LEDs and Resistors

[Brown]

So, that's the different between this. (I was looking at shift reg. and the arduino) Thanks!

[mariush]

left side: each led is limited to a particular current by its own resistor ...  ex each led is limited to 10mA , then total current is 10x8 = 80mA.

right side: all leds are limited to a current value set by the resistor.... ex r9 limits the current to 20mA, then each led gets more or less  20mA / 8 = 2.5 mA .  More or less because some leds will draw a bit more current, others will draw less.
Also note that if one led burns out/gets damaged, the rest of the leds will share the same current, for example 20mA / 7 = 2.85mA. If you're unlucky, 1-2 faulty leds can cause a cascade and burn all the leds.

=> the left version is preferred.

note that a lot of shift registers can only do about 25-50mA so you might want to use npn transistors at the outputs of the shift register (with a suitable small value resistor between output and npn base) and get the current from another input, don't power the leds straight from the shift register if you want them to be bright.

[Brown]

oh ok, thanks!

[con-f-use]

Or if you voltage is high enough you can wire them in series and use one limiting resistor for all of them. Or you can construct a constant current source with a few passives.

[Nerull]

Diodes can be subject to current hogging and thermal runaway when wired in parallel. The current through a LED is dependent on voltage and temperature. If one diode gets a bit warmer than the others, it will start taking up a larger share of the current - which makes it even warmer, leading to more and more current until the LED fails.

Current limiting resistors on each LED prevent this.