And if I may add to Melanies post, use if possible optocouplers to drive external power devices. It is the most secure method of isolation.

I had to go twice on a town up in the mountains 250Km away, for an automation once I made with 16C55 and assembly lanquage, but with no isolation and finally understood that I was too inexperienced that time in industrial automation design (long time ago, I would say...).

Ioannis