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I'm going to buy a PC tower case which has only one LED intended to be used as the power LED, thus no HDD LED is available. My intention is to have that single LED at about 50% of its intensity when the PC is turned on and there's no HDD activity, and to have it at about 100% brightness to indicate HDD activity. Thus, it would become some kind of a "hybrid" LED indicator.

I'm not sure how the LED outputs are actually constructed on the motherboards, so my initial thoughts were to use optocouplers. Please keep in mind that I'm nowhere close to being an expert when it comes to designing electronic circuits, even the simple ones. So, here's what I had in mind:

Initial design, with no resistors values calculated yet

I haven't calculated the resistors yet, though having 220 ohms in all places should be fine -- or at least it looks to me like that. Also, I haven't looked yet at exact optocouplers and their CTR values. Could the whole thing be made simpler without risking to damage the motherboard which is an expensive one?

Please advise. Thank you!

UPDATE: Based on a reply below, here's how a simplified circuit might look like. It would also handle ACPI sleep states when there's no +5 V from the PSU except the +5 VSB, and the power LED blinks in such cases. Would it work?

Simplified design, also capable of handling ACPI sleep states

UPDATE #2: Yeah, it should work as pointed out below, and it also cuts the required number of components in half. I'll try it out once my tower case arrives so I can see which brightness level works well, and I'll report back.

UPDATE #3: D1 is redundant, as the LED inside the optocoupler can't be damaged by receiving a reverse voltage coming from the motherboard's HDD LED output.

Even more simplified design

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It would be much simple if you could replace that LED with a bi colour one. For example red-green: green when powered, yellow when HDD activity (both on). –  Cornelius Apr 21 at 8:47
    
I've already thought about that option, too. However, using a bi-color LED would mean that negative outputs of power and HDD LEDs would be tied together, and I have no idea whether those are floating grounds or not, what's pretty much eliminating a bi-color LED as an option. Also, I must say I'm not that much into having a "compound" color when there's HDD activity. –  dsimic Apr 21 at 8:52
    
Common Cathode 2 Color on 3 pin would work –  user38637 Apr 21 at 10:42
    
According to hardforum.com/showthread.php?t=1125598 and case-mods.linear1.org/hdd-header-hardcore , ground is what's switched for moterboard LED outputs, so tying the grounds together might be risky. Not worth trying that on an expensive motherboard, IMHO. –  dsimic Apr 21 at 10:56
    
Then see if VCC is shared between the two LEDs instead and use a common-anode two-color LED. You're only out of luck if the mobo puts the LED in between the current-limit resistor and the drive pin. –  Mike DeSimone Jun 26 at 4:16

2 Answers 2

Seems like it would be simpler to use only one optocoupler, and shunt it with a resistor for the (dim) power-on states.

schematic

simulate this circuit – Schematic created using CircuitLab

I also think you need more than a 2:1 difference in current for it to be obvious (given the eye's logarithmic response), more like 10:1 for a modern bright LED. You may have to experiment a bit.

As one data point, car tail lights have about a 10:1 ~ 20:1 brightness difference between running and brake lights.

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Thank you for responding so quickly! Actually, motherboard's power LED output isn't the same as having +5 V from the PC power supply, as the power LED also indicates ACPI sleep states via blinking etc. However, your solution is much better even in that case, as while the PC is inside one of the ACPI sleep states there's no +5 V from the PSU except the stand-by voltage (VSB) which isn't easily accesible without splicing into the wiring etc. Regarding the brightness difference, sure thing, 50% value was off the top of my head and I'll experiment a bit and report the actual results. –  dsimic Apr 21 at 9:26
    
Actually, your solution isn't covering the blinking in ACPI sleep states and similar conditions. Could the actual power LED output be used instead of the external +5 V supply, with a resistor in series to the LED which would be shorted by the optocoupler's output? –  dsimic Apr 21 at 9:35
    
With a resistor in parallel that should work. Dim the power LED unless the HDD is active. My answer is edited. –  Spehro Pefhany Apr 21 at 9:42
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Why not punch a 5mm hole in front cover for HDD LED? –  user38637 Apr 21 at 10:45
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That would ruin the aesthetics of the case, as it has a front door and there's little to no space for adding one more LED. Also, modding the case mechanically would require more effort than adding some circuitry. Here's the description of the case: fractal-design.com/home/product/cases/define-series/… . –  dsimic Apr 21 at 17:25

Seeing as there was no recommended answer ...I'm not sure if it would help in this regard but I did something similar that could be beneficial in your needs. It will also take account for any sleep states as well accordingly and is based off a single reverse-biased PnP transistor. I used a 0.15uf polyester capacitor for a more reactive change instead but an electrolytic works just as well if needed. VCC is either of the positive leads from the Power or HDD led from the motherboard header and bases off a negative pull from the ground leads.

From testing, the motherboard I used actually supplies a steady 5v power from the positive leads but instead pulls different voltages down the negative leads, which is what it was based on.

The whole end result is that it essentially 'turns off' the power LED during activity in favour of the HDD LED. It will not combine colours at all. A larger capacitor will result in a longer 'off time' for the power led and introduce a 'fading transition' with large enough cap between them instead but will also raise the cutoff limit for intermittent HDD activity.

Source: Combine HDD and PWR status LEDs into one. - Overclock.net

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