Humidity would not be a problem.
Once it is built, the entire thing would be potted in epoxy.
That circuit is a direct replacement for the night watchman that he is already using.
3 wires enter the potting - hot, ground, and output - everything else except the RTE switch is protected by the epoxy.
The variable resistor is the only component that would potentially have any exposure to the elements, but there are ways to protect that as well.
If using a trimmer with a tweaker adjustment screw, I use a skinny straw preassembled to the pot, held in place by RTV (or hot glue as long as the straw doesn't deform too much), then leave the straw snorkelling out of the potting, trimmed flush once the epoxy cures. After tweaking is done, a blob of RTV can cover the hole to seal it, but is easily removed/replaced later if another adjustment needs to be done.
If using a 'standard' shaft pot, a seal can be made by putting a thin coat of dielectric grease on the shaft, then running a quick bead of RTV around it, making an O ring. Seal all open gaps on the pot with RTV to prevent the epoxy from fouling it during potting. It's not the easiest process, but I've done it and it works.
For ease of construction, and better fine control, I recommend the tweaker pot instead.
Steve
