I have a standard refrigerator magnet that a user will be able to move up and down across a sheet of perforated steel.  Underneath the perforated steel, I will need multiple sensors that will be able to sense the position of the magnet.  I have tested a reed switch and the following hall effect sensor( http://www.compel.ru/images/catalog/357/SS40A(50AT).pdf ) with this setup.  Both have worked unreliably. 

Please let me know of any solutions you may be able to think up and any possible errors you can perceive in the setup.  Thank you!

[dunk]

so the main problem i see with this setup is that the magnet will just attach it's self to the perforated steel, making the whole bit of steel magnetic, triggering all the sensors near the steel.
try using a non magnetic material in place of the steel. (aluminium?)

what are you trying to make anyway?

dunk.

[Liana]

The magnet needs to be attracted to the surface in order for the whole setup to work.  The steel itself is not magnetized.  You're saying the steel will still interfere with this all working correctly?  I have not had much experience with reed switches or hall effect sensors, but is there a way to have them work correctly through a sheet of steel or some other surface the magnets will be attracted to?

Overall, the user is making decisions based on where he/she places the magnet.  This all serves as the input.

Thank you for your help.

-Liana

[dunk]

You're saying the steel will still interfere with this all working correctly?

yup. that's it.

ever dropped a magnet into a pile of steel pins? when you pick it out the pins are not only attracted to the magnet but also to the other pins that are touching the magnet.

you will get similar results. (except you pin is bigger...) the reed switch will see the steel attached to the magnet as magnetic.
i'm not sure if this effect is localised to the area near the magnet or not...

try getting a cheep compas and testing the shape of the combined magnetic field of the magnet and steel.

dunk.

[Liana]

i will test that. i'm also thinking that a standard refrigerator magnet is not strong enough with this setup.  i will take a flux meter to measure the gauss with a rare earth magnet.  to my knowledge, there is no specific magnetic sensor to sense a magnet through a magnetic material.  do you know of any?  thanks again!

the specifics of what I'm making:
A 10" by 6" perforated steel plate.  Behind the plate, 5 leds, a little over 1" apart are arranged in a column.  Each led represents a certain target point.  The magnet that the user is able to move is on the other side of the perforated steel plate (it is important that there is an attraction b/t the magnet and the surface it's traveling on).  I want the sensor, whether a reed switch, a unipolar or omnipolar hall effect sensor (or other that I don't know of ) to be able to read whether the magnet is located over led 1 or led 2 or led 3, etc.  all reliably.