(word processor parameters LM=8, RM=75, TM=2, BM=2)
                      Taken from KeelyNet BBS (214) 324-3501
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                                   May 16, 1993

                                   MAGCHECK.ASC
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            This file shared with KeelyNet courtesy of Mark Mansfield.
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       Anyone need a way to check magnetic fields?  Well here's a circuit
       that will allow you to do just that!  It is simple, low cost, and
       if calibrated can be very accurate.

       It will distinguish  between north and south poles.  It does this by
       outputing a voltage ABOVE 1/2 supply  voltage  for  north  poles and
       outputing a voltage BELOW 1/2 supply voltage for south poles.  This
       causes the meter to either swing above center zero for north poles
       and below zero for south poles.

       The Hall effect sensor has a side that is the measure  side  and  as
       long as you have it toward the magnet it will tell you which pole it
       is and its relative strength.

       If you want to calibrate the instrument for gauss field reading, you
       will have to  come up with either a calibrated magnet or build you a
       circuit to do the calibration.   That's  beyond  the  scope  of this
       paper.  If you need such a circuit, let me know and  I'll send you a
       copy of it.

       The construction is straight forward and does not require a circuit
       board.  Just use  point-to-point  wiring.   Anyone  with electronics
       background should have no problems building it.

       I mounted my sensor on a piece of  2 conductor shielded wire about 3
       feet long.  This  allows  me  to move the sensor around  freely.   I
       mounted the components is a small aluminum box.  I used a small wall
       transformer to supply the power for the circuit although it could be
       battery powered just  as easily.  No big tricks or surprises in this
       circuit.  Just simple and works!













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                                                                  + 12V
                                                                  | DC
                                                                 _|_
                                                                |   | 1000
                                                                |   | ohm
                + 12Volts DC                                    |___| res.
                |                                                 |
             ___|_____           CAL. Pot.         _             _|_
           | Hall     |         ______           /   \          |   |
           | Effect   | _______| 10K  |_________/     \________>|   | 200
           | Sensor   |    |   |______|         \     /         |___| ohm
           |__________|    |       ^             \ _ /            |   pot.
                |          |_______|                             _|_
                |                               50 uA           |   | 1000
              /////                             Meter           |   | ohm
                                             zero center        |___| res.
            UGN3501U                                              |
            SEE NOTE                                              |
             BELOW                                                |
                                                                /////

       SPECIAL NOTE: The  value of the bottom 1000 ohm resistor may have to
                     be changed slightly  from the 1000 ohms.  I had to add
                     200 ohms to it so my zero control was  in  its  center
                     position when the meter was in the center position.

       Parts List:

              (1)  UGN3501U   Hall  Effect  sensor  (Available  from Newark
                   Electronics)
              (1)  10K ohm pot. for calibration
              (1)  200 ohm panel mount pot. for meter zeroing
              (2)  1000 ohm 1% metal film resistors, 1/4 watt
              (1)  50 microamp meter, preferably zero center
              (1)  regulated 12 Volt DC power source, 50 mA or more.

       Note:  If you have trouble finding the Hall Effect sensor, drop me a
              note and I'll try to help you out.

                                           Have fun!!!!
                                           Mark Mansfield
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         If you have comments or other information  relating to such topics
         as  this paper covers,  please  upload to KeelyNet  or send to the
           Vangard  Sciences  address  as  listed  on the  first  page.
              Thank you for your consideration, interest and support.

           Jerry W. Decker.........Ron Barker...........Chuck Henderson
                             Vangard Sciences/KeelyNet

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