Date   : Thu, 11 Oct 1984 09:59:25 EDT
From   : David Towson (SECAD) <towson@Amsaa.ARPA>
Subject: Re: Help with Hall-Effect

Hal - I have disassembled a couple Hall-effect keyswitches, and have found
inside each an integrated circuit chip mounted with its plane vertical.  Four
connections protrude from the bottom of each chip assembly.  These connections
allow each chip assembly to be soldered to a mother-board which underlies the
keyswitch matrix.  The mechanical portion of each keyswitch consists of a
magnet mounted on a plunger, and a restoring-spring.  When the plunger is
depressed, the magnet is moved down next to the IC chip assembly, thereby
subjecting the chip to a magnetic field.  In the case of the switches I have
disassembled, the magnet assembly is U-shaped, and actually surrounds the chip
when the plunger is depressed.  The presence of a magnetic field causes the
chip to establish an electrical connection.  The following is excerpted from
a Micro Switch "product sheet", and gives a bit more description.

     "The 12SW and 16SW Series use the revolutionary Hall effect solid state
switch pioneered and developed by MICRO SWITCH.  Solid state switches eliminate
the problems associated with mechanical switching...contact bounce, contamin-
ation, and intermittent switching at low levels.

     The switching function is performed within an integrated circuit 0.040
inch square.  The chip is actuated by a magnet to produce an analog control
voltage.  Hall voltage is converted into a digital output with a trigger
circuit and amplifier.  Each key module provides two isolated outputs."

     This little blurb is for numeric keypad assemblies 12SW and 16SW, having
12 and 16 switches respectively.  They operate from a +5-volt supply, and draw
60 and 80 MA max respectively, inplying a drain of 5 MA per keyswitch.  Each
output (there are two per keyswitch) puts out a minimum of +2.4 volts while
sourcing 160 microamps with the switch DE-ACTIVATED.  In the ACTIVATED state,
each output can sink 3.2 MA with a maximum voltage drop of +0.4 volts.  This
is obviously intended to interface with TTL circuits.

     I have "product sheets" for the following Micro Switch keyboards:
       53SW1-1
       61SW12-1
       50SW11-50
       67SW5-17
       66SW6-51
       66SW6-52
       78SW6-44
       63SW5-4
       70SW12-1
       75SW12 series
       51SD12-1
       51SW12-1
If any of these numbers match your keyboard, let me know and I'll give you
whatever additional information I have.  Don't expect much, however, as
there is little technical data in these "product sheets".


Best regards,

Dave
towson@amsaa.arpa