scott wrote:

> Load up the actual PDF spec that is linked from that page, it has more 
> detail, just stating the forward current like that means nothing.

Oh, right. I hadn't even noticed that hidden away in a corner there! I 
noticed some of the other pages had this, but in *light grey* it's not 
especially prominent.

> On page 3 there is the "Absolute max" table, the continuous current here 
> is listed as 20/25/30 mA depending on the colour.  That is the maximum, 
> above that they do not guarantee the LED will work.

I see.

> Now look at the lower part of page 3, some interesting charts there 
> (assuming the Bright Red colour, the others are on the next pages).
> 
> The one in the top right shows "brightness" against current.  You see 
> it's linear up to 10 mA and then flattens out.  This is saying there 
> isn't much point using more than 10 mA, it won't be very efficient.

Right. So it will light up with just 10 mA, just not quite at full 
brightness. (On the chart I'm looking at, 15 mA looks like it's 
meaningfully brighter, but any higher than that has negligable effect.)

> The one in the top left tells you what current will be drawn for a 
> particular voltage across the LED.  If you want 10 mA then you're going 
> to need to give the LED 2.1 V.  You also see here that even adding just 
> 10% to that voltage makes the LED current almost 3x higher!

Right. So I'm looking at 2.1 V or 2.2 V or thereabouts.

Now looking at <http://focus.ti.com/lit/ug/scyd013b/scyd013b.pdf>, on 
page 231 (which is actually page 236 of the PDF file), we see that for 
the 74HC00 I'm looking at using, we have Icc <= 0.02 mA, Iol = -Ioh <= 4 
mA, and tPLH = tPHL <= 27 ns. Now, if I actually knew WTF that means...

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