This makes me wonder what the role of these relays is. Are they necessary. You refer to switch closures. Surely you don't have switches operating the relays ... or do you?

My conclusion from your presentation of the original schematic was that the relays ARE the switches. The suggestion of a resistive voltage-divider to attenuate the input swing down to the supply at which you operate your control circuit, including capacitors, still holds, but it now doesn't look so clear.

My comment that the input circuit is low-impedance came from the fact that you had a 2K-ohm resistance in the circuit from +12 to Gnd as seen from the relays, and my conclusion was that the noise with which you are concerned comes from the cable length or ambient EMI. I didn't regard the input impedance of the optoisolator at all, since I'd omit the thing. That would leave a 40106/74C14 Schmidt-trigger inverter in place of the optoisolator, and simply have the 40106 interpret the voltage across the switch, as modified by the voltage-divider and capacitor, for you. There are six Schmidt-trigger inverters in one 40106, so you could, if you like, use them in pairs, thereby leaving the input signal to your MCU circuit in the same logical sense as the switch/relay.

Could you clarify how you apply the relay, please? If it is driven by a switch, then you could probably omit the relay, too, again, provided it doesn't violate some regulatory restriction. It's possible you might need either the relay or the optoisolator because you must be isolated from the signal source, but you probably don't need both, as either one provides isolation.

RE