Whether it's a leaking hot water heater or just a leak under the sink, you want to catch water leaks just as soon as they start, before damage occurs.
For those of you who want an extremely simple build, I've reduced it to just 5 parts:
The idea is simple enough: use extra long (30mm) male pin headers to elevate the detector above a surface where water might encroach. Water which shorts against any two adjacent pins should immediately wake up the detector so that it can report the problem. I chose LoRa because for any detected water leaks you definitely want highly reliable RF communication.
For those of you who want more of a commercial-grade, ultra high-reliability build, you can add the rest of the parts in the BOM:
and/or
This water leak detector is powered by two AA batteries.
The maximum sleep interval supported by the TLP5010 is two hours. This is set using a 170K ohm resistor. If you wish for a shorter sleep interval, select and use a lower ohm resistor. Table 3 of the TLP5010 datasheet shows a mapping of resistor values to time intervals: http://www.ti.com/lit/ds/symlink/tpl5010.pdf