What about either an optocpoupler and/or a few turns around the coil or 
a sparkplug wire. There used to be (still are) timing lights, dwell 
meters, and tach's that used stuff like that. Also airplane piston 
engines use a hall effect sensor layoing on the aluminum case of the 
magneto for tach sensor. I'm not a techie in this area, maybe someone 
else can pursue the details. :)

David VanHorn wrote:
> National, and I'm sure other vendors make linear regulators that are
> designed for automotive use.
> The standard 78XX or 340T-X regulators won't handle it reliably.
>
>
> You will need a much more robust solution for that input.
> The voltage across the points can be momentarily as high as a few hundred
> volts, and can go negative.
>   
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