I'm not sure what you are trying to do.
I can't quite understand why you'd need to measure dwell angle in an
onboard computer.
Dwell measurement is a diagnostic function, not a really useful
operational measurement.
It's pretty much like distributor advance. It's either right or wrong. If
it's drifting on you, you need
to replace or repair something. Once measured and set, it shouldn't vary
in a properly operating vehicle.

As for magnets, as I mentioned, you can get them free out of dead VCR's.
Go down the street on trash day, or
hit the local TV repair shop and ask for them. The main drum head motor
has both a tiny magnet, and a Hall Effect sensor.
If you want more than one magnet, get more than one VCR or drum motor
assembly. Otherwise buy a cheap Hall effect sensor(s)
and magnets. You can get them from places like www.wondermagnet.com or
northern tool, etc.

Otherwise, I guess if you _need_ to measure from the ignition, do like
the pro mechanics ignition diagnostic machines do.
Build an induction coil to slip over the coil primary wire. Wind it over
a AA battery wrapped in wax paper and epoxy each layer.


  ----- Original Message -----
  From: "Dumitru Stama"
  To: "Microcontroller discussion list - Public."
  Subject: Re[2]: [EE] Old cars - gadget
  Date: Sat, 2 Dec 2006 00:14:33 +0200



  My problem here are the magnets. I cannot find small magnets :(
  Also if i will do this i will not be able to calculate Dwell angle. I
  think i will try both approaches to see which one is more suitable.
  Thanks for the ideeas :)

  tec> Well, this is exactly what you want, no conversion. However, if
  it makes
  tec> you feel better. Glue two tiny magnets (or 4) and divide
  appropriately.
  tec> These Hall sensors are pretty reliable when setup properly.

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