Too much current for the Arduino to handle?

I'm working on creating a custom light show for a small (2 ft) Christmas tree. All the lights on the tree are battery-powered LED lights which I've taken the battery pack out of and tinned the wires to plug into the breadboard. I measured the resistor in each battery pack to get an idea of how much current each strand is pulling, and here's what I have:

4 X 30-light strands that ran off 4 AA batteries with a 7.5 ? resistor => 0.8 Amps each 2 X 18-light strands that ran off 4 AA batteries with a 5.1 ? resistor => 1.18 Amps each 2 X 10-light strands that ran off 3 AA batteries with a 12 ? resistor => 0.375 Amps each

Adding these up I get 6.31 Amps, which seems suspiciously high even though there's a lot of LEDs here. Am I missing something? Obviously I'll have to use different-valued resistors to account for the different voltage, but then I saw my wall-wart is rated at 9V at 650 mA, which makes it sound like I'll go way over what this power supply can handle. Is that right? Thanks in advance, electronics have never been my strong suit... :)

Bottom line is that an arduino digital output pin can safely supply only less then 40ma (20-30ma recommended). Trying to power anything requiring more then that amount of current requires an external switching device like a transistor or relay.

Lefty

Hmm well I successfully experimented around with the 4 X 30 strand lights (with a 10 ? resistor for each) which should have resulted in half an amp each according to my calculations, well above the 0.04 amps each pin is rated at. Either I'm seriously messing up my calculations or I'm missing something here.

Maybe your Arduino will last until Christmas. Maybe not.

BrandonR: Either I'm seriously messing up my calculations or I'm missing something here.

Yes you are. You are missing proper measurements taken with a multimeter. You can't reverse engineer something with just guesswork and math.

The resistor isn't the only resistance, the lights themselves are resistance limiting the flow of current, perhaps the resistors are for a sort of failsafe incase of a short on the line

winner10920:
The resistor isn’t the only resistance, the lights themselves are resistance limiting the flow of current, perhaps the resistors are for a sort of failsafe incase of a short on the line

You’re absolutely right, I forgot about the resistance of each LED. :~ I do have a multimeter and I measured the current of the 30-light strand with the battery pack to be about 95-100 mA – considerably less than the 800 mA I previously calculated. So knowing that the 4 AA batteries were producing about 6.4 V (also measured), is determing each LEDs equivalent resistance as easy as 6.4 = .095 * R, R = 67 ohms, minus the resistor in the battery pack (7.5 ohms), R = 59.5 / 30 (# of LEDs) = ~2 ohm?

winner10920: The resistor isn't the only resistance, the lights themselves are resistance limiting the flow of current

Except that when LEDs have their forward voltage dropped, their resistance is practically 0.

True, probably tho every so many leds is a current limiting resistor liie most led strips