I found some water leaked from the basement ceiling and damaged the dry wall around a lamp.
I have fixed the plumbing with the plumber’s putty, but I thought this could happen again in the future. So, instead of repairing the dry wall and painting, I wanted to put a water sensor with a warning system and attach some type of cover around the lamp to hide the damaged area and allows me to access without cutting the dry wall again.
I was debating between options: 1) water detector with a wifi module; and 2) simple sensor with a transistor and a buzzer. It seemed to be too much work for the option 1, so I chose the option 2. Always the simper the better!
Here is the schematic diagram.
Water conducts electricity, so if the probes contacts the water, the NPN transistor turns on and the buzzer sets off. The resistor 100Ω between the base and the emitter can be other value depending on what sensitivity you want. Since the transistor is normally off, the battery can last for a long time. I don’t have any tested data yet.
Here are some pictures of the assembly.
The last picture shows the sensing element. On a credit card size plastic (some old card key), several strips of adhesive backed copper tape were attached. I also added wires on the sensor terminal so that I can test the battery without open the cover. See below picture. The test probes are hidden on the lamp cover.