I am building a full sized pinball machine from scratch (not reusing an existing machine/parts). I have already designed, fabricated and individually tested all the mechanisms and sensors, and now it is time to get the electronics wired up and working. Unfortunately, I have run into an issue I didn't see coming that is very frustrating. I'll try to avoid going into the weeds, but my project uses multiple generic door lock actuators, 12 volt solenoids and logic level servos to interact with the steel ball and multiple (~40) sensors (mostly switches that connect a pulled-up line to ground) spread over ~3x4 feet of playfield, necessitating relatively long wires for both signals and power to various devices from the central brains/control boards.
I am running into problems that seem to stem from excessive noise on the switch signal lines. I designed monostable 555 circuits to trigger several of the door lock actuators (TIP120 transistors) and they all like to trigger at the same time which to the best of my hobbyist-level oscilloscope detective skills is coming from noise generated by the actuators firing. The 12v and 5v rails have capacitors on them and actually look pretty good. When I put a lead on the positive end of one of the actuators (all have flyback diodes, drain to TIP120) I see the same severe noise as a lead on the signal lines from the switches.
I have tried a number of things, but the only thing that has shown some improvement so far is adding ceramic capacitors across the actuator leads (in parallel with the flyback diodes). Now the amplitude of the noise appears reduced and the other 555 timer circuits don't always trip, but they still frequently do. Increasing the capacitance with an additional cap in parallel seems to have diminished returns or no additional benefit. It seems that something as simple as a wire moving locations when I move the table as I'm working on things can be the difference between proper and improper function even with the caps installed.
As a non-engineer I have been thinking about the following things, each of which I don't know much about. It seems like there are a lot of rabbit holes to go down, and I am hoping someone here can give me some pointers which are really worth following. Some top thoughts on problems - ground loop problems, electromagnetic interference from inductors rather than direct current/voltage effects on the lines (I don't see a path through the lines from actuators to signals)? Some thoughts on possible solutions - low pass filters on signal lines (I've got lots of signals from different places, how would this work on SPI and other communication lines?), shielded cabling (bulky, expensive?), twisted pairs with differential signal (overkill, not reasonable for number of lines from various places). All of my ideas for solutions seem overcomplicated and address the symptoms more than the source of the problem.
I am looking for a hopefully simple and cost-effective solution to deal with the noise. I'd rather cut back on the source than try to fix all the signal lines if possible. Most of the input signal lines will be handled by the Arduino, so I do have flexibility to handle some of this in software (although I plan to use input shift registers which might complicate this). I still want 5 of the actuators to run off my 555 based control board however, and I also will need communication with peripherals like MAX72xx ICs controlling a DMD and 7 segment displays not adjacent to the main control boards and am worried about whether they will even function reliably if I don't figure this out.
As a side note - I feel like there should be a simple solution or I may be overlooking a glaring mistake I've made partly because I am not the only one who has done a similar project and I don't think others have had to do anything elaborate here. (The active custom pinball community however mostly uses commercial/"real" pinball parts, control boards, etc, so my project is different enough I actually think I will get more help in a forum like this).
Thank you very much in advance for any input that can guide me in the right direction!
Sorry this is long winded (first draft was longer). Happy to provide any additional details, schematics, scope traces, etc that seem relevant.