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Date: Tue, 07 Jun 2005 09:08:36 -0400
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From: Jeff Peterson <jeffreyb.peterson@gmail.com>
Reply-To: jbp@cmu.edu
Subject: capactiance fuel gauge interface.
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LML:

I extended the wing tanks on my 360 and took the opportunity to
install a homebuilt capacitacne level sensor. I did this because the
commercial sensors are expensive, too
short and difficult to tread into the extended region of the tank. Mine is
simple, just a length of 1/4 inch tubing with holes every six inches, and a
wire thin teflon insulated wire suspeneded near the center. It has 133
pF capacitance empty and about twice that full.

Now I need to build a circuit to read the capacitance and pass along the
correct signal to some filght instrument computer. I have been told
that these computer systems take a TTL square wave at about 3000 Hz, with=
=20
a frequency carrying the level info, and that the computer gets
programmed during setup
with the empty and full levels.  so, the precise frequency for empty
or full is not
critical.  I breadboarded up a TLC555 circuit today and it seemed stable.

I have not chosen the computer.  Mabye it will be EDM 900...or a
Chelton screen...or
whatever seems nice when I get around to the avionics.

I hope the group can help by answering a few questions:

-I am correct that these systems use a 3000 Hz TTL square wave?
-Is there a published (or defacto) standard?
-Can I buy the circuit I need? I would consider paying a few hundred,
but considering that I have spent JUST $1.49 at radio shack so far,
$1000 seems rather steep.

Thanks.

--=20
Jeff Peterson