I think it's safe to consider the 1.8W and 25C to be mutually exclusive unless liquid cooling is involved.

The datasheet specifies a junction to ambient thermal resistance of 55C/W.  At 25C ambient and 1.8W that would mean a junction temperature of 125C, which is the maximum.

I would also add that most modern LEDs shine very brightly on just less than 5mA.  They last forever that way too.  25mA just leads to short lives.

Who said 25ma?

It looks like the (ULN2803A) package can dissipate a maximum of about 1.8 watts total at 25C ambient (room temperature).  You might be able to increase that a bit by using heavy traces on the PCB, or as heavy copper wire as you can if you are hand wiring.  But if you push it to it's maximum, it will be too hot to touch.

Just a suggestion here, but I would keep the total package dissipation to below 1 watt.  The spec sheet gives the Vce as around a volt if you keep the current on any given pin below 300ma and that would suggest a maximum, if load is spread out across the device, of about 1 amp total.  If you limit your LED drive current to 10ma, that would suggest you could run 100 LEDs per device with no more than about 30 on a single output.   For 20ma, cut those numbers in half.

As has been said , if you needed more LEDs than that you could stack them in series and use a higher drive voltage.  As an example, if you re using white LEDs with a forward voltage of 3.5V and drive them with 12V you could use 3 in series with a 150 0hm resistor to drive them all at 10ma.  But since you said you won't need more than 15 per output, that shouldn't be a concern.

I like the 2nd a 3rd ones.  Do you know when they will be available?

Not sure that is fair.  I doubt Aztec are set up to be, or ever intend to be, the kind of place that Mouser is.  Aztec probably carry some common parts as a convenience to their customers.  For example by analogy, one would not expect to pay the same for a quart of milk at the local gas station than at the big-box grocery store, but by the same token, you might not be able to get your gas at the grocery mega-plex.  Aztec do sell things that Mouser just do not.  As for shipping, their $5.00 verses Mouser's $4.99 does not seem to be out of whack.  They both offer free hipping over $200 as well.  Even still, that cost on that LM35 not withstanding, how many people are likely to go to Mouser for a single LM35 and pay $6.20 for it?

...and I can’t be bothered to look any further.

Pfffft..!

The swan song of the permanently ignorant.  Not that you are of course, but you are certainly singing the song.

To each their own.

As the subject line asks, where do you buy your Ardunios and other stuff and why?

A couple of months ago I was going to buy 2 Uno's and a bunch of other stuff and was looking for a seller.  I was half thinking of Sparkfun, but their price on some stuff was fairly outrageous.  A friend suggested another place, and when I checked with them, the order was going to cost about $20 less!  And I got more stuff, so, of course I went with them.

Now here's the thing.  I was recently in a email conversation with on of the folks at the shop I bought that order from about some questions I had (yeah, try that with Sparkfun) and somehow we got around to how many Arduinos they sell.  They said that since December their Arduino sales have completely tanked.  Only 1 to 2 a week, if that!  Then I checked the stock index at Sparkfun, they sold 200 in 2 days!!  Now, Sparkfun charge $30 for an Uno and Aztec (the other store) only charge $28.

So, I'm a little confused.  There are all sorts of other examples too from that order I had.

2N3094 - Sparkfun $0.75 for 1, Aztec $0.99 for 15
ATmega328 w/bootloader - Spakfun $5.50, Aztec $4.25
ATtiny85 - Sparkfun $2.84, Aztec $1.80
12mHz Crystal - Sparkfun $0.95 for 1, Aztec $0.99 for 4
LEDs - Sparkfun $0.95 for 1, Aztec $0.99 for 5
RGB LED - Sparkfun 1.95 for 1, Aztec $0.99 for 2
10K trimpot - Sparkfun $0.95 for 1 single turn pot, Aztec $0.99 for 3 multi-turn pots
400 point breadboard - Sparkfun $5.95, Aztec $3.99
0.1uf capacitor - Sparkfun, $0.25 for 1, Aztec $0.99 for 10
CDS photocell - Sparkfun $1.50 for 1, Aztec $0.99 for 3

There was more, but I think you get the point

Given then that Aztec sells the Arduino for $2 less than Sparkfun, and pretty well everything else, what's going on? I mean, I could understand if they sold maybe 10 times as many, after all Sparkfun has been around  long time and are better known, but 1000 or more times as many, and at a higher price?  I don't get it.
 

Grasping at straws here but:

Code:

void loop()  {
 
  int brightness = 0;

Should just be

Code:

void loop()  {
 
  brightness = 0;

Can't see it making  a real difference, unless it does!

This might be a silly question, but are you sure the 12V is DC?

Do you have access to a scope or a DVM?  You could check the signal going into the gate to see if the it is actually 0v when you execute analogWrite(0).

Looks like it should work unless something weird is happening.

The Americas have been occupied for tens of thousands of years.  Check your facts.

Tim

I said predominant culture.  Check what I wrote, and check your attitude.

So the age of a culture is it's dominate attribute?

Age, type of bacteria, and whether the fruit is at the bottom, or stirred in.

Yes, quite! (LOL)

So the age of a culture is it's dominate attribute?

Lefty

Well (at the risk for wandering too far off topic), by definition, quite simply, yes.  It takes time to develop culture.

To take things to an extreme for purposes of demonstration, if I created a town tomorrow, how deep would you say it's unique cultural attributes would be, by say, Sunday, when compared to a place like Athens, or Lagos (current name not withstanding), or Alexandria?

Most, really?

Well, I am also a product of NA.  However, I am open to discussion.  Given that the predominant culture in the Americas is well less than 600 years old, can you think of some other place of significance that is also significantly younger?

Seriously, we have cowsheds older than America

LOL, The phrase "North American culture" is a bit of an oxymoron when compared to most of the rest of the world.

There are batteries based on tritium that are supposed to (theoretically, not promised) last hundreds of years.  Just google "Nano tritium battery"

You need deep pockets though.  I think I read somewhere they were nearly $2000 each.

My OMS shields came in yesterday.  Hey, maybe the post office is getting their act together

Anyway, nice little boards.  I put together this enc28j60 based ethernet shield on one of them.  Very versatile.  The long 5-hole connected traces allow for using Verro-board style techniques that make them very flexible to use.  I'll be getting more for sure.

It certainly fits more snugly than the commercially available shield shown below.