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.
IC1 = 555 Timer
R1 = 1K
R2 = 100K
C1 = 2.2uf 15v
K1 = Piezo Buzzer
B1 = 3Volt Battery
The circuit is based on an Astable multi-vibrator wired around IC1 (NE 555). The operating frequency of the Astable multi-vibrator 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 multi-vibrator 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 extended, 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.