My father-in-law has one sump pump well in his basement with two sump pumps in it (a primary and backup pump)

A while ago, the primary pump failed. The back up pump eventually turned on and worked but its float was set so high that some of the basement got wet.

We lowered the float position of the back up pump.

We added a added a water level alarm.

We made this alarm to let us know when the back up pump is running. This way, he knows that he needs to go buy a new primary pump.

The guts of the alarm consist of two items:

https://www.amazon.com/gp/product/B019Q3U72M/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B073FBK9FY/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

The 3D printed housing just holds them together. I used some button head #6 x 1/2" long sheet metal screws to hold the housing together.

https://www.mcmaster.com/#92470a148/=1dbzw71

This also needs an outlet splitter, or power strip.

Plug the float switch into the wall outlet. Plug the outlet splitter into the float switch. Plug the sump pump and alarm into the outlet splitter.

Now, any time the back up pump gets power, so does the alarm (and it makes noise).