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-- H.D.D. Bicolour Activity Indicator
-- Category: Guide
-- Posted by: GideonX
-- Posted on: 2004-06-24
-- Price: ~ $N/A USD
-- Pages: [ 1 ] 2 3
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Note: This is a user submitted guide by Skylined. Spanish version is available here.

Tired of seeing that HDD LED always shining? It's kind of bland so I decided to do something with more eye candy. I saw circuits in some other sites that used two LEDs, but I wanted to use a single bicolour LED that would change its colour depending on HDD activity. I also wanted to be able to represent both information using a single LED. The circuit is very easy and cheap to do, it doesn't require too much soldering, so any person with basic knowledge is going to be able to do it without any trouble.

You may be a little scared about doing this mod because you may think you can damage your motherboard; calm down, this circuit uses a 4N25 which is an optocoupler which isolates your motherboard from the rest of this circuit. So if you have an accident with the circuit, you won't damage your motherboard.

This guide shows how to use three LEDs, I did it this way because my case has 3 holes at the front where you can put LEDs and I wanted to have them filled. Another reason why I used three LEDs is because it isn't common to see three LEDs indicating HDD activity, this circuit lets you do that, without it you won't be able to use more than one LED plugged directly to your motherboard. If you just want to use a single bicolour LED, you just have to replace the 68 resistors by 150 Ohm resistors. Three LEDs isn't a limit, you can use more, but using a different schematic.


  • Proto PCB
  • 4049
  • 4N25
  • 3 Bicolour LEDs with 3 leads
  • Resistor: x2 68 1/4W
  • Resistor: 47K 1/4W
  • IC Socket, 16 pins
  • Soldering iron kit
  • Heatshrink


Click to enlarge

The 4N25 works as a switch, when it receives a signal from the motherboard's pins in charge of the HDD's activity signal, the circuit is closed. This is when a current starts flowing from the 4N25's collector to its emitter, then it goes through the resistor and gets to the lead that commands the LED's red colour, that current keeps it's path until it reaches ground. At the same time, that current gets to the 4049's input, which is a "NOT" logic gate which inverts the signal that is presented at its input; this means that if at its input there's a logic 1, at its output you'll get a logic 0, so this way, the current won't flow from its output to the resistor that goes to the LED's lead that controls colour green.

When the HDD's activity signal coming from you motherboard doesn't exist, the 4N25 opens the circuit, so the current stops flowing from its collector to its emitter. The red LEDs stop shining because they don't get a current coming from the 4N25. At this moment, the 4049's input doesn't have a signal. As the 4049's input needs to have a logic state 1 or 0, a pull down resistor was used, which makes the 4049 to have a reference voltage: because this resistor is connected to ground, that reference signal is a logic 0, so at its output a logic 1 is represented.
The current starts flowing from the 4049's output throught a resistor until passes through the LED's lead that commands the green colour and then it goes to ground.

Get all that? Good, let's proceed...

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