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So i am realizing that I should probably learn this whole voltage and resistance and amperes thing.

Ohms law is V=IR (voltage = current * resistance)

I have a few questions.
I buy my LEDS from e-Bay, normally hong kong/china. 
It says forward voltage for a blue Led is 3.0 - 3.8 and that confuses me, cause shouldn't they have a specific voltage?
they also have forward current is 20 mA. 
I'm just curious, for example, say i put 4 of these in a series circuit, how much resistance would I need?
and then what about a parallel circuit what would I do then?
I learned this in high school way back, but i've forgotten it since then.
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I don't think you connected the grounds, Dave.
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I'm just curious, for example, say i put 4 of these in a series circuit, how much resistance would I need?
Forget resistance for now, you're going to need at least  4 * 3.8 volts of supply to start with - that's a minimum of 15.2 volts.
Do you have a suitable supply?
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plug it into the wall?
I'm not sure...I can put 4 leds in a series with my arduino and they'll work without any resistors while the arduino is plugged into the serial bus port.
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I don't think you connected the grounds, Dave.
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I can put 4 leds in a series with my arduino and they'll work without any resistors while the arduino is plugged into the serial bus port
Only if the Vf of all your LEDs add up to less than 5 volts.
Can I suggest you don't connect anything without resistors until you understand more?
It is a great way of burning-out Arduinos.
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haha, yeaaa. I'm using common anode RGB Led's most of the time and I always put a resistor on the cathode.  I'm just saying i did it once and that confused me. 
I'm pretty good about getting the right things there, I just don't understand it.
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WAIT! I do have some old power supply units from computers, think I could use those somehow?  I mean they are like 250W but those'll do right?
How would I be able to tap into those?
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Most PC power supplies don't have output voltages higher than 12 volts, so that 15V plus is still looking elusive.
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Now what you mean is something would provide that much voltage through the little DC adapter on the arduino board?
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Perhaps this tutorial will help?

http://www.antonine-education.co.uk/electronics_as/electronics_module_1/topic_4/topic_4__diodes.htm

In particular, note the I vs. V plots. In the forward direction, no current flows until the voltage is very close to the forward voltage. In the example you have given, you describe 4 diodes in series, thus the voltage drop across all 4 is additive, therefore, you will need to supply that total amount of voltage before any current will flow.

Now what you mean is something would provide that much voltage through the little DC adapter on the arduino board?

No. The Arduino board can supply only +5V max. And if you're referring to the barrel power jack, then the limit there is +12V, but it gets regulated down to +5V anyway, so there's no "extra" to be had that way.

To do what you're describing, you would need to use some other power supply. Forget Arduino for a bit, and focus on what your operative circuit needs. Then you can work out how to supply that, and control it using an Arduino.
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So if I needed to power something that needed more than 5v (b/c the arduino would bring it down anyway no matter what i put in, right?) would I plug the series into the pin on the arduino board and then have the other pin go to a power supply that provided more and then go to the ground on the arduino?  I would need some resistors somewhere in that circuit so I dont burn out the board, correct? Am I in the same ballpark?
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You would only do that if you wanted to blow up your arduino.

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b/c the arduino would bring it down anyway no matter what i put in, right?
Wrong

A voltage plugged into the power jack would remain the same. The arduino would tap off some of the power and convert it to 5V for itself and the +5V pin.
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Okay so then would I have to do something like the attached image for the series of 4 or 5 or more leds?



Uploaded with ImageShack.us

Its pretty basic, I just made it in paint


* Circuit.jpg (29.74 KB, 767x417 - viewed 5 times.)
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It helps if you read my answers. If you do that you will blow up your arduino unless the external voltage is 5V.
You need the resistor in the anode or cathode of your LED. The ground of the external supply should be connected to the ground of the arduino.

You need a transistor or FET if your external supply is higher than 5V.
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mmkay, at this point i'm only doing little tutorials so that i don't' blow things up although I did make one thing last night.
Lemme see if this was bad or not. 
So i got some fading code for RGB Leds last night that somebody shared. 
I hooked it up the way you would for 1 rgb led.  common anode.  I put 1 resistor on the cathode and then hooked up the digital pins.
But then I put a bunch more there too all connected to the led before it and then the resistors were all connected together on the +- part of the breadboard.
So all of the leds were going off of the digital pins that were for the 1st led and then all of the resistors were going off of the 1 ground.  Is that bad?  I mean it worked and nothing got warm or blew up. 
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But then I put a bunch more there too all connected to the led before it and then the resistors were all connected together on the +- part of the breadboard.
So all of the leds were going off of the digital pins that were for the 1st led and then all of the resistors were going off of the 1 ground.  Is that bad?  I mean it worked and nothing got warm or blew up. 

There just isn't enough information here. "Bunch more" isn't descriptive enough. More of the same RGB LEDs? The blue ones you referred to earlier? Resistors? Something else?

You started out posting Ohm's law. But do you understand how it works with resistances in series? Doing the tutorials can be fun, and the temptation to add more stuff to see what happens is there, but as Mike said, this is a good way to fry your Arduino, if you just do it willy-nilly without doing the arithmetic for your circuit.

Did you read the diode tutorial I posted? Have you read other threads on this board about basic electronics? Maybe http://arduino.cc/forum/index.php/topic,73872.0.html ?

People here will be happy to help, but it will be much easier if you proceed in logical steps. Doing things with electronics requires some basic foundation. Linking to the tutorials you're using will help as well.

And yes, MS Paint drawings are OK too, as long as they're clearly labeled and we can see what you're doing.
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