I have lots of LEDs, various colors, clear, RGB (common cathode just to confuse things), etc. No idea of manufacturer so no data sheet. My Mtester will tell me the VsubF so I know how much V it takes to turn them on. Primarily using Arduino so have 5V to play with, but also play around with 3V, 6V, 9V, etc. Most schematics show a 220 ohm dropping resistor used with a LED @ 5V, but I see other values(180,270,470,1K, etc.) being used as well. Yes I know VsubF and resistor voltage drop must equal less than VSS, but why 220ohm? Also there seems to be no consensus of whether the resistor goes on the anode or cathode side of the LED? So I blindly follow the crowd and put a 220ohm on the cathode side (usually directly to the negative bus rail). Is there any particular reason for this or is it just "because everyone else does".
I found some LEDs I bought over 30 years ago (yes I am an antique) and note that they are much dimmer than nowadays, but then my memory is dimmer too so... They are also much more prone to burnout. Not near as hardy as the ones now sold are.
So 2 questions, why 220 ohm @ 5V (or does V matter) and which side does the resistor belong on anode or cathode?