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Trying to adapt a fuel gage circuit

  1. Nov 22, 2009 #1
    Hi All.. Scratching my head here! does anyone have a circuit solution to a project I am working on?

    I have a WWII Fuel gauge out of a B-17 Bomber that will work on 12v DC, if I put 128K ohms inline the gauge reads "full" my Jeep fuel tank is 17 ohms full and 80 ohms empty. trying to create a circuit that will tie the meter into the tank sensor and read "E" when the tank is empty and "F" when its full! any ideas?

    Kendall ☺
  2. jcsd
  3. Nov 23, 2009 #2
    Probably going to need some data points inbetween full and empty to work that one out :)

    Perhaps something like this:

    Jeep B-17
    0% 80 ??
    10% ?? ??
    20% ?? ??
    30% ?? ??
    40% ?? ??
    50% ?? ??
    60% ?? ??
    70% ?? ??
    80% ?? ??
    90% ?? ??
    100% 17 128k
    Last edited: Nov 23, 2009
  4. Nov 23, 2009 #3
    Thanks daveg360,
    just cant think of a way to take one changing variable and creating another one!:uhh:
  5. Nov 23, 2009 #4

    You could do this with a very cheap microcontroller and a digital potentiometer.

    A simple circuit could measure the output of the potential divider made with the Jeep fuel tank sensor and R1 (in the diagram).

    The microcontroller could then calculate the resistance and output the appropriate resistance via the digital potentiometer which will be presented to the B17 gauge.

    This is just an idea. If you would like some more specific information send me a message.


    Attached Files:

  6. Nov 23, 2009 #5
    An op amp could solve this.
    With the inverting configuration, you need one pot to set offset - that's the baseline- and one pot to set gain - that's the range. That way you get perfect adjustment of the limits.
    LM386 is nice.
    http://www.national.com/mpf/LM/LM386.html#Overview [Broken]
    http://www.national.com/ds/LM/LM386.pdf [Broken]
    Last edited by a moderator: May 4, 2017
  7. Nov 23, 2009 #6
    Thanks BenchTop, I will give that a try! Sounds pretty simple.

  8. Nov 23, 2009 #7
    Benchtop,..New to this but is there somewhere I can find a detail application breakdown the LM386 or may I ask for some type of schematic from you?

  9. Nov 23, 2009 #8
    Thanks SpeedBird,
    I just got your message!!
  10. Nov 23, 2009 #9
    Hi SpeedBird.. any information you can offer would be greatly appreciated!

  11. Nov 23, 2009 #10
    http://img509.imageshack.us/img509/5784/invopamp.gif [Broken]

    There is a googleplex of sites with op amp tutorials for getting a basic understanding.
    Each company produces application notes for their devices showing lots of circuits.

    The example inverting op amp configuration, as shown, uses your sensor as an input resistor.
    R2 is a feedback resistor that sets what voltage is needed on the output to counteract the input current to make the - input of the op amp balance the reference on the + input of the op amp.

    If you test your output indicator (they are sometimes electromagnetic, sometimes electrothermal - it it's electrothermal it takes a while for a reading to become valid) you can find what voltage is needed to reach the upper limit.
    You may not need to adjust offset at all.
    You may need to add resistance in series with the sensor to keep currents low.
    You just have to stick the probes in and find out.
    Last edited by a moderator: May 4, 2017
  12. Nov 23, 2009 #11
    Hello BenchTop, Thanks so much for your help and direction.. I will do some good reading their!

    Kendall ☺
  13. Nov 24, 2009 #12
    Hello Again BenchTop, .. been reading on a wide varity of applications this little guy can handle! Found a chip that's offered by Radio Shack -

    TL082/TL082CP Wide Dual JFET Input Op Amp (8-Pin DIP)Catalog #276-1715

    Do you think this will work ok?

  14. Nov 24, 2009 #13
    You can usually find data sheets here:

    If your output indicator will work on about 20 mA or less, it looks like tl082 should work.
    If you have an unused op amp, tie the inputs to something so it doesn't oscillate.

    digikey, mouser, jameco = much better prices and selection than radio shack.
    Last edited by a moderator: Apr 24, 2017
  15. Nov 25, 2009 #14
    Thanks BeachTop, I'm familiar with digikey, I will give them a try. I should be ok on 20mA!
  16. Nov 26, 2009 #15
    Scopeman, fuel guages are linear. The only piece of information missing in your OP is at what inline resistance does your B-17 fuel guage read empty?

    Armed with that, a simple resistor bridge is the easiest solution.
  17. Nov 29, 2009 #16
    Hi mugaliens, this meter reads empty 0 ohms and full at 128k ohms using 12v the sensor in my tank is around 17 ohms full and about 80 ohms empty with 12v.
    just trying to keep this as simple as possible! your idea sounds good, do you have a diagram I can look at?

    Thanks, ☺
  18. Nov 29, 2009 #17
    To summarize:

    - Full: 17 ohms
    - Empty: 80 ohms

    B-17 Gauge:
    - Full: 128,000 ohms
    - Empty: 0 ohms

    This required a bit of research.


    Automobile fuel gauge sensors are floats connected to potentiometers. As the fuel level in the tank drops, the float drops, and the potentiometer increases in resistance.

    This checks with your Jeep fuel tank readings, but does not check with your B-17 gauge readings.

    More Background:

    Fuel sensors in large airplances use low-voltage capacitors, arranged vertically in the tank, and connected in parallel. In fact, there are 30 such capacitors in the Airbus A320. The fuel acts as a dielectric, so the circuit capacitance is proportional to the height of the fuel.

    Since fuel gauges are important, it's important to design them to be "fail-safe." If the gauge or sensor is broken, the gauge will read zero.

    In the case of your car, if the circuit is broken (infinate resistance) the gauge will read empty.

    Regardless, I think it's a mismatch between your B-17 gauge, which works on capacitance (I think), and your Jeep's fuel tank sensor, which works on resistance. If this is the case, a simple resistor bridge will not work.

    The reason I said "I think" is because I couldn't find any information to determine how the B-17's fuel tank sensor works. Fuel is a slight conductor, so for all I know, it could simply have two vertical wires. The deeper the fuel, the greater the conductances (less resistance). However, that doesn't match your gauge, which is reading backwords.

    Therefore, I suspect it uses capacitance, not resistance. If so, it's probably not a straight hookup, as there's no way for a simple DC signal emitted by the B-17 gauge to measure capacitance!

    This leaves us with one of two possibilities:

    1. The B-17 gauge includes its own circuitry to measure the capacitance of the tank sensor. However, I find this very unlikely, given what I know of avionics.

    2. I suspect there was a second device, either one at each tank or a central unit in the cockpit or cabin which translated the fuel sensor's capacitance into the 128k/0k resistance range the gauge is able to use.

    Before we proceed, we have to remember op-amps didn't exist back then! So the solution would involve a tube-based LRC circuit (the first practical application of the transistor didn't occur until 1947).

    But a transistor would work just fine. :smile:

    So, the problem boils down to the following LRC components:

    1. DC power supply: 12V

    2. Transistor

    3. Variable resistor: 17 ohms (full) and 80 ohms (empty)

    4. Gauge: 128k ohms (full) and 0 ohms (empty

    Task: Design a simple circuit using these four components such that when the variable resistor is at:

    A. 17 ohms the Gauge sees 128k ohms

    B. 80 ohms the Gauge sees 0 ohms

    It's ok to add extra resistors and capacitors, but no op-amps! They hadn't been invented yet!

    I'll play around with this, but I'm sure there will soon come along either an electrical engineer or a circuit tinkerer who'll whip out a schematic for you.

    ETA: Just thought of something: The B-17 tank could have used a simple float potentiometer, but with the readings reversed, such that it was an open circut (or at 128k, a nearly open circuit) when it was full, and a closed circuit when empty.
    Last edited: Nov 29, 2009
  19. Nov 29, 2009 #18
    Fuel quantity for airplanes in the era of the B17 used float type indicators made by liquidometer corp. They had a liquidometer float in the tank, however there were several different models, all having different arms and resistances. A manual adjustment was included for empty and full and sometime for a min fuel light. Adjusting the system with the right parts is hard enough, let alone trying to cobble together something.

    The gauge is designed to read below zero when power is off and is a wheatstone bridge circuit. BTW the B17 was a 24 volt system.
  20. Nov 30, 2009 #19
    Great info - thanks!

    As are all aircraft since somewhat before WWII, at least for the DC bus... :)
  21. Dec 2, 2009 #20
    Thanks guys,!! you can sure put some deep thought into this, but it makes sense and has to be looked at to this degree.
    The back of this fuel gauge has 2 post marked with a (+) and (C) I installed a car battery 12.44v and read 0.14A with no resistance, of course the meter pegged out. and no voltage/current = Empty and the needle has the fine adjustment screw on the face!
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