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Tri-Color LED Computer Header Problem

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:

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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 here:

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Please refer to this circuit that I have drafted:

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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. Scott.

Reply to
MotherMGA
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--
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:

+5>-----+------+------+
        |      |      |
      [75R]  [75R]  [150R] 
        |      |      |
        |A     |A     |A 
      [BLU]  [GRN]  [RED]
        |      |      |
HDLED>--+------+      | 
                      |
PWRLED>---------------+

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
Reply to
John Fields

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

Scott.

Reply to
MotherMGA

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 ------------------+-----------+--------+
Reply to
ehsjr

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