LM3914 Driving Dual color LED's on single polarity supply

Discussion in 'General Electronics Chat' started by Metalfan1185, Sep 30, 2008.

  1. Metalfan1185

    Thread Starter Active Member

    Sep 12, 2008
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    I've been playing with the LM3914 and i want to make some gauges for my car, the Power Voltage ranges on cars (my car) vary from 12.10V to 14.2V depending on running circumstances.

    My questions are as follows:

    1)I know i can drive a single color, but for nighttime, it would be nice to have all the LED's light up in one color to show fullscale, and use the comparators to drive the other color, thus using multiple color LED's. Do i need a dual polarity supply to do this, is there a simple solution to take care of the dual color thing?

    2)Will the voltage changes at the +V source for the LM3914 change the range of the steps in Voltage needed to light each segment? in other words, will the Voltage changes from the alternator change the calibration of the guage? Should i consider a Voltage regulator on the Power inputs to steady the voltage to the IC?

    Thanks, all feedback is welcome.

    Josh
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    You can do it with a single supply. If you take two op amps (or transistors) and connect your dual colour led with a series resistance across the two outputs of the op amps then when the one op amp outputs a high the other will output a low and one colour will light. If you make the low output op amp high and the high output op amp low then the polarity across the led will reverse and it will light the other colour. Its like an H-bridge driver used to reverse the direction of a motor.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You may wish to expand that range. Right after starting, your alternator might put out nearly 15v trying to replenish the battery quickly. Also, your battery still has power down to around 11.4v. If your alternator dies, you will want to know how much power you have left before you're stranded.

    If you use dual LEDs that have common cathodes, you could use a simple SPDT switch to change the anode supply from one set of LEDs to the other.

    I suggest that you do NOT use super-bright LEDs; use diffused LEDs instead. Direct viewing of super-bright LEDs can cause blindness.
    One of the LED colors should be red. Use that for nighttime. Red reduces the chance of night blindness.
    It's possible under fault conditions for the alternator to output more than 15v, which is the limit of the LM3914. It wouldn't be a bad idea to use a regulator to drop the supply voltage down to (perhaps) 8v.
     
  4. Metalfan1185

    Thread Starter Active Member

    Sep 12, 2008
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    I believe that "diffused" means that the lens color is the same as the LED color, that being said, do they make a "Diffused" Multicolor LED? What i wish to accomplish, is the have the background uif you will, of the bar light up say green, and the actual bar itself to be red, and as the Voltage increases, the bar changes in sequence from green to red.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Yes. Actually, I've bought dual-color 10-segment bargraph LEDs in a 3-inline pin configuration before. I still have nearly 20 of them. I bought them on an auction website; they weren't very expensive. Watch shipping costs.
     
  6. Metalfan1185

    Thread Starter Active Member

    Sep 12, 2008
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    I would like to make my own bars, but the LED's would be ideal if i can find something like the bottom 2 drawings on the following. Have you guys seen anything like the ones posted? (the 2 on the bottom).

    [​IMG]
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    You really don't want KAKA LEDs; you'd have a tough time trying to drive them with an LM3914.

    Here is a page from a vendor I've bought from in the past:
    http://www.eled.com/category.asp?catalog_name=leds&category_name=bi-color+-+rectangular+led&page=1
    Rectangular bi-color LEDs. The bottom few have a milky diffused lens, 110° viewing angle.

    I have some E119EGW-CA's that I'd picked up a couple years ago - but the "CA" on the end means Common Anode. The version they carry now is Common Cathode, according to the datasheet. Contrary to what I said previously, you may actually want to go with a brighter diffused LED. You can always decrease the current if it's too bright. What you don't want is a narrow viewing angle, which many super-bright LEDs have.
     
  8. Metalfan1185

    Thread Starter Active Member

    Sep 12, 2008
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    well it would be nice to wire it into the dimmer circuit somehow, i have a little researh to do before i try that. Im not exactly sure on how those things work, but i think i have an idea.

    rectangular dual color is good, i was just wondering if they were available in the configurations that i was looking at. That way i could have avoided using transistors to drive the LED's.

    Oh well. Thank You very much sir.
     
  9. Metalfan1185

    Thread Starter Active Member

    Sep 12, 2008
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    Is that why street lights signals that are green LED look a lot brighter than the red LED ones when the light changes? must have soimething to do with the wavelength of the light color
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    The human eye responds much more strongly to green than other colors. That's why it's much easier to make a green LED look bright than the other colors.
     
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