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Look at "opencores" for really building a microcontroller instead of wiring up an existing one.
Have a look at this entertaining presentation: https://media.ccc.de/v/33c3-8019-lockpicking_in_the_iot to avoid of the pittfalls.
My experience with similar constructions: even with the dedicated LM1801 moisture sensor IC that puts only AC over the electrodes, the only material that survives longer term is carbon. (Recycled from old D type batteries). Stainless steel did not survive very long. I also would recommend a snubber (RC network) over the relay contact to reduce wear, as the pump is an inductive load. I have an automatic pump in my cellar to prevent flooding after heavy rain. A bit more elaborate, everything is redundantly built, with an alarm if one of the pumps fails.
HacKeyboard, a mechanical keyboard built from scratch
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