Here is a simple water level alarm circuit using 555 timer that will produce an audible alarm when the water level reaches a preset level. The circuit can be powered of a 3V battery and is very handy to use.
The circuit is based on an astable multivibrator wired around IC1 (NE 555).
FIG 1. WATER LEVEL ALARM CIRCUIT USING NE555 TIMER
The operating frequency of the astable multivibrator here will depend on capacitor C1, resistances R1,R2 and the resistance across the probes A&B. When there is no water up to the probes, they will be open and so the multivibrator will not produce oscillations and the buzzer will not beep.When there is water up to the level of probes, some current will pass through the water, the circuit will be closed to some extend and the IC will start producing oscillations in a frequency proportional to the value of C1,R1,R2 and the resistance of water across the probes. The buzzer will beep to indicate the presence of water up to the level of the sensing probes.
The circuit can be powered by a 3-12V battery.