The type of alarm shown in the figure is useful in applications such as monitoring the water level in the bilges of a boat or in basements or to indicate when the water level in the bath has reached the required level.
Gates 1 and 2 make an oscillator which will oscillate when the sensor electrodes are bridged by water. When the sensor electrodes are open, resistor R1 keeps the input to gate 1 HIGH. This makes the output of gate 3 LOW and hence the quiescent current consumption during idling is very small.
Transistor T1 and T2 amplify the oscillate or signal and drive a loudspeaker to sound an alarm when the presence of water is detected between the sensor electrodes. The alarm will stop only when either the water level recedes down or when the supply line is cut off.