I'm trying to set up a 10 X 10 Led 1 color led Matrix by using 4 , 74hc595. 2 for the rows and 2 for the columns using NPN transistors. Now Obviously I will only be using a total of 20 pins with the 74hc595 instead of all 32. My question is with the other pins not being used and I going to run into problems? I don't really want to solder all this up if the 74HC595 will 1, have a problem going to the NPN transistors and controlling the ground rows. and 2, how will the code be different with not using the other pins on the shift reg? Oh is it possible to just use 3 and connect 20 pins to them even though on one chip there will be pins going to npn"s and pins going straight too the columns?
If you do not require too much brightness you can Charlie-plex the LEDs
using just the 168/328. I wrote up FAQ at
http://wiblocks.luciani.org/FAQ/faq-charlie-plex.html
Documentation and software examples for the 10x10 I built are at
http://wiblocks.luciani.org/PICO/PICO1TR-LED-S-index.html
(* jcl *)
Thanks for the reply. Your info has helped greatly. all I want to get out of this is three wires to control the matrix so it leaves all the other pins available. I have a bunch of 74hc595 chips and I have already wired the leds in rows and columns. I guess I prefer to use as little as possible to hook all of this up, and I would like to be able to program it as easy as possible and as little program size too. saying this. I see you said that i could get away with NOT using NPN transistors? Could you explain how and also would it be easier to code with or without them? Thanks again
Oh ok I got it. Sorry not sure on the "load" I just know as of now that I have 100 Bright blue Led 5 MM so what ever they pull. As far as powering it I haven't got that far.
Leds offer a difficult challenge as far as what they draw in current as a 'load'. Once a led 'sees' a forward biasing voltage equal to or higher then their rated forward voltage drop (blue = 3.5 volts I believe) they turns on and act like a short circuit or a zero ohm resistor for practical purposes. When using leds the current must be controlled externally from the led with either series resistors or driving them with special circuits called constant current drivers. Without such current control damage to the leds and the outputs driving the leds will be damaged beyond repair.
The software side of thing is unaware of any of this and has no real control over it, it's rather the fundamental electronics behavior of all the external components used.
Lefty