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Small LED info needed


Larry
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I made a (wooden) case for my Black Box some years ago, and since I use a CF card with an Acard, I would like an external activity LED. I moved the IDE->CF adapter LED to the top of my case. I would say it is about 1.5mm (not surface mount). The problem is that it is barely visible. The CF card adapter appears unable to drive a larger LED, since I've tried several, unsuccessfully.

 

I wonder if anyone is aware of some type of LED that I might be able to use to get better visibility? I have not even found this type anywhere.

 

Thanks,

Larry

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Normally when using an LED in a circuit, you have a limiting resistor to knock down the voltage to whatever the forward voltage of the LED in question. Different LEDs have different forward voltages, so it really depends on what you have.

 

What I would do is put a multimeter on the leads of the LED and see how much voltage you are getting there currently. Also look to see if there is a resistor in series with it. Once you have this info, you can either replace the resistor with a new value (though if you are not experienced, might not be your best approach), or replace the LED with one that matches the voltage you get from your meter.

 

Good luck!

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Thanks! Unfortunately, it is a teeny tiny SMT resistor But I can get my DVM probes on it, and I get apx 1000 ohms. That sounds high, but it is what I got. I can't do much about replacing the resistor directly, but I might be able to remove it and fill that pad with solder, then add a new resistor in line on the connecting lead. (?)

 

-Larry

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1K is what I use sometimes in my circuits for many of the LEDs I use, so that doesn't seem to be your issue. What was the voltage at the LED? You can use that voltage to match up with specs for different LED's F.V. (forward voltage) and get a better match.

 

Or you could change the resister, to something slightly lower to better match that existing LED.. but be careful not to drive the LED too high. LEDs are meant to be operated at certain voltages, hence why there is a resistor in the circuit.

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I'm not sure how I would measure that. It is a HD activity LED, so the voltage is "fleeting." As best I can measure it, the voltage is .3 - .6 volts. If I had some type of peak reading meter with a very fast response, I might do better, but I just have a general purpose (cheap) DVM. Do these values seem reasonable to you?

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Thanks! Unfortunately, it is a teeny tiny SMT resistor But I can get my DVM probes on it, and I get apx 1000 ohms. That sounds high, but it is what I got. I can't do much about replacing the resistor directly, but I might be able to remove it and fill that pad with solder, then add a new resistor in line on the connecting lead. y

 

Hmmm... 1000 ohms for a tiny SMD led is normal. They need most only 2 mA current to light up. And here could be the issue... what kind of LED you try to connect? A standard LED with wires? If it´s not a special one, they need approx 20 mA to light. Maybe 12...15 mA is enough. But 1000 ohms at 5V or 3,3V (often used at these adapter cards) could be too much.

 

For example: A standard red 3 mm or 5 mm diameter "THT" LED draws 18 mA current, operating at 5V the resistors should have 220 ohms. When power is 3,3V, the resistor should have 100 ohms.

 

Maybe you check that and try a less valued resistor or a special 2 mA low current standard LED.

 

Jurgen

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LED's are current driven devices. It's possible that the circuitry can't drive enough current to light up a "bigger" LED.

You might need to investigate the use of a "buffer" to get enough current to drive a separate LED.

 

General information on the subject of driving LED's:

http://www.evilmadscientist.com/2012/resistors-for-leds/

 

https://www.quora.com/Why-do-we-need-buffer-ICs-to-drive-LEDs

Edited by BigO
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Yes, that was a concern to me also. But it is a $5 cf adapter, so it may end up getting sacrificed in the name of "progress." :-)

Adding another 1k resistor in parallel with the current one would double the current to the LED, making it brighter. Using a low value resistor would increase the current even more, A 510 ohm resistor in parallel with the 1k ohm would triple the current to the LED.

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Adding another 1k resistor in parallel with the current one would double the current to the LED, making it brighter. Using a low value resistor would increase the current even more, A 510 ohm resistor in parallel with the 1k ohm would triple the current to the LED.

That's a good idea. If you a afraid to try soldering another 1k or 510 ohm resistor on top of the current SM resistor you could always buy an old fashion through hole resistor and look for other

points along the traces on each side of the SM resistor to solder the legs. Make sure you get a thin solder that has flux in it, and DON'T press hard!

 

DavidMil

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Even a higher value than the 1K resistor would lower the overall resistance when placed in parallel, if you wanted to be conservative about your trials and happen to have a bunch of different values of resistors laying around...like some people I know :ponder:

Edited by BigO
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Thanks guys! I now have a plan. Step one (the easiest) is to take an orange activity LED that I found on another old CF adapter that appears much brighter. All I have to do is to desolder it from the other adapter and attach new leads. I already have inline connectors for the existing LED, so this is simple. If this doesn't work, I'll go with the resistor change(s).

 

-Larry

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I just removed the surface mount led from the pcb and attached jumper wires between it and the board and mounted the light where I could see it easily and brightly... why not just moved it like that? quick use of a soldering iron or hot air gun makes it happen...tin the wires.. touch touch.... no need to play about with all that other stuff.

Edited by _The Doctor__
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