My car has a diagnostics port under the bonnet that outputs a beautifully
clean square wave with pulse width inversely proportional to RPM...

The same signal is also available at some handy screw terminals on the back
of the instrument panel. It also has outputs which represent the sine and
cosine of the speedometer and tachometer needles. Very handy.

It's a reasonably new car though, yor car may or may not have something like
it. If it doesn't I've had moderate success picking up an RPM signal from
the coil on my old (1975) car. Ber careful though as the voltages are in the
thousands...

-----Original Message-----
From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU] On
Behalf Of SHands
Sent: Thursday, 20 May 2004 8:56 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: how to extract rpm signal from car's dc outlet


I agree. It might be a red-herring, (and down to my crappy old car), but
have you tried a 'click' counter? I'm sure we've all heard the interference
from the sparks generated by cars on 'quiet' AM/LW radio stations? As the
rev's go up, so does the pauses between clicks.

Just a thought...

s

-----Original Message-----
From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU] On
Behalf Of Mike105105@AOL.COM
Sent: 19 May 2004 23:46
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: how to extract rpm signal from car's dc outlet


Actually the way to read the rpm is by measuring the ignition noise not the
alternator noise.


Mike Hillpot

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