A 600 Hz signal at a level of 2.4 Vpp, generated by the oscillator on board an LM1830 (National Semiconductor) is applied to a probe via C2. The probe is immersed in the liquid whose level is to be monitored. Because of C2, there is no direct voltage at the probe, so that there are no electrolysis problems.
As long as the probe makes no contact with the (conductive) liquid, the signal level at the input of the detector is equal to the level of the oscillator signal. When the liquid touches the probe, the detector input is connected to ground (or nearly so). This causes the level at pin 10 to drop. When it becomes more than 0.6 V lower than the oscillator signal, the detector switches on the internal output transistor in the rhythm of the oscillator frequency, since that is not suppressed by the detector.
The consequent signal at pin 12 is used to drive a simple output stage, T1, which drives a small loudspeaker, LS1.
The supply for the circuit is best taken from a 9 V PP3 battery. In quiescent operation, the current drain is 3 mA; when the alarm sounds, the current rises to about 80 mA.