security switch for level control

hi, i'm trying to find the most economical and safest solution to this problem: I already install a control system for the tank that i was asking you for help weeks ago, but i need a security switch in case the auto control failures.
I was thinking about 2 fixed electrodes, but i don't know how to implement a circuit to detect connectivity between the electrodes. Can you help me with this???
The output must be a relay contact NO, to disable the pump.
Thanks for your help

You need to state the problem so we understand it. What happens when the automatic system fails? How does this affect the electrodes? If the relay is normally open, how does it stop the pump when it closes? What does the pump do?

Heres the original thread on this topic:

https://www.electro-tech-online.com/threads/1-kw-dc-drive-simple-circuit-info-required.5736/

I thought the suggestions were quite good, it appears you are now looking for some type of fail safe circuit. You could use a dual window comparator ciruit to detect if a pair of sensors are within some pre-determined high or low range.

Here's another idea; How about the water level sensing switches as used in washing machines? They just have a hose/ tubing in the water with an air pressure sensing switch at the top end. If it works reliably in the messy enviroment of a washing machine it should do so in your inaccessible tank as well.
Its a passive system with no electrolysis problems. The hose/ tubing could be easily serviced by blowing air through it once a year.

The bubble system works well too but its an active system. No bubbles - no level indication.