Simulation? Darken second LED


My new PC has two connectors for the power LED and disk activity LED. The case has those two LEDs but they’re shining really bright lighing up the room at night. Also, the disk activity blinking around is a bit too distracting, but the information itself might still be useful so I don’t want to simply disconnect it.

So I had the idea to use only a single LED and let it light with the power signal and darken with the disk activity signal (not completely turn it off). A bit like the single Link/Act LED of a network port. But I’m not sufficiently familiar with transistors to create such a schematic without any help or references. And I don’t know how those pins are wired exactly on the mainboard. I guess they have 5 V when on and the case that provides the LEDs has a proper resistor for that. Haven’t measured it yet.

My first thought would be to wire something up in LTSpice and see what it does, then find a solution by try and error from there before assembling something with real hardware components. But every time I open up LTSpice I’m frustrated before the first component is added. And it seems I’m not alone. But that’s a different topic. Then I remembered that KiCad has some simulation in it. But from what I can find online, it’s not much better.

So how would I do this? How can I simulate this situation and find a working solution that I can then build? So far I’ve added some parts but got no simulation working. Too many issues, I can’t even describe them all here.

Is this approach even feasible? Or would somebody simply throw in a working schematic that I can just use and save me the days of experimenting? Any help is appreciated.

Just take two resistors and put one in series with each LED. Then they’re not so bright.

I had the same problem and I solved it with a black felt tipped pen. Just rubbed it over the led until it was not too bright anymore.

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Or a potentiometer? What would be a good guess? 10K?

Overly complex. Depends a bit on whether the MB supplies 5 V or 3.3 V. A few hundred ohms would be about right.
But if you insist: a 1 kohm variable resistor.

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“No, no!” said the Queen. “Simulation first–schematic afterward.”

Better stick to tape or ink to dim the LED. Well a series resistor would work too, if you can find out which wire to cut. Cut the wrong one and the computer explodes. Just kidding, that’s in the movies. :stuck_out_tongue_winking_eye:

My on/off indicator LED glows blue. I hate blue LEDs. The computer has a piece of black PVC tape over the LEDs. I’ve found other ways to work out if the computer is switched on.

The two RAM cards inside also have a row of LEDs along the edge opposite the pins. These LEDs fade from one color to another and at any time show about seven different colors over the approx. 150 mm. length. The two rows are out of sync with both the fading and colors. They are bright enough to reflect off the wall behind the computer.
I often wonder whose brain fart that was. Totally ridiculous waste of technology as well as irritating.

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Me too. But a part of people buying “gaming” PC’s spend extra money to buy parts wit extra LED’s and stuff. And quite often the LED’s are configurable by software too, but default to “something” without the software.

But back to the original topic:

A LED has a (nearly) fixed voltage, and it is between 1.5V and 2.5V depending on the color of the led. A long time ago 20mA was a rules of thumb current for all signal led’s, but leds have become much brighter over the last 20 years, and 1mA is a much more sensible current for a visible, but not too bright LED.
And you can’t simulate the brightness of a LED.

But probably more important:
It appears your knowledge of LED’s is not so big. You are probably better off with some black tape or a felt tipped pen. If you make a soldering mistake and a wire comes loose and makes a short circuit in an inconvenient place in your new PC, then that can be a very frustrating and expensive experience. It’s much better to buy an assortment of LED’s and resistors and do experiments on a breadboard.

Hm, okay, so nobody seemed to get my point.

I want the two LED outputs of the mainboard to map to a single LED that I will then connect. The desired behaviour is this:

0 / 0 → off
1 / 0 → medium brightness
1 / 1 → low brightness

I’d like to experiment with some circuit to accomplish this but I’m not sure how to start. Before unnecessarily destroying components, I thought a simulation would be a good starting point to explore different circuits. I’ve made good experience with such simulations in the past, but the available software for this is the worst that exists on this planet. Every second using that is really painful.

So, my question of this topic was: How can I simulate a circuit with KiCad? Not how I can make LEDs shine darker or not at all. I know how that works. I’ve currently attached a dark painted sticker as an immediate remedy but it looks ugly. And I already found good resistor values for both LEDs, but would still like to combine both signals into a single colour.

Is this what you want?

I/O pin 1 turns the LED on or off, while I/O pin 2 reduces the brightness of the LED.

If you want the LED to be brighter when both I/O pins are high, then just put a series resistor from each of the I/O pins to the Anode of the LED.

You don’t even know what the existing circuit is. Is the indicator connected between the driving pin and ground, or between the driving pin and +5V? Where is the resistor, between the pin and the LED or between the LED and the power rail? Although the fact that one can buy interchangeable ATX computer cases with indicators connected by a harness to the motherboard suggests there is a standard configuration.

Once you know what the circuit is, then you can design additional components to get the effect you want. This is not a problem that requires simulation, a basic knowledge of electronics will suffice. It’s like taking a sledgehammer to crack a nut. But there will be some kind of logic inversion then combination since you want the HDD pin to turn off the LED.

Yeah, for the LEDs and attached wires.

Who knows what drives the LEDs. :slightly_smiling_face:

You bloo it! I am going to fly over to your place on Jet Blew. (Misspelled name of American Airline Company.)

Yes I would not use a pot. 10K ohms is way too much. A fixed resistor of few hundred ohms sounds much better. The LED is likely to need a few mA and I do not know about the current rating of the potentiometer wiper. Actually you would be using the variable resistor as a rheostat and not as a potentiometer.

For some illogical reason, this discussion reminds me of some nonsense I read MANY years ago about how a flashlight does not emit light. Rather, it sucks dark. This is demonstrated by cutting open a flashlight battery after it has expired. When you look inside the deceased flashlight battery, it is full of dark.


Problem solved then, all OP needs to do is to replace the HDD LED with a LSD to suck light from the power LED. Might be unobtanium though. :thinking:

That PhD level thesis I had read might have been written before the day of LED flashlights. A great follow on would have been to try wiring the LED backwards to see if it begins to behave like a LSD. Of course, it could start to smoke and with enough of that you might not see anything.

You’d probably get a NED (one shot).

I did a quick search and not many people have asked what the actual circuit is and whether there is a standard. Most hits talked about how to plug the labelled wires into the motherboard. The closest I found was an electronics stackexchange post. It seems that the LEDs are floating, and the limiting resistors are on the motherboard. I’ll leave it to others to design a circuit that can be interposed to handle the specified task.

Fortunately I eschewed fancy multicoloured lights on my workhorse’s case, and the only LED shining is the disk activity LED on the control panel on the front top of the case. There’s also the ethernet link and activity LEDs at the back. All very discreet just the way I want.

Maybe. I’m not so savvy in logic circuits hence my intention to simulate things in a safe environment. I can understand that more experienced people will see this task as trivial and my plan as overkill. Good for them.

From this funny discussion, I conclude that KiCad cannot easily be used for simulations either and I will have to somehow fight my way through LTSpice again.

BTW, the ideal additional resistor for my Power LED (green) is 68 kΩ and for the HDD LED (red) it’s 15 kΩ. The LEDs of my PC case are that bright!

It can be done with an optocoupler and one or two resistors. I’m away from my computer so people can have fun working it out.

What part of the funny discussion makes you think so?

KiCad is perfectly capable of simulating things like this. I agree that ngSpice is not the easiest simulator to get to work for beginners, but it is not too difficult either. Most of the problems are with attaching the right simulation models to the KiCad symbols and solving differences in pin assignment.

And to repeat my earlier statement, which is still standing: