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Use a bi-color LED as an ammeter?

Discussion in 'Electronic Design' started by [email protected], Dec 30, 2004.

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  1. Guest

    What do you think of this idea to make a simple ammeter for my
    motorcycle - Tap into the positive lead in the battery in two spots, so
    the battery cable itself is like a shunt resistor. Run wires from
    these two spots to a bi-color LED mounted on the dash (and have a
    current limiting resistor in-line). The LED glows green when the
    battery is charging, and red when it's discharging because I have too
    many accessories turned on.

    Would this work? Crude ascii below.


    |---LED---|
    | |
    | *<-Resistor
    | |
    ----|+ ---------------------
    B |
    A |
    T |- ---GROUND
    ----|

    Thanks,
    -Ryan
     
  2. JeffM

    JeffM Guest

    he battery cable itself is like a shunt resistor
    No.
    A LED requires ~1.6V to light.

    Use Notepad to draw your ASCII prints
    then cut & paste them into your browser.
    A monospace font is required (Courier font ).
     
  3. Guest

    Yes but they are wired into 12v sources all the time with the use of a
    resistor. Are you saying the LED would not get enough power to light
    up?

    -Ryan
     
  4. Tim Wescott

    Tim Wescott Guest

    Perhaps for _your_ motorcycle. The most common color deficiency
    experienced by humans, which occurs most often in men and boys, is an
    inability to distinguish green, red and gray.

    So you can do this (after adding a suitable transresistance amplifier to
    address the 1.6V threshold problem), but if you loan your motorcycle to
    me I won't be able to tell charge from discharge.
     
  5. Guest

    The resistance of the battery cable is so low that the voltage drop
    across it will never be enough for the LED to illuminate.
     
  6. Tim Wescott

    Tim Wescott Guest

    Not to mention the fact that the current vs. voltage characteristic of
    any junction diode is exponential, so you want to drive the LED with a
    current proportional to the battery current, _not_ a voltage
    proportional to battery current.

    You _could_ sense the voltage in the manner stated, then amplify it with
    a transresistance amplifier that'll drive the LED with a controlled
    current that depends on your sense voltage -- but you'll need split
    grounds and a good amplifier and all that fun stuff. It's doable, but
    see my other post about the suitability of multi-color LED's for
    colorblind folk.
     
  7. mc

    mc Guest

    I don't think you want to have 2 volts of drop across your battery cable.
    That would be a terribly inefficient battery cable.
     
  8. I read in sci.electronics.design that Tim Wescott
    I have worked with a number of red-green blind people who had no
    problems with resistor colour codes. Can you really not tell a red LED
    from a green one? Not even as different shades of what you perceive as
    'yellow'?
     
  9. JeffM

    JeffM Guest

    The LED glows green when the battery is charging,
    That would be pretty radical,
    but I have seen techs who put in a 1M resistor
    where a 10M was supposed to go.
     
  10. Bob Stephens

    Bob Stephens Guest

    I went to school with a guy who was red-green colorblind. He bribed me with
    a 6 pack to go through his resistor collection and hand him five of each
    value. He would go to labs with a piece of styrofoam with rows of carefully
    labelled resistors stuck in it. Without that, he was helpless.

    Bob
     
  11. Green/blue problem is a very unusual defect. 15 ohms is much easier.
    Crossed eyes. (;-)

    Purple/brown can be difficult, especially under incandescent light. But
    it doesn't normally cause problems with resistors, because 2.1 and 4.1
    aren't preferred values. 75 ohms/15 ohms is a possible confusion.
     
  12. Maybe that's foveal tritanopia - where the very center of vision is weak
    on blue. The brain usually manages to fudge things other than the
    brightness of really pure deep blues since the perceived brightness of
    most blue things is from stimulation of green receptors and maybe red
    ones. But blue bands of resistors, especially 1/4 watt ones, all too
    often look like green bands.

    I have heard of it. I do not know what causes it or if this is
    something normal.

    - Don Klipstein ()
     
  13. Tim Wescott

    Tim Wescott Guest

    I'm more or less OK with color codes if it's a 5% type. Four-band color
    codes throw me off completely. Even so I always check (and I have a
    well-organized set of parts drawers) I can usually get close enough with
    an ohmmeter, even in circuit -- and if not I have my wife and children
    trained.

    And no, I can't tell the difference between green and red on a bi-color
    LED -- this absolutely drives me up the wall when some clever SOB
    designs it in, like my Yaesu FT-23 where red means "squelch" and green
    means "transmit" (or visa versa).
     
  14. Tim Wescott

    Tim Wescott Guest

    One of them -- and I think it was the green/blue deficiency -- was very
    highly prized for infantrymen during WWII because they tended to see
    camouflage easier. Ordinary folk would be so distracted by the color
    match that they wouldn't notice the tones being slightly off, the folks
    with this deficiency would then be able to see the mismatch in the pattern.
     
  15. Mac

    Mac Guest

    Here is the relevant context from JeffM's post:

    You replied:
    OK, Ryan, you are confusing me, and probably others as well. Can you
    please draw a schematic of exactly how you intend to connect these LED's
    to your electrical system?

    The schematic in your original post didn't make any sense. The
    written description makes sense but won't work for the reason JeffM
    expressed above.

    The bottom line answer is that you can sense whether your battery is being
    charged or discharged, and light up LED's in response to the direction of
    charge, but doing so will involve more than just LED's and resistors.
    Also, it may or may not be practical to use a length of cable as the shunt
    resistor in this application.

    --Mac
     
  16. Kryten

    Kryten Guest

    I concur with other posters that the circuit doesn't look plausible
    whichever way I try to allow for ASCII character shifting.


    No that definitely won't work.

    Regarding coloured LEDs, about 10% of males are red/green colour blind.
    Actually, 'blind' seems a bit of a misnomer.
    One of that 10% said he found it hard with dark greens and reds but bright
    ones were okay.
    Everyone fades into monochromatic vision at low intensities, so I would
    guess that such people just have a brighter threshold for those colours.

    10% is a significant percentage of chaps so I always add extra stimuli.
    Such as steady green = okay, flashing red = warning.


    In an application such as described, I would have two separate LEDs.

    You can get triangular body LEDs, which would be nice for indicating the
    direction of flow.
     
  17. After solving the amplifier problem, just add two extra green LEDs.
    Configure the bi-color LED to produce a horizontal red or green bar and
    the auxiliary green LEDs to add green squares above and below the bar.
    Now you have a red '-' sign for discharge and a green '+' sign for
    charge.
     
  18. I read in sci.electronics.design that Tim Wescott
    Those are usually called '4-band'. Brown, black, brown, gold.
    I think you mean 5-band. They confuse practically everyone. Brown,
    brown, black, black, brown? Or is that brown, black, black, brown,
    brown?
    Noted. What colour do you see? The same colour as [insert name of
    familiar coloured object].
     
  19. Guest

    OK, Ryan, you are confusing me, and probably others
    OK I've put up a crude drawing at
    http://myweb.cableone.net/zirconx/LEDammeter.gif
    LEDs are available that run off as little as 2ma of current.

    If I had two wires to run power from the battery to the bike, current
    would run through both of them. This is essentially what I am doing
    here, but one wire has much less resistance than the other and the
    other has an LED in it. Could I get 2ma would flow through the other
    wire?

    -Ryan
     
  20. I read in sci.electronics.design that wrote (in
    You could, but the problem is that you need around 2 V (minimum) voltage
    drop across the wire carrying the charge/discharge current. That is far
    too high; you battery won't charge properly from the alternator and the
    starter won't work at all, probably.
     
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