> From:         D. Schouten[SMTP:d.schouten@QUICKNET.NL]
> Sent:         Wednesday, May 19, 2004 3:59 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE]: how to extract rpm signal from car's dc outlet

> 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
> more advanced technique necessary like an FFT?

> Thanks!

> Daniel...

I've never heard of this, but I suspect that the problem will be getting any
kind of sizable signal. The presumption is that the AC output of the
alternator will put pulsations on the battery bus, but the amplitude
is unpredictable. You will have to determine empirically the relation
between the pulsation frequency and the RPM. Also, ignition noise will
have to be dealt with, since it may occur at a different mutiple of the
RPM. If there is a signal, the processing sequence would
be filtering, amplification, and extraction. For extraction, I would use
a phase locked loop. No FFT should be necessary.

John Power

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