Hydro alarm electronic project

Electronic circuit diagram below is a very simple hydro alarm, which detects the absence or reduced amount of water in a container.
The electronic circuit is very simple and works as follows: N1 Schmitt trigger works as an oscillator and produce a frequency of about 1 kHz. If in the tank is still enough water, then alternative voltage go from electrode A to electrode C. After rectified of diodes D1 and D2, at the pin 13 of the N4 reaches trough the D3 a logical "0". By this, the oscillator sound N4 and buzzer are disconnected.
If the water level drops into such an extent that electrode C is no longer in contact with water, then blocking of N4 is suspended and buzzer starts to sound at regular intervals. Alarm duration is determined by the oscillator capacitor N3 and C6.
The alarm stops as soon as the tank fills and electrode C is again in contact with water. If S1 button operates before filling D5 LED lights as soon as water reaches the electrode B. D5 turns off after about 10 seconds to load is not too much power.
Electrodes use may be as pieces of wire. Because monitoring level is made with AC signal, electrode oxidation is reduced.
Supply voltage can be between 5 and 15 V. If you choose a voltage lower than 9 V, the electrode surface C must be much higher than that of electrode B. This necessary because there is loss of signal from the electrode C in diodes D1 ... D3.