I created a Raspberry Pico based PLC board which worked perfectly alone, however once I placed it into a distribution box and started switching a fairly large (7.5kW) contactor, the on-board seven segment display started glitching by displaying random digits and eventually turning off. This happens every time I power on the system and must restart it manually once the display turns off.
The contactor is switched on 230V mains by a +5V relay and the relay has flyback diode across its coil, so it should not create much noise theoretically on the low voltage side.
After inspecting the traces between the Raspberry Pico and the MAX7219M seven segment driver IC, I noticed that the clock signal, as well as the +5V are really noisy as you can see here:
So far I tried removing all unnecessary connections and upgrading the +24V 2A PSU (which feeds the on-board +5V switching regulator) to a 10A one, but the problem still persists.
I did some research and realized that I probably should have added a ground plane to the PCB to reduce EMI, however I already ordered them and look for a solution that does not require redesigning and reordering them.
Any suggestions how to minimize the noise?