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From: ssave@caen.engin.umich.edu (Shailendra Anant Save)
Newsgroups: sci.electronics,rec.autos,rec.autos.tech
Subject: Digital Tachometer.
Keywords: Digital Tachometer Car Automobile Engine RPM
Message-ID: <1990Oct23.222654.25781@engin.umich.edu>
Date: 23 Oct 90 22:26:54 GMT
Sender: news@engin.umich.edu (CAEN Netnews)
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Distribution: usa
Organization: The University of Michigan, Ann Arbor
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    Has anyone built a digital tachometer from parts?

Here is what I have:
4 cylinder car. It has wires that go to the spark plugs.

What I want:
Some way of counting the RPM.

How I have thought of approaching this:
Method 1
  Wrap a coil of wire around the spark-plug wires and
get a voltage across the terminals which would be the
average DC voltage. Then DC voltmeter caliberated to
this, would give me the RPM. 

Problem: How linear is this?
         How accurate is this?
         How easy is it to caliberate the voltmeter?


Method 2
   Wrap a coil as above. Schmitt trigger the pulses into
square waves.  Gate and count the number of pulses in a
time (1/10 sec). Display the count.


Method 3
   The fan blades are such that there is a light path
between each blade. Use infra-red transmitter -- detector
pair and get pulses. Count as in 2.

  Anyone have any experience with these?  The thing is 
that I want digital. Analog ones available are about 
$35 each. I am hoping that I can make one that costs 
even less. 
   Please send any other suggestions to me.

				Thanks,
                                      --Shailendra
				ssave@caen.engin.umich.edu
				The University of Michigan
					Ann Arbor.