This is a block diagram of what you get when you buy a $1
Panasonic Hall sensor. See page 205 in the Oct December 99
Digi Key catalog.
 
These are tiny devices about 
0.18 by 0.18 by .08 inches thick. The wires coming out of it are
only .4 inches long. It looks like a small transistor.

This is called an open collector circuit as the output
transistor has no resistor load. So you need a 180 ohm
resistor. The place to put the resistor is back
at the instrument panel where the selector switch is mounted.
This reduces noise pickup in the wires but it is a good idea
to use shielded wires anyway. 

A nine volt battery will last almost forever.
This Hall effect device will work with voltages in the
range of 4.5 to 16 volts.

Digi Key has a $25 dollar minimum so buy a few of these 
in case you burn one up, the C&K selector switch for about
$5 bucks and a few resistors. Also buy the $2.50 booklet on
Panasonic Hall sensors. They will also sell you muffin
fans for one stop shopping.
 
Paul Lamar
 
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