To monitor the filling of a bath, a water tank, or a swimming pool, or to warn when a gully is overflowing, here’s a very simple water level detector built around a CD4011 CMOS quad Gates IC1. A and IC1.B are wired as an astable multivibrator. The oscillator frequency is determined by C1, R2, and preset P1.
When quiescent, resistor R1 pulls the input to gate IC1.A down to logic low, which therefore by default blocks the operation of the oscillator in the absence of water. When water is present between the e+ and e− electrodes, IC1.A is taken high, enabling the oscillator. The output signal from gate IC1.B is shaped by IC1.C to obtain a rectangular waveform. Gate IC1.D inverts the signal so that transistor T1 is held off in the absence of water, which avoids current flowing in the primary of transformer TR1 when the system is at rest. TR1 is a 12 V 1.5 VA AC power transformer wired as a step-up transformer i.e. with the low-voltage winding connected to T1. The transformer’s step up ratio affords ‘passive’ amplification of the signal present at the drain of T1. The transformer’s high voltage winding is connected to piezo sounder BZ1 (e.g. Murata; the ‘28’ indicates the diameter) which produces the audible warning.
In order to optimize the sound output of the unit, you’ll need to adjust P1 so as to set the oscillator frequency to the resonant frequency of the piezo transducer; this setting can be done by ear. The electronics and batteries can be housed into a salvaged case (for example, the kind of oval box found inside giant chocolate ‘surprise’ eggs). The electrodes, formed from simple rigid copper wires, pass out through the case; the join is made watertight using epoxy adhesive.