handy little circuit boards if you are interested in
monitoring for the presence of a liquid (rain, leak, etc.).
They use Darlington Transistors which enables these to work
with very clean (low conductivity) liquids. You can use
this to detect rain and turn off your automatic sprinkler
system or to monitor for a leak around your washing machine,
fish tank, water heater, etc. It detects the presence or
absence of liquid at the tip of the sensor which can be
simply 2 wires stuck into a dry sponge that when the sponge
gets wet, the on-board relay is activated. When the
relay is activated, the voltage that is applied at the input
of the circuit board will be switched from the normally
closed contact of the relay to the normally open contact.