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dual color LEDS, 2 signals to drive, need help

Discussion in 'Electronic Basics' started by John, Feb 24, 2005.

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  1. John

    John Guest

    I have a dual color led that I need to control from a programmable
    device. I have 2 signals (LEDYAGC and LEDGAYC) to control if the led is
    yellow or green.
    I'm OK with all that.

    I need to figure out the voltage and drive strentgh of the programmable
    device (or the buffer if it needs that) that drives the LED signals. I
    also need to figure out the series resistor I need to use. I've never
    used this type of LED, so I just don't know where to start. Here's the
    schematic of the LED and it's requirements. The programmable
    device/buffer IOs can be 3.3v or 2.5v.



    LEDYAGC ------
    |
    |
    -----------
    | |
    -- --
    yellow \/ /\ green
    -- --
    | |
    -----------
    |
    |
    LEDGAYC ------





    LED Requirements

    Description Absolute Maximum Rating
    Green Yellow Unit
    Wavelength At Peak Emission, If=20ma 565 590 Nm
    Dominant Wavelength, If=20ma 566 588 Nm
    Lens Clear Clear ---
    Luminance Intensity, [email protected] 14 5 mcd
    Capacitance, If=20ma 15 20 C
    Forward Voltage, Vf=20mA 2.5 2.5 V
    Reverse Voltage 5 5 V
    Peak Forward Current 140 140 mA
    DC Forward Current 25 30 mA
    Reverse Current, Vr=5V 5 5 V
    Power Dissipation 105 105 mW


    Thanks, John
     
  2. Bob

    Bob Guest

    You need to add one series R from one of the IO pins (it doesn't matter
    which one).

    Set the IO strength such that its outputs go all the way to the supplies
    when they're delivering the desired current (into the LED). The series R
    will then determine the LED current. Read the specs for the IO driver to
    convince yourself that the chosen IO strength will provide enough drive to
    insure that the outputs get close to the supplies under this load (i.e.,
    within at least 200mV, or so).

    Assuming that you want to operate the LED at 20mA (this will be VERY
    bright), and the LED has 2.5V across it (at 20mA), then this implies that
    you'll want to use 3.3V IO. The series R will, then, be the controlling
    factor for the current.

    The LED current will be (3.3V-2.5V)/R. You can do the math.

    Bob
     
  3. Rich Grise

    Rich Grise Guest

    I've found that an LED is visibly just as bright at 10 mA as at 20, at
    least I couldn't tell the difference. From there down, the apparent
    brightness is pretty much linear with current.

    Cheers!
    Rich
     
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