Interference from main 120v

Hey there,

I'm finishing one of my project, A led controller.

The main casing for this project is an old ATX case that i remodeled to install all the led driver and what not, also i use an ATX psu to power a few led and the arduino itself.

The arduino is controlled over serial (USB) from a distant computer (Vb Form). The arduino output to 2 Shift Register (74HC595), which control 12 relay (1x 8 relay board + 1x 4 relay board). Those shift register are mounted on a DIY pcb. The uno is powered from the DC jack with psu 12v and the shift register board from the psu 5v, both share the same groung.

The casing got a main 120v power input to power several led drivers. I do not have a schematic of the whole thing so i'll try to explain the best i can.

So when i power up the arduino from the psu and leave the main 120v unplugged, it all work great, i can power on/off every relay. But as soon as i plug the main 120v, it all start to act strange, like turning on every relay. Why is it acting like so?

The main 120v input is different from the psu input. The only thing i can think of, is main 120v wires touching or being close to the 12v wires. The signal cable coming from the shift register board to the relay is the same one as those we use as HDD/DVD wires (flat grey wire, sorry dunno the name!!).

Can someone help me figure this one out.

Regards

Bill

Upon further testing, i discovered the system to be more stable when sending command through the serial monitor of the Arduino IDE instead of the VB project. Is this relevant?