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I have been following http://arduino.cc/en/Tutorial/Fade to fade and LED. But I don't know why they are using a 220 Ohm resistor. According to Ohms law if LED is of 2.2 V and it runs on 0.025Amps and is connected to 5V pin of Arduino then the resistor should be of 120 Ohms but why we are using 120 Ohm resistor ? One more thing. How can I collect value of resistor to be used with different output and input devices ?
Thanks in advance
If the LED has a forward voltage of 2.2V and is rated for 25 mA, then you're correct that a 120 Ohm resistor should be OK. It's theoretically calculated as the voltage across the resistor divided by the desired current in the LED. (In practice, it's often more complicated than that, but that's a good starting point.)
It's important to note that not all LEDs are rated the same though. I imagine the 220 Ohm resistor was selected for the tutorial to provide a bit of a safety margin, as that should be OK for most LEDs a hobbyist is likely to use. If you don't have access to the full specifications then it's probably better to under-drive it. The LED might be a little dim, but at least it's unlikely to be destroyed.
Here's a useful online tool for calculating theoretical resistor values for one or more LEDs:
For other devices, the resistor values required can depend on a huge number of factors. The best thing to do is to check the datasheets. For some components (such as ICs), you'll often find an "Applications" section towards the back which shows various ways you might want to connect the item to an external circuit. If things like capacitors and resistors are required, it will often explain how to select the correct ones for a given situation.
