74hc595 Current

So I am planning a project where I have groups of LEDs based on different colors, and I only need each group to light as a whole. My idea was to connect groups of LEDs in parallel to the shift register's pins. My question: how many LED's can I run on each pin of the 74hc595, and what kind of resistors should I be using? (I need the LEDs to be as bright as possible) Thanks!

74's are at best 20-40 ma depending on what they are and who made them

or in other words squat, they are logic chips and not really decent driver chips

try looking this article over

I have already made an LED Matrix (LED matrix projector – tinkerlog) that worked perfectly well with 74hc595s and a darlington driver. If that worked to my satisfaction, could i drive up to 8 LEDs per pin? I would purchase another chip, but i already have 74hc595s.

In the matrix you made you are probably switching the LED's on and off rapidly. This is very different than having them full on all the time.

the 595 shiftregister can handle max 35 mA pr. output pin, so you could not even drive two stanard LEDs (20 mA each) constantly on pr. pin.

Further more the total power dissipation of the 595 IC is 500 mW, this means that you can probably not even run all the pins at their maximum current rating at the same time. This gets even worse if the temperature exceeds 65 (c).

EDIT:

Check Grumpy_mikes site for power calculations:

http://www.thebox.myzen.co.uk/Tutorial/Power.html

Please check out the M5451 LED driver chip. It drives 35 at 20mA, 12 volts (its a big chip). So you can put your leds in series instead of parallel. Or since it doesn't seem like you really want to run 35 outputs you could tie a few together to get higher currents.

Google Arduino M5451:

I also have boards with 2 M5451s on them...

As I said, I don't want to purchase another chip because I already have 74hc595s, and also I only need this project for one day. Would it be possible to drive multiple LEDs off of an arduino pin itself, or would it be better to work with a shift register. 2 LEDs per pin is all i really need, if that will work for this short term project.

You can stack up 595s on top of each other to increase the output current.

Arduino pins can source 40mA each, but again they can't do that all of them at the same time, because the board as a whole can only source 200 mA.

You could daisychain serveral 595's and just have one LED pr. pin, with a suitable current limiting resistor for each LED.

that worked perfectly well with 74hc595s and a darlington driver.

You need your shift register AND darlington drivers to get the current output you need. If you already have the 595s then just get more darlingtons and connect the LED to +ve through a resistor to the darlington drivers acting as a current sink.