I am putting together an 8x5 led matrix using two 74hc595 and 5 2n2222 npn transistors. I am aware that the 74hc595 chip can only output a max of 70mA but I am wondering if I use millisecond delays if that would indeed work. As of right now on the rows of the leds I control the anodes straight from the 74hc595, and on the columns I send the output of the 74hc595 to the base of the transistor to switch the ground. This works properly and all is well. I am able to light up all of the leds in the matrix at one time(using a total of 40 1 millisecond delays) and it seems to be working like i would intend. I guess what I am asking is that is it safe to use these millisecond delays so that only 1 led is on at any instance or is that still dangerous for the chip to be switching that fast?
You can switch much faster. Like 500uS. Go thru all 40 LEDs every 20mS.
ok perfect thats what i needed to know. I just wanted to make sure the chip could handle this fast of switching and the current at any given time would never be over a single leds worth because of being left over or something
"the current at any given time would never be over a single leds worth because of being left over or something"
You'll have to address that in your software.