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Bicolor LED - 3 leads in a 2 hole

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thatdecade

New Member
Hi All,

I made a new enclosure for a MP3 player as a project. I would like to replace the red/green status LED on a MP3 player with several red/blue or blue/white LEDs. The status LED only has 2 leads so I assume it is the bi-directional type. I plan to use a simple push pull circuit to drive the additional LEDs [Boosting the Drive Current].

Ok, so the problem is I can only find 3 lead (common lead) dual color LEDs for purchase for my color selection. Is there a simple way to connect a 3 lead in place of a 2 lead? I searched around and found a post detailing how to accomplish this with diodes. I am under space constraints in the new enclosure and need to do this with as few components as possible.
I was hoping there was a way to do this with a trick of wiring.

Alternatively, does anyone know where I can buy 2 lead bicolor LEDs for red/blue or blue/white?

Any comments are appreciated, thanks.
 

Hero999

Banned
I've not had any luck with finding teo lead through hole red/blue bicolour LEDs but I've found some in SMT packages.

Your Search Results | Farnell United Kingdom

Another option is to use a three lead red/blue LED and use logic gates to convert the push-pull signal to light either one LED or the other.
 

thatdecade

New Member
Wow, thanks~! The logic gate idea is great! I hadn't thought of that. I didn't mention it, but I also have my own uc in there. I left two pins on the board free for future use. An analog input and a digital output.

I can wire it up with the input from the MP3 status LED to the analog input, and a single transistor on the digital output to drive the additional LEDs. Then it is just software to make it all work.

Here's a neat dual color trick for you. How do you control a dual color LED with a single digital output?
You connect the resister to the LED on the common lead. This wiring only allows one color to light at a time, if both colors are powered the one with the lower forward voltage wins.

Do either a quick experiment or take a quick look at the LED's datasheet to figure out which color to power with the system on and which will be switched on and off.
 

Hero999

Banned
You need at least two outputs free on the MCU to control a three pin bicolour LED.
 

Hero999

Banned
Sorry, I didn't read the thread you linked properly, this will work, but you'll need to omit the zeners.
bi-color-problem-gif.21849
 

marcbarker

New Member
connect the resister to the LED on the common lead. This wiring only allows one color to light at a time, if both colors are powered the one with the lower forward voltage wins.

I see :) green anode to vcc, common cathode to ground via res., and the red anode to the uC. The driven red one shorts out the green one. cool!

Also, LED Vdrop & light wavelength are inversely proportional to eachother.

PS, if you waggle the red, you make a third coloUr: yellow.
 
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Hero999

Banned
Oh yes, I get it now, marc's explanation was more clear.

It's no problem as long as the voltage across the dropper resistor doesn't exceed the reverse rating of the red LED. You can't turn the green/blue LED off though.
 

marcbarker

New Member
It's no problem as long as the voltage across the dropper resistor doesn't exceed the reverse rating of the red LED.

You can't turn the green/blue LED off though.

what's the problem with an with LED reverse breakdown? (apart from 'violating' AbsMax) Does it harm the LED? (not a trick question, I've always been curious of this)

Why doesn't it turn off the green/blue LED? I think it does. (says I with deja vous feeling.....)
 
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Hero999

Banned
I mean you have to turn on the red LED to turn off the green/blue LED so you can't turn both LEDs off without using another pin.

Exceeding the reverse breakdown voltage doesn't seem to damage old red LEDs but it breaks the newer blue/white LEDs.

The reverse breakdown voltage will probably be much higher than the 5V, normally specified on the datasheet, anyway.
 

thatdecade

New Member
LED Vdrop & light wavelength are inversely proportional to each other.
I didn't know that, thanks. The color with the lower wavelength has a higher voltage drop. Cool. So activating the lower wavelength color starves the higher wavelength color for current because the voltage drop across the resister decreases. or something like that lol

Wavelengths: Violet > Indigo > Blue > Green > Yellow > Orange > Red

PS, if you waggle the red, you make a third coloUr: yellow.
I have actually been doing something similar with red/blue LEDs. Single common resister, blue always powered, red powered by uc with PWM. I set the frequency to make a nice purple. :)


I looked into my blue/white LEDs and my plan may not work. The forward voltage rating is the same for both colors. That doesn't sound correct, it may be a specification mistake. I ordered a few to experiment with, should be here later this week.
 
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