[analog31]

You're on your way!

Part of the joy of process control is thinking about all of the ways that things can fail. Adding to your woes...

Relays can fail, and it's actually a common failure mode for them to get stuck in the "closed" position, meaning that your pump runs forever. I learned this the hard way controlling heaters.

Amusing anecdote: I wanted to set something up to water my tree while my family was on a trip, and I ended up with a large pail of water next to the tree and a siphon tube running between them. Totally passive.