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Looking for assistance - LED fader...

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The Eagle Maint

New Member
Hello, this is my first post here. I apologize in advance for it's length. :)

I have been modifying my computers for a few years now. Mostly just simple case mods, but recently I've been getting more and more 'advanced'. Well unfortunately, I don't have much experience with electronics, so everything so far has been additions to the case itself. In general, what I am looking to do here is to have 1 LED on my computer that appears dimly lit when the computer is on, fades up to full brightness when the HDD is being read, and fades back down when the HDD stops being read.

I'll try to explain this in as much detail as I can. I bought one of those really nice cool white LEDs from radioshack. "Typical MCD is 1100. Size is T-1- 3/4 or 5mm. Clear lens color. 20mA (max). Typical Voltage is 3.6V. Comes as package of 1." (That's what it says on their site, anyway.) On my computer's motherboard there are 2 connectors; one for the power LED, one for the HDD LED. They each provide about 2.9v. What I am aiming for if possible, is to have the LED lit up to about 1v, or about 20-25% of it's maximum brightness (powered by the power LED connector). Then when the HDD LED power comes in, I would like the white LED to quickly light up to it's full brightness. When the HDD LED power goes out, the white LED should slowly fade back down to it's dim brightness again.

I don't know a lot about electronics really. I can use a soldering iron, read a multimeter, read (simple) schematics, read resistor colors, and I have a vauge understanding of volts/current/amps/ohms... very vauge actually... :oops: Anyway, I'm just wondering if anyone here could possibly write up some kind of schematic for this, if it isn't too complicated. If it is, I'm willing to throw in a little cash tip for a well drawn out explanation and plan as to how I could make this happen. :D The main reason I don't want to just play around and try to figure it out myself is that it's dealing with my computer... I'm not prepared to fry my motherboard just yet. :twisted:

If this is actually a complicated thing to do, then don't worry about it because it's just for fun, nothing really important. But I would be very appreciative if it's simple to do and someone can help me out. Thanks for your time. :)
 

Exo

Active Member
The 5V and GND connections you need to get from a harddisk (or floppy) power cable (the red-black-black-yellow ones). GND is a black one, +5V is a red one.

The harddisk led output from your motherboard goes straight to the optocoupler (because a current limiting resistor is already on the motherboard). Measure the HD-led output to locate the + and the - of it. The - should go to the optocoupler's pin 2, the + to pin 1. The power led output on your motherboard is left unconnected, since we take power directly from the power supply.

When powered on, the led will be lit dimly trough resistor R4, so the value of R4 sets the intensity when dimmed.
when the harddisk is active, the optocoupler gets switched and pulls the base of T1 low, putting T1 in a conductive state. 5V will go trough T1 and the R3 to the led, since R3 is small the led will light brightly (so, R3 sets the brightness for hd-active, but i shouldn't make it much lower then 220ohm). When the HD is inactive capacitor C1 will discharge and make the led fade back to the dimmer state, so C1 controls the fading.

I used an optocoupler to make it system independant because i can't know how the leds are connected on your motherboard. This way it should work on all motherboards.
It should work with other opto's then the TIL111, but that's the one I have lying around in my junk box.

You can always test it with a 4.5V battery first (to prevent computer damage if you've done something wrong). In this case connect +5V and gnd to the battery ( the led should light dimly). When you connect pin 2 of the opto the the battery's - and pin 1 to the + trough a 330 ohm resistor it should light brightly.

I tested it and must admit, quite a nice effect.
 

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