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LM3915 as LED fuel gauge?

Discussion in 'Electronic Basics' started by Glenn Ashmore, Feb 2, 2005.

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  1. I want toadd a set of gauges to my fuel management system. It will require
    one for each tank and 3 standard gauges take up to much room and cost a
    bunch so I am going to DIY it. Follow me through on this:

    The sender is a variable resistor with a range of 240 Ohms empty to 33 Ohms
    full. If I put a 240 ohm resistor in series with the sender, feed it 3.3V
    and measure the voltage across the 240 Ohm resistor I will get a voltage
    from 1.65V empty to 2.9V full. That is a range of 1.25V in a more or less
    logarithmic curve. If I feed this voltage to an LM3915 LED driver I can
    display the tank levels on a 10 segment LED array.

    There is only one problem. How do I set the zero to 1.65V?

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  2. Use an opamp to subtract the 1.65V. An old CA3140 will do.

    VCC
    +
    |
    +-+-------+----------------+
    | | |
    .-. .-. | ___1k
    | |33- | |10k +-----|___|-+
    | |240 | | | | |
    '-' '-' | | |
    | | 1k ___ | |\| |
    | +-----|___|--+-|-\ | out
    | | | >------+----
    +----------------------|+/ CA3140
    | | |/|
    | | |
    .-. .-. |
    | |240 | |10k |
    | | | | |
    '-' '-' |
    | | |
    ++-------+----------------+
    |
    ===
    GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    If your environment is noisy you can reduce the 10k resistors and/or put an
    elco 1-10uF parallel to the lower 10k. You can make a zero adjust with a
    100-220 Ohm trimpot between the two 10k resistors, its tab to the 1k input
    resistor of the opamp.

    petrus bitbyter
     
  3. Thanks. That should work and not be effected by minor supply voltage
    variations.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
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