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
No. 1.5A. Ten times less. It would help if you said which LED you are using. The usual little LED that goes on a PCB can blind you at 1mA these days.... I don't know what charlie plexing is.
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).
If you do that, you can have brightness control too.
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?
That's right. These are not "power drivers", but logic chips. If you want to drive a hungry load, use something like the ULN2003.
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?
Yeah but just re-think your approach first. Tell us what kind of LEDs you are using and what kind of light output you want.
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.
Traditionally, a long time ago, it was easier for chips to sink current. They were only able to source small amounts of current. There are entire classes of outputs with no sourcing at all, like "open-collector" outputs. This is why people prefer sinking. At least I do.
Thank you in advance, and apologies again if I sound like an airhead. :D
> When it comes to these things, well... I am!
Your post belongs in the basics group but overall it's better than the usual spam, politics, or trolls in here.
Just tell us what size of project you are working with here. Is it 6 feet tall and needs to be visible from across the street, or is it a christmas card that sits on a table? Once we have that, we'll be able to judge the current.
You don't want a Christmas card that will leave a retinal after-image for half an hour, do you??? :)