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HDD LED Transistor-based Switch

post #1 of 2
Thread Starter 

This is posted on 4/01/14, but is not an April Fool's joke. See the Corsair YouTube channel if you are looking for humor.

This is a simple electronics mod thread that answers the question:

How do I make a string of LEDs light up when my hard drive activity LED flashes?

If you have never asked that question, then this may not be interesting to you. I have asked that question, and this week I took the effort to make a circuit that does exactly that. When the HDD activity LED flashes, this circuit makes a 12v string of LEDs flash in parallel. It uses a separate 12 volt input and a transistor to make the magic happen.

Here is the circuit diagram:

HDD LED Circuit by samwisekoi 2014

Here is how it looks on a one-inch square PCB or prototyping board:

HDD LED PCB by samwisekoi 2014

This is a tiny PCB, with only a few inexpensive parts, including a fifty-cent NPN transistor. It will work if your motherboard pulses the HDD ground (as they normally do) or if it pulses the HDD 5v (positive) line. The optional LED in the circuit is flashed by the motherboard as a replacement for the normal case LED. You could wire D1 to your case LED, or place a 5mm LED directly on the little PCB to show that the switched circuit was replicating the normal HDD activity LED. Or you could leave it off entirely.

I am using this to illuminate the interior of a small NAS box when the array is used, but you could use it for any purpose. In fact, you could wire it to pretty much any 5v circuit to flash an LED strip in parallel.

I designed a 1x1" mini circuit board, and if a few people are interested I could have some prototypes made. They would be about $5 each in green or $10 each in black. Of course, they'd be less than a buck if I have a whole bunch made, but really, how many of these can one man use in a lifetime?

In the meantime, I am making my prototype on a scrap of breadboard. I'll post some pictures and a Mouser parts list, probably tomorrow.

If anyone besides me is interested in having some actual PCBs made for this circuit, reply in this thread (or in the parallel thread over on GeekHack.) Otherwise it shall remain here for posterity and future searchers who want to make some LEDs flash when their hard drive activity LED flashes.


- samwisekoi
post #2 of 2
Thread Starter 

So this circuit works. It works perfectly, in that it flashes a 12v string of LEDs when the 5v HDD activity LED circuit flashes, It does that great.


Because HDD activity circuits pull HDD- low instead of pushing HDD+ high, the circuit requires the use of the HDD- pin as ground for the 12v string of LEDs as well. Again, this works, but it does mean that 12v (20mA in my case) comes back through the HDD- pin. On the single low-cost ITX motherboard example I tried, this worked great. For about 30 minutes.

Then it stopped flashing. That is, the motherboard HDD- stopped being pulled low, so all the LEDs stopped flashing. In fact, even just a case LED no longer works on this motherboard.

I do not know if this is an isolated instance for this one low-cost ITX motherboard, or if it is universal. I do know that I don't want to drop it on an ASUS ROG Z77 board to find out.

So, this circuit would work well when the triggering circuit was the HDD+, and the main ground could be the drain. And it might be fine for other boards. I expect, in particular, that it could work really well for keyboard LEDs where the string being powered was 5v like the trigger. However, before I attach it to another HDD activity LED circuit, I am going to add an isolation circuit so that I can use the main 12v+ and ground to power the LED string.

Hopefully that will only add another transistor, so the total cost remains trivial.

- Ron | samwisekoi

p.s. Along the way, I discovered that Windows software mirroring spins the disks essentially 100% of the time, so in addition to everything else, this circuit turned out to be less interesting for this use case than I expected. It is kind of dull to have a string of LEDs flash during boot-up, and then just stay on forever. It would be more interesting to drive them from a 3-pin fan connector so the LEDs would turn on when the motherboard brought the fan circuit up above the turn-on voltage of the LED string. That might have the effect I was intending in this use case. I'll solve that later.
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