35mA led with 25mA pic output?

[skimask]

People sometimes forget that most LEDs will produce full brightness long before maximum rated current is reached.

As is the case with my parts box full of LEDs...good luck finding any sort of datasheet for any of them.
Just start with a high value resistor (1K-ish) and work your way down until it's good enough or you hit the current limit (47 ohm is my low limit and it's worked for me for years). Hit the current limit first? Put that LED in a different box (i.e. garbage can) and grab another LED.

[xnihilo]

As is the case with my parts box full of LEDs...good luck finding any sort of datasheet for any of them.
Just start with a high value resistor (1K-ish) and work your way down until it's good enough or you hit the current limit (47 ohm is my low limit and it's worked for me for years). Hit the current limit first? Put that LED in a different box (i.e. garbage can) and grab another LED.

Right, thanks.

[falingtrea]

Looks like the LED you are using can work as low as 12.5mA. So you have a broad range for the current. I would at least try running it at 18-20 mA and see if the light level is sufficient. One other thing you should remember about LEDs, is the higher the current you run them at, the more you reduce the life.

[mister_e]

You can still use a pair of I/O in parallel to drive up to 50mA. It's been widely used in the past. I'm not a fan of it... but sure it may work. You just need to play with the WHOLE TRIS register value.

[rmteo]

There are LEDs and then there are LEDs. They can vary widely in their brightness output. For example, these are 2 green LEDs in a 5mm, T-1 3/4, thru hole package. Both are rated at a test current of 20mA.

http://search.digikey.com/scripts/Dk...me=511-1191-ND
This one has a rated output of 100mcd. This is typical of commonly available LEDs, 50 -300mcd.

http://search.digikey.com/scripts/Dk...FPG1-15Q-A3-ND
And this one has a rated output of 34,000mcd.

The output of the 2nd. one is 340 times that of the 1st. And it is not even a matter of price - both of them are about the same. So if the LED is bright enough for you at 20-25mA, just go for it. If not, get a brighter one. No need to mess with paralleling outputs, using BJT's or MOSFETS etc.

[skimask]

The output of the 2nd. one is 340 times that of the 1st. And it is not even a matter of price - both of them are about the same.

Almost...
The 1st one, at 10 degrees off center still looks about 70% of it's total.
The 2nd one, at 10 degrees off center is only about 30% of total.
So while the one may be brighter, it's only brighter in the middle, it's got a bit more focus...if that makes any sense...

[rmteo]

Almost...
The 1st one, at 10 degrees off center still looks about 70% of it's total.
The 2nd one, at 10 degrees off center is only about 30% of total.
So while the one may be brighter, it's only brighter in the middle, it's got a bit more focus...if that makes any sense...

Sure does. My point is that if you are using an LED as an indicator, 20-25mA is plenty to get a sufficiently bright display with the right LED. A high efficiency LED at just 1mA can be almost blindingly bright.

[xnihilo]

There are LEDs and then there are LEDs. They can vary widely in their brightness output. For example, these are 2 green LEDs in a 5mm, T-1 3/4, thru hole package. Both are rated at a test current of 20mA.

http://search.digikey.com/scripts/Dk...me=511-1191-ND
This one has a rated output of 100mcd. This is typical of commonly available LEDs, 50 -300mcd.

http://search.digikey.com/scripts/Dk...FPG1-15Q-A3-ND
And this one has a rated output of 34,000mcd.

The output of the 2nd. one is 340 times that of the 1st. And it is not even a matter of price - both of them are about the same. So if the LED is bright enough for you at 20-25mA, just go for it. If not, get a brighter one. No need to mess with paralleling outputs, using BJT's or MOSFETS etc.

34000mcd... awesome! I didn't even think such exist
What if I put one on my car ...

[skimask]

34000mcd... awesome! I didn't even think such exist
What if I put one on my car ...

34000mcd (http://en.wikipedia.org/wiki/Candela) sounds really bright and really impressive, and they are fairly bright when viewed straight on...but it's not all that.
These are what I used in my toobz...
http://www.philipslumileds.com/pdfs/DS05.pdf
70mA drive current with a 1/2 power viewing angle of 90 degrees.
They aren't the Luxeon super bright, multi-watt, power LEDs, but they do well.

[xnihilo]

You can still use a pair of I/O in parallel to drive up to 50mA. It's been widely used in the past. I'm not a fan of it... but sure it may work. You just need to play with the WHOLE TRIS register value.

Unfortunately I don't have enough pins. One pin only can be dedicated to a led. I will use the led at lower voltage as many suggested in this thread.