I'm working on a project that has a micro-controller controlling a servo(it's battery powered so its not an issue with it's local power supply), it's connected to a control panel about 3m away with 3 long wires. The wires are open signal, close signal and ground. The panel simply has two relays that switch open or close between 5v and GND. Everything works nicely but when a large linear power supply in the control box is switched on it causes the controller to do strange things like close and then open the servo. We figured that the 3 wires are acting as an antenna and picking up the inductive spike from turning the psu on(we even ran the signal 5v and ground from a battery in the control panel to isolate the circuit so it's definitely the wires that are picking up the noise)
Originally I was not sure if it was a spike on the close line that was causing the weirdness so we disconnected the open and close wires and just left the ground wire connected, same problem. I also confirmed this on my 'scope and during switching there are no spikes on the open or close lines during switching. Thus I have come to the conclusion that it is common mode noise on the lines that is causing the problem, as even when it is only the GND wire connected the spike causes GND to float up momentarily and reset/mess with the micro?
My solution that I'm going to try next is to add some opto-couplers on the inputs so that the two electrical systems can be completely isolated. I also want to add a common mode choke on the three wires as this should damp the noise? Can anyone suggest how I would go about choosing a choke and/or any other suggestions on how to noise proof this micro?