From Team Practical Projects on Hackster.io:
A leaking washing machine, overflowing tub or sink, or leaky pipes or toilet can cause considerable damage to a home if not detected and shut off early. Years ago, a washing machine water valve stuck open while washing clothes late at night. I awoke in the middle of the night to find a flood in the washer/dryer vestibule resulting in $12,000 in water damage. Since that time, I have been reluctant to run the washing machine at night or at any time when someone is not home to watch over it. This is not always convenient and it would help to have a solution that can detect a water leak and signal me via a local alarm (for when I am at home) and/or a smartphone notification (for when I am out).
Leaky or humid basements and leaking water heaters present similar problems. Adding a second water leak detection sensor, as well as temperature and humidity measurement to the project, can cover all of these bases.
The Water Leak Detector (WLD) is designed to detect leaks and to notify the user in time to contain the damage while it is still minor. To this end, the WLD provides both a local audible and visual notification and a remote notification to the user’s mobile phone. The WLD is designed to support two sensors, each of which may be placed anywhere that can be cable connected to the WLD electronics.
The WLD will alarm with a puddle as small as 2 mm deep. An alarm condition exists whenever either of the sensors detects the presence of water. The alarm condition persists until both sensors are dry, in which case the WLD automatically clears out the alarm condition and re-arms for subsequent triggering of a water leak alarm.