Here is a schematic of a simple water or liquid level sensor relay switch circuit, which can be used to switch on or off electronic appliances at the desired level of water. The circuit is ideal for automatically switch off the water pump when your water tank, pool or aquarium is filled. The circuit is very easy to build
and contains only few parts. The NPN transistor 2N2222 is working as a switch, when the two probes shown in the circuit will detect water the transistor become switch on and activates the six volts relay For operating the circuit with 12 volts use a 12 volt relay.