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Controlling LED lights by differant ohms reading.

Discussion in 'LEDs and Optoelectronics' started by bbarnumboy, May 26, 2011.

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  1. bbarnumboy

    bbarnumboy

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    May 26, 2011
    first of all this is my first post so hello. Looking for some insight and help on a custom setup.

    I have a project which uses 8 LED lights, each running off different ohms reading.

    The lights will indicate a empty and full appearance by color from left to right. My plan is to have all of the lights running when the source reads full ohm range. and turning off as the source gets empty.

    As the level drops down, the ohms range will change, in hopes of turning off lights as it decreases.

    Think of a fuel tank in a car. Reading 110 ohms at empty and 3 ohms at full, from the sending unit.
    when full at 3 ohms i want all of the Leds illuminated, and to turn off as the ohms climb, ending in 110 ohms for the last light.

    So here is my question. How would i go about doing this? I am a noob at this and I am having a hard time with an easy way to set this up.

    The signal is a 1 wire source and i know i will need resistors, but how do i control when the lights come on and off?

    if anyone can help with this it would be appreciated. My project is on hold until i can figure this out.
     
  2. poor mystic

    poor mystic

    1,074
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    Apr 8, 2011
    Hi bbarnumboy (smile)
    It'd be easier to be sure how to go about this if you let us know exactly what it is you are measuring, since there are lots of ways to measure resistance (in Ohms) and different techniques apply in different situations.
     
  3. bbarnumboy

    bbarnumboy

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    May 26, 2011
    Sorry i had this worded differently for another forum which was no help to me at all


    I am deleting the stock fuel gauge from the gauge cluster in my vehicle.

    Instead of a new gauge, i would like to make a fuel level meter using a few LED lights in the dash. I would like them to all be lit up when full, and as the tank goes down (lets say 1/8th) i would like a light to turn off, and so on until all lights are off meaning the tank is empty.

    The ohms range for the sending unit in my gas tank reads 110 ohms when empty and reads 3 ohms when Full. all other ranges can be determined through math i would assume to find where the intervals are at between the ohm range listed above.

    The source is a standard 12V Auto power source.

    Hope this made it easier to understand. Thanks for any help guys. I am stumped :confused:
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    One way would be to pass a current through the sensor then use the resulting voltage to drive the input of an LM3914.

    In fact there seem to be quite a number of people who use these things to do exactly what you want to do.
     
  5. bbarnumboy

    bbarnumboy

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    0
    May 26, 2011
    Interesting, do you know of any meter such as that, in a very small size? i have limited room
     
  6. bbarnumboy

    bbarnumboy

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    May 26, 2011
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    No, that one is a VU meter and has a logarithmic response.

    You want one designed around the 3914.
     
  8. bbarnumboy

    bbarnumboy

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    May 26, 2011

    can you explain how i could easily do this?since the sending unit is in the gas tank, can i do this from outside the tank?
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Easy. The concept is that you connect a resistor to your battery,connect that to the sender and the other side of the sender is grounded. As the amount of fuel in the tank varies, the voltage between the resistor and the sender varies. This is translated by a LM3914 into one of the LEDs.

    Practically, you need to determine a suitable series resistor, and you need to set the input of the LM3914 so that the empty and full indications are correct. You may find some non-linearity in the readings.

    An alternative is to pass a constant current through the sender (a resistor does this poorly) which may result in a more linear response.

    Of course, you don't want to (in either case) send enough current into the sender that you could risk a source of ignition. I think this almost goes without saying.
     
  10. bbarnumboy

    bbarnumboy

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    May 26, 2011
    well the sending unit should already have a source of current from the battery to power it in the first place, correct?

    I may be able to just tap into the current thatis already leading to the sender, and use that as the current.
     
  11. bbarnumboy

    bbarnumboy

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    0
    May 26, 2011
    disregard that last post. i found some really good info and i think i understand now.

    my last question is about this LM3914

    is this a full build board with LED's on it that i can do this with? or is the LM3914 just a chip and i have to build a board using it?

    if it is a full unit, do you know where i can purchase one.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    An LM3914 is an IC.

    Try googling for "LM3914 kit" or similar (try ebay too) and you will find people who sell this IC, boards, parts, and even the odd kit to do something.

    The problem is that you will have to determine the range of voltages you will see from your fuel gauge, then determine how to use this with an LM3914. The datasheet will be of help here, but you may have trouble understanding it.

    If you can find a kit for an expanded scale voltmeter, that may be able to be modified to suit (it will have the ability to set the upper and lower voltage, although you may have to tweak the circuit to get the range you require).
     
  13. bbarnumboy

    bbarnumboy

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    May 26, 2011
    ok so i ordered up a kit and have everything to make this work, the only thing i cannot figure out is determining the resistor size to get it to read my range of 103 ohms to 3 ohms? any thoughts
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, is it 3 ohms when full, or 3 ohms when empty?
     
  15. bbarnumboy

    bbarnumboy

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    May 26, 2011
    sorry i was out of town and i appreciate all of your help with this. it is not really my area of expertise. so it is running a 12v system and should be 102-118 ohms when empty and 1-5 ohms when full. These are the ranges from the manufacturer, so safe to say 3 at empty and 103 at full.
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, if the LM3914 is set for a 1.2V reference, then you want the sender to produce a voltage of roughly 0 to 1.2V.

    so if we were to send a current of 1.2/103 A (between 11 and 12 mA) the voltage would vary between close to 0 and 1.2V. The only problem is that the gauge would read backward.

    Maybe someone can come up with a simple way of making it work the right way around. I guess you could reverse the high and low voltage references on the LM3914, but I've not tried that trick and it does not appear to be mentioned in the datasheet.
     
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