I am sure that this has been asked before but since I dont even know what the problem might be my searching the interwebs was quite unsuccessful (and frustarting). So please bear with me if this is going to be one of those dreaded "gaziilionth asked sand stupid" questions.
I am quite new to electronics but once I saw a LED cube in action I wanted to build one by myself. I have been experimenting for the last 6 months and learned quite a few basics.
I started out with a couple 3x3x3 simple led cubes to practice soldering and general setup. I used very simple software side multiplexing with 9 pins on the arduino (9 for colums and 3 for layers). This actually worked quite nicely and after a little break a couple weeks ago I thought I was ready to go to the next level.
I started reading up on RGB LEDs and how to multiplex a lot of those (since I was planning on doing a 5x5x5 RGB cube this time). I stumbled across ShiftPWM and found it to be excactly what I was looking for. So got myself the parts needed (mainly the LEDs and a couple of 74HC595B shift registers) and started experimenting.
My initial tests went on smoothly and I figured Ill try the demo that is described here: http://www.elcojacobs.com/using-shiftpwm-to-control-rgb-leds-with-arduino/. (see the schematics). I set this up and after a painful realisation got it to work finally. I tested it with the sample sketch that is provided with ShiftPWM and everything worked as it is supposed to...
I was running the setup off of the arduino 5v and ground. Since I obviously cannot run 125 RGB LEDs off of my arduino I connected my bench supply and then the trouble started...
When connected to the bench supply there seems to be some kind of "noise" on the pinouts of the shift registers, i.e. even though only one LED should be led there are others (some directly next to it but even the most two apart) that light up randomly with all three colors. Once I shift the supply back to the arduino everything works just fine. What is up with that?
I thought maybe that had something to do with the capacitors I initially left out of the circuit but this does not seem to be the case. I read up on that and once I understood what a decouling capacitor is installed the 100nF ones across the rehisters and the 100 uF one across +5V and Ground, but the problem still persist.
Can somebody may help me with that? I have a couple of basic tools like a few beginners meters and an old analog 20 MHz scope (which I am not very good at operating) if you need more information from me.. Let me know if you need anything else - I guess I could supply some images if needed.
Thanx in advance, Chris
TL;DR: Setup is 3 74HC595Bs and 8 RGB LEDs. Everything works fine while power is supplied by the arduinos 5V/Ground. When using my bench supply LEDs which are supposed to be off "flicker" randomly. Why?