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When wiring an LED you have three options
For an example we will use a single Ultra Bright blue 5000mcd 3mm LED.
Our supply voltage will be 5V from USB (could be 12V or
whatever supply you are using)
Calculated to exactly 85 Ohm
*safe pick means you should have a buffer over the minimum, in reality you can run a much higher wattage resistor without issue most readily available are 1/8-1/2 WATT or 0.125-0.5. Do keep an eye on this number when calculating multiple LED's. If you run a wattage higher than your resistor is rated to run it will get hot and fail at best, burn your finger, melt plastic, start a fire at worst.
So to wire the above example, we find the LONG LEG of the LED which is POSITIVE and attach the proper resistor to it. Connect the ground to the SHORT NEGATIVE lead.
This is the method that adds up quick for power draw. But it often can be the easiest, just connecting leg to leg of the LED's, great when you need a small number close together. Use the same forward voltage and mA as above and choose 4 LED's resistor calculator , you'll find that there is not enough voltage. If you up the voltage of the source to 12V and lower the quantity of LED's to 3 it works.
This is our preferred method for connecting multiple LED's. With a comparatively simplified install with a single resistor and the ability to handled 4 or more LED's this was our choice for our Xbox 360 controller mod amongst others.
Bundle all the like legs together slap a resistor on the long positive leg and away you go. Using our example resistor as above 20mA 3.3V if we enter a supply voltage of 5V and a quantity of 4 in resistor calculator we find that a 22 Ohm resistor will serve our needs.
You can make many combinations that will give a proper
resistor value but keep an eye on the wattage of the resistor required, its easy
to go over capabilities of cheap and readily available resistors. (e.g.
1/8 WATT=0.125 WATT, 1/4=0.25, 1/2=0.5)
Rule of thumb values to use for calculating unknown LED's
20mA for all colors