Just took it for a test drive. The speaker turned out to be very useful. I took it to about 50 MPH and it worked fine, with one caveat. The waveform seems to be a bit screwy.
Judging by my ear-scilloscope, the waveform is something like this:
Whereas I think I want something more like this:
It would seem that my problem is that the hard drive magnet (or fraction therof) has a strong field on either side, but nut so much inbetween. The fields are both oriented so as to be able to trigger the switch every time one passes by.
This could be solved by just using the (bigger) magnet that was on the treadmill
Oh, BTW, the way I have the reed hooked up now is this:
+9V---reed---speaker---ground
(powered by a 9v battery for this test)