I started working on the sensor cluster that will be placed inside the sump pump container. I anticipate that this sensor group will do several things:

1. Sense low solution level and send (publish) a request to Node-Red to turn on the reserve pump to begin refilling the sump.
2. Sense high solution level indicating that the sump is full and send (publish) a request to Node-Red to turn the reserve pump off.
3. Sense the temperature of the nutrient solution in the sump.
4. Possible test the electric conductivity of the solution to indicate if the nutrient solution mix needs correction.

This sensor needs to be relatively simple as there will be some mechanical aspects to it that could fail. It must also be water-proof as it will be submerged in the nutrient solution.

My first design idea is to use a ridged plastic tube that holds two magnetic reed switches, each at the appropriate point to indicate low solution level and sump full. A float made out of something that floats could hold a neodymium magnet. This float would follow the solution level and the magnet would activate one of the two magnetic reed switches as required. I found two different corks as well as some high density foam to test as floats. I looked for rigid plastic tubing at the hardware store but the best that I found was grey water supply line. It isn’t really rigid enough and has a slight curve which could be a problem with smooth operation of the float. It would also be nice to get a clear tube to observe the location of the internal components. Ideally I can find a set of tubes that are rigid, clear, and easily nest. Then I could cut a small section of one to insert into the float to act as a sort of bearing surface.

This tube could also house the DS18B20 temperature sensor providing the needed water-proofing.

I’m trying to decide about the solution conductivity portion. It would be fairly easy to put two electrodes extending from the tube but I need to do some experimentation to see if this is valid. I have a small handheld testing unit that shows the level of dissolved solids but I have a feeling that it is more sophisticated than simply measuring conductivity. It’s possible that, although conductivity might not be a “laboratory” measurement, it might be enough to signal that the nutrient solution has changed.

I've also included the relays that I plan to use to control the two pumps.