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Date: Thu, 2 Oct 2008 20:32:17 -0600
From: Bob White <bob@bob-white.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: How to use an automotive tach with Tracy's EC2 (or EC3)
Message-Id: <20081002203217.4dc60b62.bob@bob-white.com>
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John Downing contacted me several months ago asking if there was a way
to use an automotive tach with his EC2.  I built a circuit that was
triggered from the rotor 1 and rotor 2 primary EFI signals (Tracy uses
one of them to feed rpm data to the EM2).  The output was conditioned
to provide an output pulse of constant width each time either rotor
primary fuel injector was triggered.

If you have been following John's messages to this list, you will know
that it didn't work.  Even though the tach is connected to the low
voltage side of the coil, it use the higher voltage spike that is
generated there when the coil field collapses.  There may be tachs out
there that aren't set up that way because of the prevalence of
computer controlled autos, but this type of tach is still common.

After some web searching, I found a web site where someone had
modified his tach to operate from a low voltage signal on a Prius.
see http://www.techno-fandom.org/~hobbit/cars/tachack/

John's tach looks exactly like the one described in that web site, but
there were some differences in the circuitry.  For one thing, the
input signal first went through a diode, then there were a couple of
capacitors.  None of these were on the circuit described on this
site.  Also, the resistor values were different throughout.  Much of
the circuitry was similar however.

Using this information as a guide, I made the following changes to
John's tach.

1) Reduced the voltage divider ratio as described on the web page.
2) Bypassed the input diode.  The tach wouldn't respond to the low
   voltage pulse through the diode.
3) Doubled the value of the timing capacitor.  This mod was needed
   to get the correct reading with only one pulse per revolution.  The
   typical 4 cylinder engine outputs two pulses per revolution but it's
   much more convenient to use one pulse per revolution with the EC2.
4) Adjusted the calibration pot to read 6000 rpm with 1 pulse per 10
   ms. (100 pulses per second = 6000 pulses per minute.)

I will also use a simple opamp to double the 5V signal from the EC2 to
10V.  There is probably some way to get it to trigger down to 5 volts,
but after trying several mods I couldn't find it. Changing the voltage
divider ratio didn't do it (much to my surprise).  After all changes,
the tach quit working as soon as I hit 6V, so instead of fighting it
I'll just add the opamp.  The opamp will also provide a higher level of
isolation between the tach and the coil trigger signal.

The web site does an excellent job of explaining the theory and
circuit operation for this type of tach.  I haven't given specific
values because the values needed will depend on the specific design
of the tach used.

Tracy uses the injector signal to measure rpm with the EM2, but the coil
signal will be used to trigger this tach because the injector signal
changes pulse width as the engine load changes.  If the pulse duty
cycle gets too long, it could cause an error in the tach reading.

FWIW, and YMMV

Bob W.

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