Using a LED fan as a HDD indicator

[CLHatch]

I have an AC Ryan Blackfire4 fan on my side panel... This fan is UV reactive with 4 UV LEDs, and lets you use separate power connectors for the fan power and the LED power (so the LEDs don't dim when the fan slows down). It uses the standard 3-pin fan power connectors for both.

With the position of my case, I can't see the HDD indicator, so I got to thinking, and hooked up the power connector for the LEDs to the HDD LED header (I have the case turned to the side, so I have a clear view of the side fan). Low and behold, my fan is now my HDD indicator!

Problem, though (I figured I would run into this problem): It's rather dim. The LEDs are expecting 12V power (obviously, they use resisters in the fan), it's only getting probably about 5V or so.

For now, this will work, but this is my goal: With no HDD activity, have the LEDs dim (5V, 7V, whatever), with HDD activity, full power (12V).

I know a circuit should be easy to come up with, powering the LEDs from the power supply (where it was hooked up to about 30 minutes ago), using the the input from the HDD activity header to change the voltage supplied to the LEDs.

Could someone help me out with this? It's possible that this could be done by combining the voltage from the HDD header with the voltage from the power supply (don't know how safe that would be, so any info you have...), or it's possible a circuit that uses the HDD activity as a "switch" would be needed. I just don't know enough about the electronics to know for sure, and obviously I don't want to screw up my motherboard. At the same time, I'm not afraid to experiment if I can get verifiable information, and I figured this was the perfect place to get such information.

 

[AnnonUSA]

Why not just add an LED somewhere inside the case that is visible from your viewpoint?

 

[CLHatch]

No reason other than that I think using the side panel fan as the indicator would look cool, and be rather unique (I have a fan grill coming in the mail now). If I can't figure out how to do what I want, I may mount a LED on the acrylic fan adapter (I had to put a 92mm fan in a 120mm spot because my heatsink was in the way... even had to dremel off the inside corner of the 92mm fan to make it fit).

 

[CLHatch]

No reason other than that I think using the side panel fan as the indicator would look cool, and be rather unique (I have a fan grill coming in the mail now). If I can't figure out how to do what I want, I may mount a LED on the acrylic fan adapter (I had to put a 92mm fan in a 120mm spot because my heatsink was in the way... even had to dremel off the inside corner of the 92mm fan to make it fit).

 

[Zepper]

A simple low-power transistor should be adequate to do what you want. Just bias it to get the minimum glow from the fan LEDs and wire in the voltage from the HDD LED header to bump it to full conduction. Or just remove the dropping resistor(s) from the fan LED cirsuits and run directly from the HDD LED header.

.bh.

 

[Harvey]

If you have any experience building electronic circuits, and you know which end of a soldering iron to hold, it would be very easy to build a circuit that connects to the 5 volt logic that controls the HDD LED to control the 12 volt LED circuit in the fan. The easiest way to build a sturdy project would be to use a piece of perf board with copper solder pads on each hole. The solder pads are more visible if you click Larger Picture Available at this link.

Most boards control the usual front panel LED's with a circuit that connects the postive lead of the LED through a resistor to +5 volts and grounds the negative side to light the LED. When the LED is supposed to be off, the negative control pin is essentiall open. In this state, the maximum voltage on this pin rises to the 5 volt supply, but no current flows.

The following assumes you are using an LED that drops around 2 volts across the LED and requires around 5 ma for the brightness you want. Adjust the value for the LED you use. It also assumes the fan assembly includes current limiting resistor with the LED.

The simplest (and most risky) circuit would be to connect positive lead of the fan LED through a current limiting resistor to the 12 volt supply and the negative lead to the HDD negative (-) control pin on the motherboard.

For a 12 volt supply, subtract the LED voltage (~ 2 Volts), and calculate the resistor value:

• 10 Volts / 0.005 Amps = 2,000 Ohms.

This should work, and it won't be destructive as long as the voltage drop across the LED and resisitor keeps the voltage at the negative control pin under 5 volts when the LED is supposed to be off.

You can avoid any problem of the voltage exceeding the rating of the chip that controls it by adding a small 6.8 Volt zener diode in series with the resistor ground. Type 1N5235B would work. This would reduce the voltage applied to the series resistor and LED to a maximum of around 5.2 volts. Subtract the LED voltage (~ 2 Volts), and calculate the resistor value:

• 3.2 Volts / 0.005 Amps = 640 ohms.

The closest standard resistor value is 620 Ohms.

Remember, a zener works by being reverse biased. In this circuit, that means the cathode (the end with the stripe) is connected to the most positive voltage.

1. Connect the cathode of the zener (the end with the stripe) to the 12 volt supply.

2. Connect the anode of the zener to the resistor.

3. Connect the resistor to the anode (+ terminal) of the LED.

4. Connect the cathode (- terminal) of the LED to the - pin of the HDD LED connector.

To insure that this combination never applies more thana 5 volts to the HDD LED logic, add another resistor from the junction of the zener and the first resistor to ground. This will insure that the zener is always on, even when the LED is off. About 1 mA should be enough. Subtract the zener voltage (~ 7 Volts), and calculate the resistor value:

• 7 Volts / 0.001 Amps = 5,000 ohms.

The closest standard resistor value is 5,100 Ohms.

If you're really paranoid, it would easy to build a circuit using one inverting comparator or CMOS inverter with an open collector/drain output rated at or greater than 12 volts, or one comparator with a voltage output driving an NPN transistor, that reads the logic state of the HDD control LED (negative) terminal, but if the first solution works, this would be overkill.

Any of the above solutions would cost less than a few bucks.