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How to synchronize fuel level and LPG level gauge signals

Discussion in 'Automotive Electronics' started by milkoni, Apr 27, 2016.

  1. milkoni

    milkoni Member

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    Hi friends!
    I need help to make a circuit for synchronizing two signals - fuel level gauge and LPG level gauge in my car. I would like to have an information on the car Instrument Panel for fuel level when I'm driving on petrol and the LPG level when I'm driving on LPG.

    The problem is that the fuel level gauge have a resistance form 0 to 285 Ω as follows:
    10 Ohms -full
    55 Ω - 3/4 tank
    98 Ω - 1/2 tankΩ
    155 Ω - 1/4 tank
    285 Ω - empty

    The LPG level gauge have a resistance from 0 to 90 Ohms.
    0 Ω - empty
    22.5 Ω - 1/4
    45 Ω - 1/2
    67.5 Ω - 3/4
    90 Ω - full

    I'm attaching a principal circuit.

    Thanks for your help in advance!
     

    Attached Files:

  2. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    In one case you have a linear relationship and an inverse non-linear one. It seems like someone should make a universal solution at least the ability to adopt senders to existing gauges.

    In one system I knew of, there was a pulsing regulated voltage and a fluid filled ammeter. It's also why, the gauge doesn't bounce around and it takes a while to register when first turned on,

    Liability is the main reason, I'm sure people don't take to this project well.

    There's another way that's used an it also compensates for low battery voltage.

    Measurement of open circuit voltage and short circuit current and any waveform present helps to get started as well as if there are any "regulator modules" and/or multiple terminals on the gauge.

    I have been contemplating a similar issue, but not with fuel gauges.

    A microprocessor is likely the "way to go". Your problem is way simpler than mine.

    I think when you mean "LPG level gauge" and "fuel level gauge" you mean sending unit or simulated sender.

    So, in effect, you have to "measure the sender's" resistance and convert to a current source (voltage across a resistor) through a look-up table to drive the existing gauge.
    Unfortunately, you can't assume much.

    An Arduino would surely have the smarts to do so.
     
  3. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Do you want to use the petrol gauge to indicate either the petrol level or the LPG level?
    If so, then only the LPG signal needs to be modified, is that correct?
    It would help to know the resistance of the petrol gauge.
     
    Last edited: Apr 27, 2016
  4. dave

    Dave New Member

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  5. milkoni

    milkoni Member

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    Yes, I'll use the petrol level indicator to indicate both levels. You're right - only LPG signal needs to be modified.
     
  6. alec_t

    alec_t Well-Known Member Most Helpful Member

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    It would also help to know if that gauge is supplied with the car's nominal 12V or with a stabilised lower voltage, e.g. 5V, and whether the gauge is the old air-coil (crossed coil) type or the modern stepper-motor driven type.
     
  7. milkoni

    milkoni Member

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    The gauge is supplied with 5v stabilized voltage. The both gauges are old air-coil type.
     
  8. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    So, we still need to know the equivalent circuit of the gauge. Can you make the following measurements? Here is how to connect it up for the required tests:

    fu.gif

    Temporarily use a 2W pot to simulate the Gas sender resistance. Use the 10 Ω resistor to prevent ever shorting the gauge. Record the Voltage between Sender and Gnd, and the resistance of R1+R2 required to make the gauge read Empty, 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, 7/8, and Full.

    You sort of did that before, but knowing V(sender) that corresponds to the marks on the gauge is needed...
     
  9. shortbus=

    shortbus= Well-Known Member

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