Using this circuit diagram can be designed a very simple liquid detector using common electronic components. The liquid detector may operate an active buzzer, to make a sound when is reached a certain level of water.
Because water sensor and control circuit for buzzer are located on the same printed circuit board, indicator, together with 9 V battery and buzzer can be mounted in a compact case.
When water reaches the sensor, the base of T1 is connected to the positive supply terminal. Therefore, T1 and T2 are open, so that buzzer BZ1, will be activated. Sensitivity reduction of the circuit can be done by increasing the value of R2.