If your main issue is protecting the pump, perhaps investing in one with its own float (if the pump is an immersion type) or flow (if it’s external) switch is the way to go. That’s the only thing that needs to work at line voltage. In other words, separate the problems and make your life easier. Protecting the pump doesn't need to be part of level monitoring. Instead, it should be a proven, self-contained and low-tech approach. With that out of the way, I'm going to assume you don't wish to cut any additional holes in the side of your cistern; that any level monitoring will be installed from the top. That in mind, here's some ideas for monitoring level: * Multiple [optical prism sensors][1], in a dip tube * [Capacitive tank sensor][2], used in the RV and boating trades for water and wastewater monitoring. Again in a dip tube. * [Measure distance to water with a time-of-flight laser][4]. Seems problematic. Could maybe be hacked to use a reflective float? * [Hydrostatic][3] (pressure) sensor, suspended from the top. Popular. This is arguably the best method for cisterns. With the hydrostatic approach you have lots of options for display, including wireless monitoring. With a vertical tank the math is simple: pressure is proportional to tank fill height, so you can calculate % of full, or give capacity in any units you choose (gallons, cu. ft, liters, m^3, etc.) With suitable math processing you can calculate dynamic usage, and even detect leaks. This applies to capacitive and laser sensor approaches too. More here: https://electronics.stackexchange.com/questions/700725/is-it-possible-to-do-water-detection-with-a-single-electrode/700742#700742 [1]: https://www.electroschematics.com/optical-liquid-level-sensor/ [2]: https://tankedge.com/downloads/How_our_exterior_Moda_tank_sensors_work.pdf [3]: https://blog.wellaware.us/blog/how-to-monitor-water-level-in-a-tank-complete-guide [4]: https://electronics.stackexchange.com/questions/521420/is-there-a-way-to-measure-distance-to-a-surface-of-a-liquid-water-for-example