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Tri-Color LED Computer Header Problem

Discussion in 'Electronic Basics' started by MotherMGA, Sep 19, 2005.

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  1. MotherMGA

    MotherMGA Guest

    Ok. I've built a PC and crammed it into a Nintendo case. What I want
    to do is hook up a single Tri-Color LED to my motherboard and mount in
    the place where the nintendo has its power LED. The reason I want to
    do this is so I can 1) have my power light on when the PC is on and 2)
    the LED will flash a different color when the Hard Drive accesses.

    The LED is a 4 pin, 3 color LED, of the common annode variety. You can
    check it out here: LED.htm
    I also have a common cathode LED like that one but I can't think of a
    way to make it work with the circuit.

    The transistor I'm using is a BC178, which is a PNP. I don't know much
    about transistors, so any help would be hot. You can read about it

    Please refer to this circuit that I have drafted: Circuit.gif

    Looking at the circuit, the two +5V cells and the
    two grounds are simply 4 pins on my motherboard header. I labeled the
    grounds by the names that are on the header. The switches drawn next
    to the grounds are part of the header, which I can't control.
    everything else is my circuit. I drew the 4pin LED as 3 separate
    LEDs, but the +5V that connects to each in the diagram is really the
    single annode on the LED.

    I originally hooked it up without the transitor, but I found out that
    for some reason the HDLED circuit only measures 2V instead of 5V like
    the header is labeled. I tried testing against different 5V pins, but
    it always measures 2V. The PWRLED circuit measures 5V the way it
    should. The 2V is a problem because the Green and Blue LEDS need
    3.5V, and it barely lights when hooked up.

    So, my goal is to use the 5V circuit which lights the red, blue and
    green leds perfectly, but have a transistor control the circuit that
    lights the green & blue LEDs for the HD access. With the transistor
    in place, Red lights, but dimly and the blue & green LEDs don't light
    at all. I thought I may have confused teh Emmitter/Collector on the
    transistor and I swapped them, but it still didn't work.

    Am I going about this the wrong way? What can I do to make this work?

    Thanks for the help.
  2. John Fields

    John Fields Guest

    I'm confused by your description but if, as you say, those switch
    symbols are on the motherboard and are connected to the ground
    symbols, that would indicate to me that when the hard drive's LED
    was supposed to turn on the HDLED switch would connect to ground,
    and when the POWER LED was supposed to turn on the PWRLED switch
    would do the same.

    If that's true, then your circuit should look like this:

    | | |
    [75R] [75R] [150R]
    | | |
    |A |A |A
    [BLU] [GRN] [RED]
    | | |
    HDLED>--+------+ |

    It doesn't make any difference which side of the LED the resistor is
    connected to, but note that you need a separate 75 ohm resistor for
    each of the red and blue LEDs
  3. MotherMGA

    MotherMGA Guest

    Thanks John. I was making it more complicated than it needed to be.
    your solution worked perfectly. Thanks for your help!

  4. ehsjr

    ehsjr Guest

    John Fields gave you the answer to get your LEDs to light
    the way you want - no transistor is needed.

    I'll comment on your transistor circuit. It shows a PNP,
    as you stated in your post. However, you need an NPN in
    that circuit, instead of a PNP. Again, that circuit is
    not needed with common anode.

    Here's a common cathode circuit, using 2 PNP transistors:

    +5 --------------------+-----------+
    | |
    e/ |
    PowerLed ---[2k2R]---| Q1 PNP |
    (Gnd) c\ |
    | e/
    HDLED ---[1kR]---- | ---------| Q2 PNP
    (Gnd) | c\
    | |
    [150R] +--------+
    | | |
    | [75R] [75R]
    | | |
    Red\ / Blu\ / Grn\ /
    ----- ----- -----
    | | |
    Gnd ------------------+-----------+--------+
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