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:
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?
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.