How do you determine the brightness of an LED?
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 1st August 2010, 10:42 PM #1 Headfi Fanatic   diyAudio Member   Join Date: Jul 2010 Location: Chicago How do you determine the brightness of an LED? Here are the specs VAOL-5MSBY2 VCC Standard LED - Through Hole The LED will be used as a power indicator on the front panel. My power source will be 24V. I don't know how much mA I should use as this will help me determine what resistor I should use. 20mA is 1500 mcd right? So that's probably 75 mcd/mA. I don't have anything to reference it to. How bright is 1500 mcd, 1000 mcd, 500 mcd, etc. @ 60 degrees? I don't want it to be blinding when looking directly into it nor do I want it to be too dim under medium to high ambient light conditions.
 1st August 2010, 10:50 PM #2 korben69   diyAudio Member   Join Date: Nov 2006 Location: Lyon Hope this will be helpfull : LED calculator for single LEDs According to your Led specs, R will be about 1.2K/1W
 1st August 2010, 11:19 PM #3 Headfi Fanatic   diyAudio Member   Join Date: Jul 2010 Location: Chicago That's the calc I used as well, however I will not run it at 20mA; too bright and will have a shorter lifespan. In order to decrease mcd, I will need to decrease mA by increasing resistance, which is what I'm getting at. If I wanted 750 mcd as opposed to 1500 mcd, then I would get 2.2 Kohms @ 1/2W. The problem is, how bright is 750 mcd? I can't find any references online.
 2nd August 2010, 06:03 AM #4 lgreen   diyAudio Member     Join Date: May 2003 Location: San Diego, USA This is one thing where "learning is doing," where I mean you have to use trial and error. I found that with as little as 5mA the LEDs were still very bright. Try a low voltage source like a 9V battery, use the same equation and do 3-5-10 mA to see what you like. Then use the same mA for your final install. Or just experiment with your final install in the first place using a variable resistor or something. What is stopping you from this- no LED or resistors? Just planning right now and have no parts? With 24VAC you have to be careful about power dissipation, might need a 1/2 W resistor- but you are all over that issue. __________________ Retired from DIY (2010) but still lurking now and again. My DIY audio projects- PartTimeProjects.com.
 2nd August 2010, 07:58 AM #5 theAnonymous1   diyAudio Member     Join Date: Feb 2004 Location: Anonymityville I use a 10k trim pot and slowly lower the resistance until it's at the brightness I want. I then measure the pot setting and replace it with a suitable resistor. Perfect brightness every time.
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Join Date: Jul 2010
Location: Chicago
Quote:
 Originally Posted by lgreen This is one thing where "learning is doing," where I mean you have to use trial and error. I found that with as little as 5mA the LEDs were still very bright. Try a low voltage source like a 9V battery, use the same equation and do 3-5-10 mA to see what you like. Then use the same mA for your final install. Or just experiment with your final install in the first place using a variable resistor or something. What is stopping you from this- no LED or resistors? Just planning right now and have no parts? With 24VAC you have to be careful about power dissipation, might need a 1/2 W resistor- but you are all over that issue.
Weird. I asked my dad if 20mA is bright and blinding (if looked directly into it). He said not at all. This is a 3.5V diffused LED. I'm guessing the ones you were using were the clear type?

I'm ordering stuff from Mouser and Digikey so I would like to have everything in one package. I would like to avoid "The Shack" as much as possible.

Quote:
 Originally Posted by theAnonymous1 I use a 10k trim pot and slowly lower the resistance until it's at the brightness I want. I then measure the pot setting and replace it with a suitable resistor. Perfect brightness every time.
That's a great idea! The horror of ordering every resistors from The Shack just for testing purposes. Thank god trim pots were invented.

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