You first line of defense is avoidance - avoid running your wires parallel to wires carrying current ie:240vac etc. Also avoid running your wires parallel to your telephone wires as you may pickup interference from these as well as cause interference to the telephone. What distance? As much as possible! Use shielded cable if possible even though it is more expensive than CAT5 twisted pair. Connect the shield to your mains earth at one end only. Shielding is preferable especially since many of us have mobile phones - the long wires act as a nice antenna picking these up! So a household installation is more like an industrial installation these days!

The second line of defense is protection: Since you are only communicating pushbutton presses, something like 100R in series and 100nF to 0V forms a low pass filter that will take care of most nasties then add a transzorb (or zener diode) to clip the high voltage spikes from thunderstorms and static discharge.

The third line of defense is to debounce the input via software so that a closed condition must exist for a certain amount of time to register as a valid operation. ESD (electrostatic discharge) and lightning are very fast spikes so creating a 'dwell time' filters out these.


These methods should make your system fairly robust. I've been involved professionally in lighting control and security systems and these methods have been applied successfully.

There was an instance that an installer ran an alarm wire in parallel to a 3phase feed to a refrigeration compressor. Every once in a while the system would give false alarms - especially when the compressor started. Re-routing the alarm cable solved the problem.