"Hybrid" LED combining - Power + HD on a single LED

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kenc3dan

n00b
Joined
Nov 12, 2005
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It seems simple..
Power on = dim LED
Hard drive activity = bright LED

I figure, just tie the two positive leads together add a resistor (maybe around 180 ohms) and I should be close. Maybe even throw in a pot instead of a resistor so I can goof with the current to get it just right.

Turns out the motherboard doesn't give us switched positive voltages as I expected.
Turns out the motherboard switches the ground leads on the LEDs.
When either the power or drive LEDs light up, current flows from the always on 5V positive leads to the switched grounds. Can't very well combine the grounds together...any time ANY LED is switched on all the voltage flows to one of the grounds and the LED is fully on.

Can anyone help me make this "hybrid" power/HDD LED idea work?

Thanks!
Ken
 
I once bought a bunch of multi-color LEDs that had 3 legs, apply power to one leg and it was red, to the other it was blue, and to both and it was something else. Either way, you could apply power to both and there was on predominate color that became brighter. You could try looking into something using that route.

I beleive I bought them at http://www.lsdiodes.com/


edit: Looks like there's a white and red one, that'd probably work good. Put the power on the red leg, and whenever the harddrive activity wire puts power on the white leg, it should become a brighter red. You'd have to buy some of the LED's and test it out though, I'm not sure how well it'd work.
 
Thanks for the speedy response and the link!
Considering the way the bi-color LED would need to be wired though I wonder if this will work for me?
I have what amounts to one 5V supply and two switched grounds coming from the motherboard. If I run the ground on the LED to one ground and the positive to the 5V, I then can't see where the other positive on the LED will run to. This is such a wierd situation.
Thanks for your help!
Ken
 
This actually shouldn't be too hard. Connect the positive leg of the LED to either positive voltage. Then connect two resistors to the negative leg of the LED. One resistor goes to the switched GND for the HDD LED, the other to the switched GND for the PWR LED. Something like this:
Code: 
         /-----^^^^-----GND for HDD LED
5V---LED-
         \-----^^^^-----GND forPWR LED
I'd choose the two resistors so that if only one is on, the LED will draw about 10mA. So if we have a (for example) blue LED, you get the following:
5V-3.5V = 1.5V dropped by the resistor
V=IR so R = V/I = 1.5/.010 = 150Ohm
If both are conducting, then you'll get (roughly) twice the current, and a much brigher LED.
 
Assuming the grounds are only active when the original LED was on.

The way I understand it, the grounds are grounded at all times, then the +2.2V for the LED is on when the LED is to be lit.

I could be wrong, and the above diagram won't hurt anything if it doesn't work. The worse it'll do is nothing.
 
Not understanding where you are getting 2.2v and the original poster has mentioned he has tested and it's using switched ground rather than switched power, which is pretty common for low voltage switching.
 
I am sure that the 5V side is always on (relative to any ground on the motherboard). Maybe it is possible that the ground is not necessarily switching to "GROUND" but is actually switching from unconnected to 2.2V, making the difference 2.8V and lighting the LED.
Does THAT make sense?
How would that affect this LED combining sheme?
Dang, this stuff is complicated.
Thanks to everyone.
 
kenc3dan said:
I am sure that the 5V side is always on (relative to any ground on the motherboard). Maybe it is possible that the ground is not necessarily switching to "GROUND" but is actually switching from unconnected to 2.2V, making the difference 2.8V and lighting the LED.
Does THAT make sense?
How would that affect this LED combining sheme?
Dang, this stuff is complicated.
Thanks to everyone.
Click to expand...
It means the MOBO or case manufacturer already included the current limiting resistor. If the built in resistor is on the ground side then Mohonri's suggestion might work if you eliminate the resistors.. except the current might be too high. If the resistors are on the positive side, it won't work... will have to think of another approach.
 
Frank4d said:
It means the MOBO or case manufacturer already included the current limiting resistor. If the built in resistor is on the ground side then Mohonri's suggestion might work if you eliminate the resistors.. except the current might be too high. If the resistors are on the positive side, it won't work... will have to think of another approach.
Click to expand...
Good point. I'm pretty sure that motherboards have built-in current-limiting resistors. It's easy to find out whether they're on the + or - side if you've got a multimeter--just measure the voltage from GND to each pin of the LED when the LED is lit. I suspect that the + will be at 5V and that the - one will be somewhere between GND and 5V. That means that the switching transistor and the current-limiting resistor are on the emitter (-) side of the LED. In that case, just hook the cathode of the LED to both - pins, and you're set.
 
Thanks for correcting me. I always thought the ground pin was grounded at all times, and the + pin was 2.2V when the LED was set to be on.
 
Switching the ground is pretty standard cmos logic design because cmos can usually sink more current than it can source. You'll see the same thing with most any cmos i/o, from simple gates to modern microcontrollers. In a sense it provides inverted outputs, meaning that voltage is present at the gate pin when then deivce (such as an led) is to be off, and no voltage is present when the device is to be on (chip output grounded).

It is also very common for the led limiting resistor to be located on the mobo. I'm not sure I recall ever seeing pc led's requiring external limiting resistors. This can cause problems for some people trying to install larger or different color led's that require more current, because the limiting resisitor is not always easily replaced.
 
agent420 said:
This can cause problems for some people trying to install larger or different color led's that require more current, because the limiting resisitor is not always easily replaced.
Click to expand...
For the drive activity LED, there's no need to replace it, the LED will just get a bit less current and LEDs will glow enough for an indicator on anything over about 1mA. Looking at my old mobos, the resistors are 220-330 ohms. If you want to add several LEDs, use an optoisolator in place of the standard LED and switch a separate LED supply.

This shows how the IDE Activity system works, with the one LED for both IDE channels.

The power LED can be run from any molex, plugging into the motherboard header is just a convenience.

A single LED can be used for both power and drive activity by connecting it as power LED and taking an ordinary silicon diode from its anode (after the resistor) to the HDD Activity cathode pin on the motherboard (the other HDD header pin (LED anode) is not used). When the HDD is active (low), the diode shorts out the higher forward-voltage LED so it flashes (Off & On rather than the usual On & Off).
 
agent420 said:
Switching the ground is pretty standard cmos logic design because cmos can usually sink more current than it can source. You'll see the same thing with most any cmos i/o, from simple gates to modern microcontrollers. In a sense it provides inverted outputs, meaning that voltage is present at the gate pin when then deivce (such as an led) is to be off, and no voltage is present when the device is to be on (chip output grounded).

It is also very common for the led limiting resistor to be located on the mobo. I'm not sure I recall ever seeing pc led's requiring external limiting resistors. This can cause problems for some people trying to install larger or different color led's that require more current, because the limiting resisitor is not always easily replaced.
Click to expand...
they seem to have enough power for most things, the blue led that is my power light is fuckin spotlight.
 
This works perfectly.
-Dim LED when only power is on
-Flashing birghter with drive activity
Perfect!
You rock!

Mohonri said:
This actually shouldn't be too hard. Connect the positive leg of the LED to either positive voltage. Then connect two resistors to the negative leg of the LED. One resistor goes to the switched GND for the HDD LED, the other to the switched GND for the PWR LED. Something like this:
Code: 
         /-----^^^^-----GND for HDD LED
5V---LED-
         \-----^^^^-----GND forPWR LED
I'd choose the two resistors so that if only one is on, the LED will draw about 10mA. So if we have a (for example) blue LED, you get the following:
5V-3.5V = 1.5V dropped by the resistor
V=IR so R = V/I = 1.5/.010 = 150Ohm
If both are conducting, then you'll get (roughly) twice the current, and a much brigher LED.
Click to expand...
 
Works great. The resistor values are a little odd.
I found that 880 Ohms on the Power LED ground and 2 Ohms on the HDD LED ground gave a nice balance.

led_combiner.jpg
led_combiner.gif
 
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