At 08:25 PM 9/5/2012, you wrote:
>I have a pin coming out of a chip that indicates an error condition. Pin
>will be at +5 or ground. (e.g. not a 3.3 volt system)
>
>The output requires at least 4.7K ohms resistance, or risk pulling more th=
an
>the iMAX rated current for that pin.
>
>PCB already done, no room for a transistor to amplify the output (next
>revision).
>
>So the question is this: What LED would you use to get the most noticeable
>indication from 5 volts through a 4K7?
>
>I'm thinking very small, but diffuse red.
>
>But then I notice the ones with the highest mcd rating are always clear...
>
>Any experienced advice?


Hi, James:-

(oops.. sent before finished)


I think it depends a lot on the package. With a Vf of 2.1V, you could
get maybe half a mA, which should be sufficient for normal room lighting.

Tinted will give you more contrast. Tinted clear gives more brightness
viewed head-on (assuming a lens at the end) but diffused give a lot more
viewing angle. Small SMT ones are pretty disappointing, but there are some
with lenses on top.

In 3mm radial types- SSL-LX3052ID is one possibility. $21/100 maybe 1mcd.

This one: WP710A10SRD/E from Kingbright looks very good.. $9/100 and
should give >10mcd with 5V and 4K7.

If its really important to you, I'd buy a few samples of various types
and try them out.. just to be sure.

BTW, usually CMOS outputs have lower drop pulling down rather than
sourcing, so it's better to put the LED from +5 to the output (through
the resistor) rather than from the output to ground.

--sp



>--
>James Newton at massmind.org
>1-970-462-7764
>
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