Re: [Gumstix-users] Lighting a Led with Breakout-gs

[Dave H. <dhy...@gm...>]

hi Deniz,

>      I will try to light a LED with Breakout-gs, but before i try i think it
> is better to ask the list. Are there any difference using any CN? I will
> connect the LED directly  to GPIO and GND I guess, and echo set and clear
> the GPIO. I affraid to burn gumstix becouse of possible high current. Can i
> try this scenario directly? What are the things that i must care about?

You'll also need a current limiting resistor, otherwise the LED will
live a very short life.

Here's a page that walks you through figuring out the correct value:
http://members.misty.com/don/ledd.html

A typical run of the mill red led will have a voltage drop around
1.7v. The GPIO pins on the PXA255 are only rated to 10 mA, so we can't
exceed that. 15 mA would be a bit better, but you should use a
transistor or buffer driver if you're going to exceed the specs.

So now, we use ohms law. V = I x R, where V = 3.3 - 1.7, I = 0.010,
and we're trying to find R. So R = V/I = 1.6 / 0.010 = 160 ohms.

So you need a resistor with at least 160 ohms. Higher is fine, it will
just cause the LED to be a bit dimmer. 180, 220, 270, 330, 390, and
470 are common values above 160.

As far as which CN to use, no it doesn't really make any difference. I
would avoid the FFUART.

-- 
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/