Super-bright Radio Shack 3.7V blue LEDs: a quick guide

[Crazy Jayhawk]

These LEDs are quite bright, but there's a problem. They use a very odd voltage figure, which can be hard to incorporate into a computer. If you want to hook these LEDs up to the 5V or 12V lines, you're going to need a resistor inline with the LED.

First, let's find out what the effective resistance of the LED is. The package lists that it runs at 3.7V and about 20 milliamps. Using Ohm's law solved for resistance, 3.7V/20mA = 185 Ohms.

Now let's find out what total resistances would be needed to achieve this same 20mA figure in a 5V or 12V circuit. These LEDs are able to handle up to a 30mA current, so I'll also calculate the minimum series resistance.

5V/20mA = 250 Ohms. 5V/30mA = 167 Ohms.
12V/20mA = 600 Ohms. 12V/30mA = 400 Ohms.

Subtract the LED resistance from these values and you have 0-65 Ohms for the 5V and 215-415 Ohms for the 12V. These LEDs have a max voltage of 4.5V, so they could conceivably last a while given 5V directly, but this is a much safer approach. Mine are hooked into my 12V fanbus, and have 390-Ohm resistors in place. This is well within the 12V resistance figure.

Hope this helped somebody. :)

[IFMU]

Well Im sure that if I had a clue as to what you were saying, it would......:rolleyes:

[Warlord2]

coo


I am going to make a fanbus and this would work nicely

thanks for the info

[KeyboardCowboy]

very cool, i love blue leds

[Crazy Jayhawk]

Originally posted by ifmu
Well Im sure that if I had a clue as to what you were saying, it would......:rolleyes: Tell me what's confusing you and I might be able to help you. :p

[mark]

I tried to go to Radio Crap for LED's but they were always out of the white and blue ones. So I looked around, and http://www.caseetc.com carried them for the same price as Radio Smack does. I also got the 470 Ohm Resistors too. ( I am running the LEDs from a +5v line with that resistor inline on each LED.
Then one day I had to go into Radio Scrap to get some elect. tape and a molex kit. They had the LED's so I got a blue and a white one. I hooked them up to the +5v line without a resistor and they had them SAME brightness. (if you run without a resistor for long periods of time they will get very hot, and will break.)


So if you are looking for LED's check both places.

Good luck
mark

[Oni]

I plan on eventually switching my LED's to blue. If so, I'd wanna dim them down a bit. Could I run a lower voltage into them (say 2volts) and get a weaker light?

[Crazy Jayhawk]

Originally posted by Oni
I plan on eventually switching my LED's to blue. If so, I'd wanna dim them down a bit. Could I run a lower voltage into them (say 2volts) and get a weaker light? Yep. All you need is the appropriate resistor.

[bdf24]

Do you know what voltage the LED's running off your motherboard for power and HDD run?

[Crazy Jayhawk]

Originally posted by bdf24
Do you know what voltage the LED's running off your motherboard for power and HDD run? I think it's 5V. So you could get some 5V blue LEDs from the 'Shack and hook them up directly.

[el]

most hd and mobo LEDs are around 2.0volts. sometimes this is not enough voltage for higher voltage LEDs. I have a blue 5volt LED and it work on two boxes but the third I had to use a molex pass thru with the blue power LED! I just finished a RED box! One epox 8k7a with LED readout and some other red stuff. A red LED power and Red HD indicator adn now I jsut added a red floppy LED! now it looks sweet. Oh yeah I almost forgot I got a window with a red cold cathode tube too!!! the blue box is going to be so f@cking bright!!!!


btw a modders best friend is a multimeter

[scap]

i have one of those led i used on a project i turned it into a flashlight it was so strong.
Thanks for the info
scap

[Oni]

Originally posted by Crazy Jayhawk
I think it's 5V. So you could get some 5V blue LEDs from the 'Shack and hook them up directly.

I bought my blue LED's today. I think I'm gunna have to dim them a bit. My computer runs 24/7 in my room, and w/ my cable modem, my mouse, and my computer on, its pretty bright in here. But we'll see