changing the hdd activity light

cybertechcomputers used Ask the Experts™
Hi, i have recently built my own computer case and am wanting to make my own hdd activity light. i currently have a row of red LED's wired up to a molex connector to the power supply. Can someone please advise of an easy to follow guide or point me in the right direction of what needs to be done. Keep in mind im not to familar with resistors and how it all works. so a step be step guide would be appreciated.

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The HDD activity light is just a pair of wires that attach from the HDD light in the case to the motherboard.  All you need to do is download the motherboard manual and check the wire/plug configuration of the case lights. It should be really straight forward to follow.


yes it is straight forward if i only want to use 1 LED. but because i have a row of LED's i need to increase the voltage going to them. each LED i think powers about 3volts, the socket on the mainboard is 5.5volts. So i can power 1 maybe 2, but anymore they start to get dimmer.

I have tested with the current setup with the LED's i have. it works if i just connect the 2 wires, but very very faint. i need to be able to route a power connect from a molex socket to the LED's to draw the power
Principal Software Engineer
The built-in resistor on the motherboard limits the current on the HD activity line so that only one LED can be driven.  This is easy enough to get around.  Driving the base of an NPN transistor from the HD activity line can switch enough current to run multiple LEDs.  See the diagram here.  Since this is a switching application, most any NPN transistor will do.  A 2N3904 would be a good starting point.  They are common, cheap and available at any Radio Shack.  If it overheats, try something with a larger current capacity.
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Top Expert 2009

you are going to need one of this

it is an opto-isolated solid state relay

what you do is, hook up the HDD led signal from the motherboad to it, and put 12 V in the feed. when the signal is high the output pins fire up and you can drive up to 2 Amps of load.
Top Expert 2013

so you want only to light up 5 led's instead of one?
then DrKlahns answer is ok for you, if you want to light them sequentially (like a light bar) that needs another circuit


ok, i will explain a different way, the guide DrKlahn explained is for several individual LED lights. My setup at the moment is several LED lights already joined together and is acting as 1 big one, not individual lights. So at the moment there are no fuses or anything connect, just 2 cables going into a molex adapter.

What i need to be able to do is connect those 2 cables to a resister which then connects to the molex adapter, this hopefully should then be powered from the power supply but flashes like the hard drive LED normally does
DrKlahn's circuit will still do the trick, with some modifications.  Think of your series-connected LEDs as a single LED.  Connect a 1k resistor in series with them and use this in place of R2 and LED 1.  Skip the other LEDs and their associated resistors.  Instead of connecting the 1k resistor to +5V, connect it to +12V.

You can adjust the brightness of the LEDs by adjusting the value of the 1k resistor, lower value gives brighter LEDs.
One point of clarification.  You mention that the 5 LEDs are connected together, but not whether they are connected in series (sequentially, end-to-end) or parallel (all LED + sides connect together, all LED - sides connect together).  The parallel connection would be very unusual.

My comments assumed series connection.  Keep in mind that LEDs are polarized.  That is, there is a + side and a - side.
Top Expert 2009

should then be powered from the power supply but flashes like the hard drive LED normally does

then you have to use a solid state relay.
like i mentioned in my previous post.

these links should help you get an idea.
keep in mind, that knowledge in elementary digital electronics and knowing how to solder are required.
Can you clarify your post?  DrKlahn's response seems to fit that description well.  The transistor is controlled by the hard drive LED signal from the motherboard and should cause the LEDs to follow the normal HD LED operation.
Top Expert 2009

a transistor can blow because of a variety of reasons, i am not debating on its probability to blow.
an optoisolated circuit is inherently safer by its basic design - even if the 12V side gets damaged, 12V will NOT get into 5V circuits.

the question asker has specifically stated the he is not very good with electronics, that is the reason why i suggested so.

i think it would be prudent to spend 3 dollars more to keep a 200 dollar motherboard safe from a 12V surge in the 5V signal line directly coming from the southbridge / storage controller IC.

i am not an electronics expert, but i have worked extensively with 12V and x12V systems, solar, wind and like. keeping signal lines and power lines isolated seems to be a good thumb-rule - never let me down.

i would be very delighted if you all can provide a circuit (isolated of course) for having multiple LEDs powered by 12V, switched by a 5V signal.
I would agree that an optoisolator will do a better job of keeping the 12v and 5v lines away from one another.  I would also argue that the transistor solution is certainly adequate.  Even if the transistor shorted any two (or all three) pins together, I'd expect nothing serious.  If the base shorts to the collector, 7V (12-5V) will drop across 5 LEDs and the resultant voltage will pass through two 1k resistors (the signal out to base resistor and the resistor I suggested in series with the LEDs).  The resistors alone will limit the current such that any sort of reasonably designed output circuitry would not be damaged.

If the collector and emitter short together, the LEDs will illuminate as if the HD signal is on.  Nothing too remarkable there.

Nevertheless, given that this is only being built once, erring on the side of caution (with a moderate price penalty) may be the more reasonable approach.


Thank you for the help, i ended up using the transistor you suggested

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