Max ratings for the BBB outputs...

Hi Fredrik,

I think you are reading it correctly. Most LEDs can be driven brightly for indication with just a few mA, so in that case you should be ok - I've not fried my BBB with just a few LEDs (see here) and neither has this guy here. For higher loads, driver circuits should be considered.

I don't disagree, leds with suitable resistors will be fine.

However, for comparison, page 6 of the BBB schematics has this for the onboard leds.

Hehe yes I too noticed that. Nice little 6-pin devices for Q1/Q2. I wasn't sure what voltage blue LEDs need (not really something either of us use a lot of! ;-)

Which reminds me, I still need to remove them off another BBB. I might go for sunflower yellow this time ;-)

Thanks, that answered my main concern...

As long as I'm aware of the limitations, I don't see 6mA as a limitation, it's really just a matter of designing for it...

shabaz wrote:

I wasn't sure what voltage blue LEDs need (not really something either of us use a lot of! ;-)

I recall the voltage is slightly higher, but not much.  I think they did it this way so that they could make those blue leds really, really, really bright

selsinork wrote:

 

shabaz wrote:

I wasn't sure what voltage blue LEDs need (not really something either of us use a lot of! ;-)

I recall the voltage is slightly higher, but not much.  I think they did it this way so that they could make those blue leds really, really, really bright

According to Wikipedia, Blue LEDs have a voltage drop from 2.48V to  3.7V.  I've usually seen numbers like 3.1V.  It's problematic driving these from 3.3V, since the chip will only be able to pull down to 0.1V to 0.2V or so, so good luck selecting a resistor to get just the right amount of current.

So the board pulls the blue LEDs up to 5V.  The problem here is that if you connect the LED cathode to a SoC GPIO, if that I/O is off the LED can pull the pin to 5V and blow the I/O if it's not 5V-tolerant -- I think only the USB pins are 5V-tolerant.  If you're lucky, the GPIO's VDD clamp will turn on and the LED will turn on weakly.  I think the transistors are there to protect the SoC pins.

John Beetem wrote:

 

According to Wikipedia, Blue LEDs have a voltage drop from 2.48V to  3.7V.

I just did a search in farnell's catalogue for blue leds which reveals this at the top end with 4.9v

http://uk.farnell.com/chicago-miniature-lighting/cmd15-21ubc-tr8/led-blue-1-5mm-x-2mm-45mcd-430nm/dp/1345388

and these at the bottom end with 2.6v

http://uk.farnell.com/optek-technology/ovm4b/led-blue-2-26cd-463nm/dp/2075213

I'd probably been thinking of around 2.5v, so it seems I'm out of touch with blue led specs.  In my defense, I do hate blue leds with a passion

selsinork wrote:

 

In my defense, I do hate blue leds with a passion

They're only a problem at the start.  Once your retinas are nice and crispy, the issue disappears, literally.

Morgaine.

selsinork wrote:

 

I'd probably been thinking of around 2.5v, so it seems I'm out of touch with blue led specs.  In my defense, I do hate blue LEDs with a passion

I like blue LEDs in moderation, but then I remember when practical blue LEDs were one of the "holy grails" of the 1980s and everyone was quite impressed when someone got the first carborundum LED working in 1989.  This in turn reminded me of the wonderful old saying my father taught me: illegitimi non carborundum, which means "don't let the bastards grind you down".

I digress... a lot.

So, have you tried painting the tops of your blue LEDs with various colors of phosphor?  With blue as an exciter, you should be able to get any color you want short of violet.  Cheap white LEDs use yellow phosphor, which combines with the blue to produce bluish-white light.

Guess it's a personal preference, but I love the blue LEDs.

And I do remember the whole search for the Blue LEDs.

It really is amazing how much brighter with less current the "modern" LEDs are, compare to the ones in the 80-90s...

And I guess they are only going to get brighter in the future...