[mike31154]

Ya, the instructions are somewhat lacking in the wiring details. They really should include a schematic diagram of the wiring. Way too easy for someone with no electrical theory or training to mess this up, hence the warnings in the instructions to let it be if you don't know. In any case, not including a schematic diagram with the kit is criminal IMO.

The 12v wall transformer is used to supply low voltage through the float switch. When the contacts on the float switch close, 12 volts from the wall xformer will activate/deactivate the relay (depending on how it's wired) thereby switching the 115 VAC to the pump you are running. This is safer than what some people do & have the float switch shut the pump off directly by interrupting the 115 VAC without a relay. This has the potential of 115 VAC permanently at/through the float switch, in the water. Not a good idea. I also don't like the way all the components are stuffed into the project box, not a very elegant design.