[alfa] Coil negative terminal waveform

[Joe Elliott <jee@xxxxxxxxxxxxxx>]

I have a fairly open-ended assignment in one of my classes where I 
need to design a simple circuit with a microcontroller.  I'm 
considering making a tachometer.  (Yes, I know this can be done 
without a microcontroller, but that's not the point.)  I know that 
electronic tachs get their input from the negative terminal of the 
ignition coil, but I don't know what they're actually measuring. 
Frequency of some wave?  If so, is it some kind of waveform that 
could be replicated in the lab with a signal generator?

Thanks,
Joe
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