Ok so here goes, I saw the usb pen light that Sparky made, and thought to myself Hey! that would be cool! So I set out trying to make one. I pm'd Sparky with some questions which he answered, then we got the the resistor, well why do I need and resistor and which one? which he promply explains here --->
What your seeing there are either coated LEDS or really poor mcd LEDS. LEDS are rated by MCD to measure the amount of light they put forth. If an LED is only rated at 2000 mcd you can pretty much guess that their crap. The ones you have purchased from dragonmodz.net are rated @ 8000 mcd which is considerably brighter (top notch). Where you see that your LEDs are rated at anywhere from 3.3v to 4.5v foward voltage, those are part of the equation to which your going to need.

Im not sure if your really familiar with electricity or ohms law for that matter, but ill go ahead and give you the basics of whats gonna happen. There are basically 3 main parts to electricity into which you need to know in order to understand how your LED will function. The first is Voltage, which comes in two standards of AC (alternating current) and DC (direct current). This is referring the sign wave that ac or dc is measured by. If you were to measure the amoung of currnet that is moving through a strand of wire, and the voltage to which it is being carried is ac you will notice a certain pattern to which it fluctuates or "alternates"; but with dc you would just see a straight even flow of current "direct". This may all sound confusing right now but by the end you should be able to grasp the most basic concepts. To finish about voltage i want you to just think of it as a truck or car.

The next major part of electricity is Current, or Impedance, or most probably know as Amperage. This is what packs the load in electricity, makes the light emit light. Amperage or current is actually the think that hurts like hell when ever you get shocked. PURE VOLTAGE DOES NOT HURT YOU! But one cannot exist without the other one soooo dont get shocked. As i also stated about the differences in AC to DC above you can pretty much guess which type of voltage is supplied to your LED. If you said DC, youd be correct because as was stated before its a direct shot of current and if you somehow hooked up ac to it , the light would alternatly come on , as in blink. Youd most likely not be able to see the blinking but hopefully you see my point. The last thing to remember is to think of current as the driver of the truck(voltage).

Next we move on to Resistance, or ohms which you can pretty much guess what it is going to do. Resistance exists in all schematics whether there are resistors present or not. Why? Because all things electrical and all wires carry a certain amount of resistance. What resistance does is restrict the flow of voltage which in turn adversly effects Amperage, raising your amperage. Imagine that resistance is the road in which the truck is driving down.

Now for the finally which hopefully make things easier on you. The truck(voltage) is going down the road(resistance) and the driver(current) is just relaxing and goin with the flow of things. When suddenly the truck is slowed down by a mudhole in the road, we'll call this mudhole a resistor, so what does the driver(current) do? He stomps on the gas.

The next scenario is the truck just got out of the shop and now has a 472 big block installed in it. What does this mean to the driver and the road? Nothing to the road but the driver feels the need to slow down and take it easy because he such a big engin now.

What have i been getting at you might be asking yourself right about now, well what ive been doing is explaining Ohms Law. Ohms Law describes electricity and its effects in one simple equation E=I∙R. The E in this equation stands for voltage and the I stands for Current and the R stands for resistance. What my little scenarios were meant to do was show you the effects of changing one of these value, as you can see if you plug some numbers into it than all of their outcomes will change a second time if you change either E I or R. Lets plug in your LEDs numbers into it. First of all lets go with the voltage your going to be using from one of your molexs off your Power supply unit which is 5v, so if your LED is rated at 4.5 than subtract 5 from 4.5. Which is as you can see .5v. Now plug that into your equation under E. The next part is your current, so since you wanna use 4.5v than you have ,current so 20ma is what your gonna use, which when convertated to decimal form is equal to .02. So now plug that into your equation. Now for resistance, well thats unknown so thats what you gotta figure out. So what were lookin at so far is .5v=.02∙R. Im not sure if you know algebra but what is needed next is to get the R by itself, once that is done the equation is now R=.5v/.02, once the math is done you can see that 25 ohms(resistance) is what you need to achieve. This can be done with a 25 ohm resistor or whatever is closest. What would happen if you didnt do this? Obviosly youd be overvolting your LED and causing it to burn out. So far the Led pen that i made has been on constantly for about 6 months now and hasnt weakened or dulled any. An LED can last up to 10 years if its wired correctly.

Also instead of supplying 4.5v to your LED id suggest using 3.3v that way youll trully achieve the 8000 mcd that these LEDs are rated at because you can use that higher end amperage without any negative results such as shortening of life. BTW your equation for that would be 1.7v=.03∙R. Therefore R=1.7/.03 which in the end would mean that R=57 ohms. So you would need a 57 ohm resistor to solder inline with the power wire going to your LED.

If you have anymore questions im sorry but i will not be able to answer them cause im leaving for the Gulf for about a month and a half.....Hot sun, camels, and the occassional terrorist? What more could a guy ask for....hehehe ILL be leaving on Sunday but wont have much time to be on Pimprig, sorry. <-- Thanks for the help Sparky! So I go shopping online for a resistor, and run into another brick wall (for me anyway) I found a web site that sells 57 ohm resistors, but they also talk about wattage! and nowhere on the stats for the LED's or in Sparkys little monster truck equation does it talk about wattage! so is it really that important? Heres the link to the resistors i found. If you scroll down to the bottom of the page parts no. 149a, and 149 are both resistors, but one is 1 watt and the other is 2watt. http://www.rfgco.com/catalog/index.html

Sorry for the long post but I'm an idiot.

Kr1cH

The wattage rating of the resistor needs to be the same or greater than to the wattage rating of the device you're powering, e.g. if you were aiming to reduce the voltage going to a fan (usually around 2.8W), you'd get a 3W rated resistor.

A single 20ma LED consumes 0.07 watts at 3.3v (3.3 x 0.02), so you can use a very low rated resistor. the smallest/cheapest kind (about 10c each) tend to be rated at 0.5W - higher than you need but that's about as low as they go.

Basically, you don't need to worry about what wattage resistor you get - lower rated ones will be cheaper and smaller, but a 1W or 2W resistor will do the job just as well - just cost a little more and be harder to hide.

As the risk of confusing you, LEDs are fairly tolerant about the voltage they can handle, and the 57ohm resistor Sparky's maths gave as the best match is just an ideal number - if you can find a 0.5W resistor that's a close match (56ohm or 68ohm are the closest match afaik), that would be fine too. 56ohms would mean the voltage supplied to the LED would be a _tiny_ fraction higher than ideal, 68 ohms would mean that it would get a little less voltage, but the effect on the light emitted would be negligable.

I couldn''t be bothered to look throught the site you linked to, but I suspect you're looking at wire wound resistors?
The ones you want are metal film resistors: