I'm replacing the relay logic of an old flipper with arduino uno. My first idea was not to totally replace the logic but just some functions. The relay logic is based on relays (of course) and motor operated cams contacts. In addition there are big solenoids to reset flags and move flipper fingers. I used coax cable to connect contacts and I connected the shield to ground, but even if I place the arduino board outside the flipper it gets enough noise to activate the interrupt and related programming. I used a pull up resistor on the interrupt pin. Flipper counters relays are operated through optoisolated relays so I don't think that noises could come from there.
I then decided to cut out the relays and cam contacts and to use only programming logic but I'm afraid that would not be enough because of the other solenoids I must keep.
Question is : how can I shield arduino? Should I use filters and if so how to calculate values?