On Sat, 03 May 2014 15:54:10 -0400, wrote:

On Fri, 2 May 2014 23:26:14 -0700 (PDT), whit3rd
wrote:

On Thursday, May 1, 2014 9:12:52 AM UTC-7, Tim Wescott wrote:
Problem statement:

Mount a small (nominally 3.2 x 1.6mm) NdFeB magnet in an aluminum model
airplane spinner backplate (for a tachometer).

Problem restatement: get a rotation indication using a Hall sensor.
So, put the magnet on the fuselage, the Hall sensor between the magnet
and the backplate, and drill/tap the spinning part for a small
ferrous screw. When the screw nears the field, it will cause
a spike in the magnetic field (as it becomes magnetized). Works
with a seek coil, too.

Loctite will hold the threaded part.
Much simpler to use variable reactance sensor - fine coil around a
magnet, makes signal when ferous screw passes by.
Male that variable "reluctance"