It is (almost) that simple, some things to keep in mind are:
1. The alternator is a 3-phase bridge-rectified device (6 pulses per 1
alternator revolution).
2. The pulleys determine the ratio of engine RPM to alternator RPM.
3. During coasting the RPMs may drop far enough that a fully charged battery
doesn't show ripple.
4. The higher RPMs will have very high frequency, and very short ripple
peaks.
5. State of battery charge will have an effect on the amplitude of the
pulses.

KF4HAZ - Lonnie

----- From: "D. Schouten" <d.schouten@

> Hi all,
>
> I want to build an rpm meter for my car with an acoustical rev limit
signal.
> I have heard that you could quite easily extract the engine's rpm from the
> AC component riding on the DC of the cigarette lighter plug/outlet. As a
> matter of fact, there are some commercially available aftermarket in-car
> display units which can do the same from the DC outlet. However, these
units
> are too extensive for my application.
>
> Has anybody have any experience with this kind of application?
>
> Is it just a matter of AC coupling the car outlet's ripple to some kind of
> comparator before feeding it to the PIC for further processing? Or is
there
> a more advanced technique necessary like an FFT?
>
> Thanks!
>
> Daniel...

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