Resistors on the cathode/anode

I've been reading a wiki titled "Starter Kit for Arduino". I had some confusion when I got to the "Lesson 2 LED Blink", and their placement of the resistor on the cathode end. So I did some searching on this forum, and found that it does not matter which end you put the resistor on.

However in the next section "Lesson_3_Rolling_Light", it shows them using 8 resistors. Which leaves me questioning; are 8 resistors really necessary? Couldn't we just join all of the cathodes to a common rail, and then use a single resistor to connect it to ground? What might be some of the fail cases in that?

Yes they are all necessary. This is because if you only have one resistor the voltage it drops depends on how many LEDs on. Therefore the overall brightness will depend on what LEDs are on.

Okay that makes sense, thank you.

This, and similar articles can be useful http://www.ledsupply.com/blog/wiring-leds-correctly-series-parallel-circuits-explained/

If not all of your LED's have the same forward voltage (manufacturing tolerances) one may hog all the current until it burns out http://electronics.stackexchange.com/questions/174584/is-it-ok-to-put-leds-in-parallel

Even if you supply current to each of your LED's in turn, a mistake in your sketch can cause more than one to be on at the same time! So its best to play safe.

NB always use a bigger resistor than that needed to provide the maximum current - 5ma instead of, say, 20ma will still be quite bright enough for most purposes, and you reduce the risk of exceeding the overall capacity of you Arduino.

Thank you, that helps a lot!

FYI; One link led to another, and I found this amazing simulator.

skds1433:
FYI; One link led to another, and I found this amazing simulator.

Just come across this - the visual representation is brilliant, thanks for pointing it out