This simple and inexpensive circuit sounds an alarm Ca whistle) when water level in a Container, such as an over- head water-tank, reaches a predetermined level.
The basic circuit is an audio oscillator with positive feed- back. The output from Tl transistor is fed back to the base of T2 transistor via the mass of water connecting the two probes and the capacitor C. Consequently, when path is open the oscillator does not operate. When water-level reaches a pre-selected height, submerging the two probes, the feedback path is complete and an alarm is sounded over the loud- speaker. The pitch of the whistle may be altered by changing the value of capacitor.