Tundra10
Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario
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 Posted: Wed Apr 27, 2011 12:09 pm Post subject: VM1000C and Capacitive Fuel Senders |
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A datapoint for the archives, in case someone else runs into this.
I helped a friend solve a problem with his Vision Microsystems VM1000C
and VMC capacitance probes.
Before I was involved, one tank had been mis-wired and then corrected.
That probe no longer provides a signal, so my conclusion was that
the electronics in the probe are not reverse polarity protected. This
is a sad state of affairs, since a repair would involve removing the
wing.
The fuel system in this airplane has both wing tanks plumbed together,
and generally an indication derived from one tank is sufficient, along
with a separate indication for the header tank. Fortunately, a probe
had been installed but never wired up in the other wing tank, so we
could use that.
I used an oscilloscope to confirm a 5V square wave output from the
probes when powered by the 5V supply of the VM1000C. I used a signal
generator to confirm the VM1000C operated.
The maximum frequency the VM1000C would handle was just above 14KHz.
The header tank probe is very short, perhaps a foot and when empty,
and generated about 11KHz empty. Frequency decreases as fuel is
added, so there is no problem with the frequency being out of range of
the VM1000C.
So with a 5V square wave being generated by the probe, and the VM1000C
confirmed to accept it, everything should work, right ? One might be
forgiven if they assumed that both products, offered by the same
company might be compatible. Sadly wrong.
The VM1000C put too much load on the signal from the probes, and
converted it into a 1 volt peak-peak triangularly shaped wave 
JPI now owns the Vision Microsystems product line, and provided very
limited support. Although the tech seemed to want to be helpful, he
did not have access to the schematics or any knowledge of the input
characteristics of the VM1000C. I am quite sure this was not the
first non-functional installation, yet he had no knowledge of these
issues. Before I was involved, the owner sent his VM1000C in for
testing. The tech indicated that the unit would have been connected
to the same quality test equipment as new units off the production
line. He indicated that a number of wiring harnesses had been shipped
that were incorrectly wired, and that new wiring harnesses have the
same error. Although a new harness was promised to be shipped when
the unit went for testing, it was not included, so I was not able to
confirm this. I do not know if the harness in the airplane was
provided by VMC or fabricated. The tech is at a different location
than the factory and seemed isolated. For example, he could not look
at a harness to identify if it was correct or not.
My solution was buffer the signal with a transistor, and everything
works fine.
If anyone needs to do this, what worked well is the following (repeat
for each fuel tank). Resistor values were optimized by observing the
resulting waveform with the oscilloscope.
The drawing is crude, so the description is:
Connect the signal from the fuel probe to the base of the transistor
with a 2.7K resistor. Connect the collector of the transistor as the
signal input to the VM1000C. The collector is also pulled up to +5V
with a 1K resistor.
The emitter is connected to the ground.
PROBE VM1000C
Red +5V ------------------------------------------------- +5V Pin 11 or 12
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#
# 1K resistor
#
|--------------- Fuel Signal 9 or 10
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/ C
2.7K resistor |
White Fuel Signal ------####----------B| 2N4401 NPN transistor
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V E
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Black Ground -------------------------------------------- Ground Pin 3
Jeff Page
Dream Aircraft Tundra #10
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