ian wrote:
> just to clarify what I said about magnets, I've drawn a simple sketch:  
> http://www.iasmith.com/pickup.gif

You might crank the signal up a bit more by splitting that stack of 
magnets in half, and clamping or gluing them on opposite sides of an 
iron slug, with the magnets repelling each other at the attachment to 
the slug.  The slug should sit in the center of the coil.

The coil generates voltage in proportion to the flux that is passing 
radially through the coil, and this arrangement crowds a lot more of 
the flux in that direction.  It also makes the total magnetic field 
balanced, so that the Earth's magnetic field cannot push or pull on 
the magnets as it changes.  This reduces interference from things like 
solar storms.

Your present design wasts a lot of flux that passes out the hole in 
the coil and generates no signal.  Making the change I am suggesting 
might double your signal while reducing some sources of interference 
and not increasing resistive noise at all.
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