I am looking to power my UNO with a 9V battery through a snap connector connected to the power jack. I've heard that if you do this, the board itself is powered at 9V? So the output is 9V? If this is the case(and it overrides the usual 5V output of the UNO), what resistor would I need for my LEDs? I am using a red led, which I think draws 2V.
I figured it would be: (9V-2V)/0.02A = 350ohms.
so would I need to get a 350 ohm resistor? (or go a bit higher, like 390)? or do i just keep the 220ohm resistors
The Arduino has a voltage regulator so the Arduino itself, and any LEDs it's driving are running from 5V (assuming they are connected correctly).
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Do not use a PP3 battery as it cannot supply much current.
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Connect the external power supply to the Arduino power jack.
Okay thank you, just to confirm, I should just stick to the 220 ohm resistors as usual because the Arduino will reduce the voltage from the 9V battery down to 5v?
What do I use other than a PP3 9V battery? It's just a project which uses 8 LEDs and an LCD display(though not at the same time).
And yes, I was thinking of connecting it there
6 1.5V penlites in series will do.
But 5 penlites will do just as good.
How would I connect them in series and then to the power jack?
Battery holder.
jack
jack and plug sets
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Great thank you. I just saw this on digikey:
the first image is a 6 pack battery holder but the second one is a 4 pack. The voltages would be above 5 anyways for either one but the second one has a custom mounting style(first is chassis) and it also has a cover with an on/off switch. Is it fine to get the 4 pack, despite less batteries? The device won't be running for a long time and is just for short term entertainment purposes.
Unfortunately, no. The Arduino has a built in regulator that needs a MINIMUM of 6V, but preferably more like 7. Your 4 batteries will meet that 6V requirement briefly, when absolutely fresh.
Ah right, so after a while the batteries will just become insufficient?
Would x4 3.6V batteries be overkill? should I just stick to the x6 1.5V
Yes, and the worst part of that is it will likely make your Arduino erratic. If it just shut down that might be okay, but it's more likely to restart, hiccup, etc.
Okay, and yeah i just remembered the 7-12V input to the arduino 
, great thank you for the help! I think that's all I needed to know at this point 
If you have a car that can do 180km/h, should you drive it at 180km/h?
Just use a 1k resistor, and lower the value if the brightness is not enough.
Or use 10k if it's too bright.
Leo..
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