On a sunny day (Sun, 02 Nov 2008 15:11:24 +0100) it happened Jollino wrote in :
Hello,
>apologies if this question is very stupid. :)
>I have been toying with electronics for a little while, but I know I
>lack some basic knowledge to wrap everything up together so... here is
>my doubt.
>
>I am designing some Christmas decoration with a bunch of LEDs driven by
>an array of 74HC595s (controlled by an ATtiny45). The whole thing will be
>
>I did something similar in the past, using three 595s to control three
>7-segment LED displays, but I did it fairly randomly: I just connected
>each output to each segment, with a resistor between the two.
>Since this thing I'm planning to do is going to be simple yet much
>bigger -- involving twelve 595s -- I'd like to do things properly.
>The 96 LEDs could and probably will be all on at some point, so
>considering a current draw of 15 mA for each LED, the whole thing would
>draw a total of about 15 A,
100 x 12 mA = 1.2 A
1A = 1000 mA
In any case, let's assume for the sake of the example that I want to be
>able to drive eight LEDs with a single 595. The datasheet tells me that
>the maximum current load on each output pin is 35 mA, and the current
>load on the supply pins (Vcc and ground) is 70 mA. I am not sure how to
>interpret this: does it mean that the sum of the output currents can't
>be more than 70 mA?
Exactly.
Run the LEDs at 70 / 8 = 8.75 mA. Modern LEDs give a lot of light for that current.
Also, I'd like to finally understand whether it's better to source
>current from a pin or sink current into a pin.
Depends on the port.