Need help with a Simple 2-LED Driver

Discussion in 'The Projects Forum' started by jesse b, Sep 13, 2009.

  1. jesse b

    jesse b Thread Starter New Member

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    I am a novice in the world of circuit design. I have spent several hours googling a solution, without success. I suspect that this is an easy project for some of the participants of this forum.


    I need a circuit that lights one of two LEDs depending on the value of an input voltage. A blue (or green) LED lights as the voltage increases from zero to a given threshold. (It doesn’t need to light at very low input voltages.) As the voltage approaches the threshold, the blue LED begins to fade and a red LED begins to light or slowly flash (perhaps the frequency increases as the voltage increases). At the threshold, the blue LED is off and the red light is fully on. The threshold value should be adjustable, perhaps by a small pot.

    If the input is insufficient to light the LEDs, I will need a circuit to amplify that input and power the LEDs via a small battery.

    Precision is not critical. Compactness and low-cost are important.

    I will appreciate any assistance on this project and will be able to build it from a schematic.
    Last edited: Sep 13, 2009
  2. Bernard

    Bernard Senior Member

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    At threashold , red is fully on, what does red do above threshold?
  3. jesse b

    jesse b Thread Starter New Member

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    The red LED stays on above the threshold.
  4. Bernard

    Bernard Senior Member

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    There's hope yet. The blue LED[ really a white] dim at 2V, bright at 3V, dim again at 4.6V. Next the red , will start at 3V, bright at 5V. Used 2 OP Amps so far, next, one for red,& one as comparator from a TL084, quad bifet op amp. Using 12V for now.In put based on a 0-5V signal.
  5. Bernard

    Bernard Senior Member

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    Operating on 9V battery, ckt draws about 30mA. A 5k pot used to simulate input signal of 0-9V.Need to adjust V divider to suit your own refference points.'dropped 5V comparator which was to completely turn off blue and turn on red at 5V. Diodes on inputs 3,10 to keep outputs from rising on low input V. Have fun.

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