Hej Guys,
I'm want to control a lot of LED's with three 74HC595 Shift Registers. Now I'm wondering how to connect them the proper way. Many will be installed seperately on seperate pins of the register, but I was planning to put 11 LED's on a single pin, since I will always turn them on and off together.
Now I'm wondering what kind of Resistor Setup I have to use, especially for the 11 LED's on a single pin.
My questions are:
1 - Should I put the 11 LED's in series, or should I connect them parallell? Can I actually do any of those, or should I never put a lot of LED's on a single pin?
2 - And how should I use resistors? Is one before all of them enough? Or do I need one before each LED (Which kinda makes no sense when they are in series.
3 - I was wondering if I could use a single resistor for all of my LED's, just before they go to ground. But I read somewhere you shouldn't do this. Is this correct? It would save me a lot of trouble if I could use only one resistor ...
4 - Anything else I should look out for with this kind of setup?
Thanks a lot in advance for your suggestions 
74HC595 should only drive 1 LED with about 8 mA of current. It should only drive 70mA of current total before the part is overstressed.
Use aTPIC6B595 instead, it can sink 150mA per output, so you could have 11 LEDs in parallel, each with a current limit resistor, and 13mA of current each. Or Wire a bunch in series from up to a 50V source and 20mA thru them all. TPIC6B595 is controlled the same as HC595, but it only has Low outputs - shift in a 1, the output goes low to sink current thru an LED from a positive supply (vs sourcing current thru a device to Gnd).
LEDs in parallel each need a resistor.
LEDs in series need a resistor for each string of LEDs.
- You don't have the voltage to put more then 1 led on the output of a 595
- If leds are parallel, always best to use a resistor per led
- Correct. Because then the number of led's that's on will determine the brightness
- Yes, a 74HC595 isn't able to supply a whole lot of current, it can supply a max of 35mA per output BUT only 70mA all outputs combined! For 8 outputs that makes less then 10mA per output. Not a problem if you use them as indicator leds though because then something around 3mA will be more then enough for modern leds. But, this also counts for the pin you want to connect 11 leds to. That leave less then 0,8mA per led and that's if I use the absolute max of 70mA total (which is better to stay under by a margin).
In other words, a 595 to drive a couple of indicator leds at mA each, not a problem. Driving 11 leds from a single output, not going to happen if you want some current ;)*
*) Although, if you indeed run them at 3mA it's fine to have one output drive 11 leds. 11 x 3mA = 33mA (okay, bit close to the absolute max) and 7 other outputs with a single led takes 7 x 3mA = 21mA. So a grand total of 54mA
Thanks a lot for the fast responses guys, I'll look into the other Shiftregisters I guess 
If you want a bunch of LEDs to go on at once and are at risk of overloading the 595, why not put a Darlington pair on each output. Check out the ULN2803.
That's still a fine option as well Max per ULN2803 (from the top of my head) is like 800mA (no, you cannot just sum the 500mA...).
@SoldierOfSodom, do you really need those blindingly bright leds?
For pins driving 1 or 2 (possibly 3) LEDs you may use the pin directly. For more LEDs drive them via a transistor. Google "using transistor as switch".