I agree with GoForSmoke
If you use a float switch as shown in post #15
or better yet, the type that mounts horizontal, but you have NO or NC depending if the float rises or falls to open the contacts. you just spin it 180 degrees.
Some PVC pipe some Tee fittings and some elbo fittings, and some end caps.
you can mount the switch on the end cap
mounted pointing up and it has the switch closed, or NC
pointed down and the switch is open or NO
in the lower tank. the switch is pointed down. the switch is open until the water is high enough to float the switch.
so there is no signal from the lower tank unless the water is high enough.
in the upper tank, you would mount the switch and point it up.
it would always close the circuit, unless there was water.
these connect to the relay and a power source.
when the lower tank is empty, the float is down and does not pass power. the relay is not energized.
when the upper tank is full,the float is up, and does not pass power, the relay is not energized.
when the lower tank water level floats the switch, the switch passes power to the float on the upper tank.
when the upper tank is empty, the float is down and it does pass the signal.
therefore, with the lower tank full and the upper tank empty, the signal will pass.
the relay will be activated and the pump will run.
When either switch changes state and opens the circuit, the relay will de-energize and the pump will stop.
the relay can be 12 volts or 5 volts and you will only need resistors to make the circuit part to activate the optocouplers on the relay board.
The beauty of this is that the unit does not draw any power unless the system is calling for the pump to run.
There is not need for any special power supply either. and you can add an LED into the unit.
this will run, day or night, anytime that the lower tank is full and the upper tank is not.