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Firstly, apologies for what I am sure is an incredibly novice question -- I have very little general electronics experience.

Having previously built my home/hobby PCs from pre-existing components, cases, cables, etc. I am currently attempting a very custom build with a bespoke case and an unfamiliar motherboard.

The motherboard documentation (page 33 onwards) describes the pin header of the front-panel case connectors. It has the usual power and reset switches, and power and HDD LEDs, which I am familar with, as they are common to most cases. All of these use a standard, two-pin Dupont-type connector, and look to be easy enough to wire in with a kit (presuming I can get my +/-, ground, standby, etc. terms the right way around). Similarly, it has two NIC activity LEDs, which look to be the same as either the power or reset LEDs.

However, the front-panel header also includes pins for what appears to be a multi-state indicator for identification and alerting for overheat/fan failure conditions, where it can be:

  • off (no issue)
  • solid orange/yellow (overheat)
  • flashing orange/yellow (fan failure)
  • solid blue (identification)

I appreciate I can run without it but, if I were looking to be completist, I am struggling to determine what that indicator is and how it would wire in. It looks to only require two pins, although it labels these as simply Pin 7: Blue UID LED; Pin 8: OH/Fan Fail/PWR Fail LED (where the others have Pin x: +3.3 Stby; Pin y: the LED). I'm guessing these aren't some kind of single-pin LEDs, because surely they need both a + and - connection of some kind?

I expect that, if I were to use a chassis/case from the same manufacturer, it would be an proprietary/embedded part of that case, but for a bespoke build, I'm not sure what I could substitute.

I can certainly see various bi-colour LEDs for sale, but I assume it is not likely to be a simple as soldering one onto a pair of wires and crimping a Dupont connector on the end? I'd guess there'd have to be a small custom board, the correct resistance, some microcontroller to handle amber/blue, on/off/flash based on...voltage(?)

Given the information available, is there enough to determine what should be used here (and, if so, what); or is somehow acquiring or salvaging the proprietary board from a manufacturer's chassis the only real option (besides running without)?

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2 Answers 2

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Looking at some other mbs for similar, I'm sure it's a simple bi-color arrangement. The UID led pin should have 5V when UID is not on, and the grounded when on. The OH/Fan Fail/PWR Fail pin will be 5V (maybe 3.3V) as well when no failure and grounded when failure. Both sides are controlled by the mb instead of one side tied to ground or VCC directly.

So when normal, both pins are 5V and neither led are on. When you press the UID switch on front/back/via the service processor, pin 7 should go low. Use a multimeter to check. It's harder to check the failure led behavior.

You can make your own bi-color led by putting two leds in reverse parallel and adding an appropriate resistor. Even if the mb has one already, add one anyway for safety. 400 to 600 ohms or so should be fine enough. Low current to prevent issues if we are wrong and won't light up your entire room if the leds light up.

Update: schematic courtesy of https://forum.arduino.cc/t/connecting-dual-led-to-arduino-board/200282 and example from my desk:

enter image description here

enter image description here

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  • \$\begingroup\$ Ah, so each of those LED should use the pin of their counterpart as ground terminal? That's a clever trick to save pins, if you don't need to light them at the same time of course. Hadn't heard of that before. Makes good use of their diode property. \$\endgroup\$ Apr 8, 2022 at 7:12
  • \$\begingroup\$ @BartvanHeukelom yeah. Thats why, at least on their server boards, they have a jumper to decide which is more important to show. I know dell also used that trick for their external "status indicator led cable p/n HH932" that's used as part of their poweredge server cable management arms. \$\endgroup\$
    – Passerby
    Apr 8, 2022 at 8:16
  • \$\begingroup\$ Frankly, if I can't get something off-shelf for this, I might as well run without it. I certainly don't have the experience or confidence to put something together myself. \$\endgroup\$ Apr 8, 2022 at 17:57
  • \$\begingroup\$ Hang on -- does rapidonline.com/… look like it might do the job? The combination of terms 5V, 3.3V and flashing (and it having only two pins) certainly suggests it, but I don't know for certain what I am looking for... \$\endgroup\$ Apr 8, 2022 at 18:20
  • \$\begingroup\$ I mean it's just 2 leds, a resistor and wire/connector. The bare minimum of electronics. And no that led has a built in microcontroller that makes it flash automatically. You want a normal bi-color led. @jimbobmcgee \$\endgroup\$
    – Passerby
    Apr 8, 2022 at 19:34
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Edit: this is probably wrong and Passerby is probably correct


To me it looks like that section describes two LEDs, an orange one and a blue one. Not one with two colours. You're right that there's no such thing as a single pin LED. Presumably the other pin of each may be connected to any available 3.3 V pin. The document is a bit unclear and inconsistent here

Also strange is how it defines a single "OH/Fan Fail/PWR Fail LED" in text, but the pin diagram shows a separate "Power Fail LED" and "OH/Fan Fail LED". Edit: ah, the Power Fail LED is described 2 pages later.

By the way, you generally don't want to simply connect a regular LED to a voltage source, it needs a current limiter, usually just a resistor. So the question is whether that's already provided for each 3.3 V pin here (in which case a single pin won't support more than one LED) or whether it's supposed to be included in the "LEDs" you connect to it.

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  • \$\begingroup\$ I was also confused by the wording of the separate Power Fail LED. Is it possible that the proprietary case cable is some sort of 4-wire cluster, one for each of the three LEDs, and the fourth for a shared 3.3V pin? \$\endgroup\$ Apr 8, 2022 at 0:05
  • \$\begingroup\$ @jimbobmcgee possible but seems unlikely. You can sometimes see two wires in one dupont connector but more would make it prone to failure. See my answer. \$\endgroup\$
    – Passerby
    Apr 8, 2022 at 1:34

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