arduino - Square Wave pulses generating EMI

Following on my unanswered question about bluetooth noise, I have now believe that I have learned a bit more about my problem.

I have built a Arduino controlled circuit to replace the pots and switches on an electric guitar. You can find full information at the webpage.

I communicate via Serial Port Protocol. A bluetooth module is connected via shielded cables to the circuit board.

The audio part of the circuit is fully opto-isolated from the command part.

At each data exchange, I can hear a faint click out of the amp.

After quite some searching, it seems that this click is a result of radiation from the 5v square waves passing along the rx and tx wires, despite the shielding.

I further shielded the cable with a copper mesh sheath, and this reduced the click intensity. As did wrapping the connectors in tin-foil.

I am wondering:

• Why/how the shielded wire lets the EMI radiation out?


• If changing to 3v square waves would further reduce the intensity of the click?


• Why there are no clicks when using a USB cable to communicate instead of the rx/tx?


• If there is any way of preventing this EMI from being radiated?

Here is my best attempt at showing the physical layout. Note that the cable to the BT module is now shielded.