arduino driving multiple leds

How many leds in parallel could one digital out on the arduino board power? meaning could i hook up 25 led's in parallel to one digital output? thanks for all the help and im using the arduino delimonlove with the atmega 328 if that helps

How many leds in parallel could one digital out on the arduino board power

Two Each output should not be asked to supply more than 35mA. Each LED to have its own resistor.

Each resistor to have its own resistor.

When does it end?

When does it end?

That's recursion for you ;)

Original post edited. :)

The qty could be higher as long as the current limit is respected, no? I would submit that 10 columns of 2 LEDs and current limit resistor for 2 mA in each column could be supported with a 5V arduino. Say if one wanted to cover the face of a guitar for example. Saw that on TV, not sure what group, they had a bunch of LEDs on the front body of their guitars that sequenced across, turned off/on in time with the music, etc. Kinda neat to look at.

Lot of resistors that way. I would go with a higher voltage if available, put the LEDs in series and cut down on resistors.

Or 2 in series at 20mA, could be obnoxiously bright if they were 5,000-10,000mCD LEDs! I am using 5,000mCD LEDs in my fencing scoring displays, but only driven at 2mA as they are so bright. Have room to get brighter if needed.

Or add a transistor buffer and really get the current up there (but that is really added scope to the question).

Depends on the planned use I guess.

The qty could be higher as long as the current limit is respected

Yes quite right. You can get low current LEDs but 2mA is pushing it a bit.

Again in series would increase things but the original question did say parallel. You could probably get away with two in series especially if they were red.

You should try some of the 5,000-8,000 LEDs. Those suckers are bright! I have some 10,000 mCD Ultrawhites, I measured the current at 1.8mA and my wife said they were still too bright!

so if i installed a transistor using 9 volts as Vdd to that transistor, how many leds could i then have in parallel? thanks

If you have the arduino output connected to the gate of a mosfet transistor, such as International Rectifiter F3707Z, the arduino will need to source very little current out to turn it on. Per its datasheet, with the Source at ground and the Gate at 4V, one could get near 50A thru a properly heatsinked part. Rds of the part is 0.0125 ohm, so with P=I^2xR, 50A would be dissipating 31W in the part. 10A would just be 1.25W, which wouldn't be nearly as hard to heatsink away. If the parallel LEDs were each dissipating 20mA, thats 500 LEDs. If could connect your LEDs into columns, say 5 in series per column with a 12V supply, then 20 columns would give 100 LEDs (and a lot less resisters!) and the MOSFET would only be sinking 400mA, dissipating just 5mW.