There are two popular types:

1. A Mechanical Device - Upon cantacting water, the sponge will swollen up and witch off, shut down the feed water:


leak detector stop


2. Electronic Device: In contacting water (by leak from RO water filter or sink or garbage disposal), it will turn on the alarm and shut off the feed water (It also shut off water when the 9V battery dies):


Savant AS414BRLeakDectorAS414BR illustration