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, which is pretty much impossible to handle.
> Correct? I will probably have to investigate charlieplexing to such a
> big scale, or maybe make it so the 595s are working one by one at a fast
> rate (by pushing a series of logic zeros down the data line).
>
96*15mA is "only" ~1.5A, and lots of modern leds will be plenty bright with much less than 15mA try it.
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?
yep
If that is the case, I suppose that I could use transistors. Would
> connecting each output to the base of an NPN and the LED (with a series
> resistor) between Vcc and the emitter work?
resistor from output through a resistor to base, emitter to ground, led and resistor from Vcc tc to collector
> Also, I'd like to finally understand whether it's better to source
> current from a pin or sink current into a pin. I would think that
> sourcing is "cleaner" because one doesn't "overdo" the chip -- if the
> current is not enough, it simply doesn't work -- but I'm not sure about
> that at all, because on many schematics I have seen LEDs sinking into
> MCUs' ports.
TTL was much better at sinking than sourcing, guess that just stuck
-Lasse