I bought the arduino starter pack, and found that the blue led requires more resistors than the green led does or it blows fairly quickly. The starter pack didn't give any heads up about this. All it says on the Kit Contents paper is that the led's are 3mm which is misleading because it gives the impression that they are all the same.
I've been using multiple resistors for the blue led, but to simplify my breadboard i want to just use one. The problem is, I don't know which one to use or purchase. I went to Radio shack and asked the people there. They didn't know, and they tried a Google search but were unable to tell me which resistor is needed.
I'd like to get all these resistors in one trip.
Green led appears to work well with a 220k
Which resistor is needed for the red led? 220k is my guess but I want to make sure, these leds cost $2 a piece
Which resistor is needed for a yellow led?
Which resistor is needed for a blue led?
All these led's that I have are the ones that come in the standard Arduino starter packs. I also have a clear one but it doesn't have three prongs like the 5mm, tricolor RGB... It only has two prongs. I haven't messed with it yet, but it must have come in the other starter pack. I do have some plastic strips that I think were designed to wrap around the led so you can make it whatever color you want. If that requires a different resistor please let me know as well.
A typical LED has about a 2.1 volt drop across it and runs at 20 milliamps current. Depending on the color of the LED, the voltage drop will be less (red ones) or more (true green and blue ones). But 2.1 volts and 20 milliamps is a good "catch-all" that will safely work with any LED.
So, you need a resistor in series with the LED and the voltage source. Using Ohms law, R = E / I, you figure out the resistor you need.
Say you want to run the LED from a 9 volt battery. The voltage you need to drop across the resistor is 9.0 - 2.1 = 6.9 volts.
The current is 20 milliamps or 0.02 amperes. So, R = 6.9 / 0.02 = 345. The nearest common value is 330 ohms - good enough.
Now make sure the power rating of the resistor is sufficient. The power the resistor will dissipate is R = E * I,
We need to figure out what the current is because we used a 330 instead of the calculated 345 ohm.
I = E / R, I = 6.9 / 330 = 0.021 amps. Therefore the power is 6.9 * 0.021 = 0.144 watts.
You need a 1/4 watt (0.250 watt) resistor or larger (a 1/2 watt or 1 watt is OK, a 1/8 watt is too small).
So, LED in series with a 330 ohm, 1/4 watt resistor connected to a 9 volt battery = happy glowing LED.
Hope this helps.