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This is a simple circuit for liquid level alarm. It is built around two BC547 transistors (T1 and T2) and two 555 timer integrated circuits (IC1 and IC2). IC1 and IC2 are wired in unstable multi-harmonic mode. Timer IC1 outputs low frequency, while timer IC2 outputs high frequency. As a result, a beep will sound when the liquid tank is full. Initially, when the container is empty, transistor T1 does not conduct electricity. Therefore, Transistor T2 is conducting and pin 4 of IC1 is low.
This low voltage disables IC1 and it doesn't oscillate. IC1's low output will disable IC2 and it won't oscillate. So no sound is coming from the speaker. But when the tank is full, transistor T1 will pass through. Therefore, transistor T2 turns off and pin 4 of IC1 goes high. This high voltage turns on IC1 and oscillates to produce low frequency at pin 3. This low frequency output enables IC2 and also oscillates to produce high frequency.
The result is the sound produced by the speaker. Using the VR1 preset, you can control the volume of the speaker sound. The circuit can be powered by a 9V battery or by a power supply using a 9V power adapter. Assemble the circuit on a multipurpose PCB and place it in the appropriate cabinet. Install two metal strip water level probes so that one end touches the bottom of the tank, the other touches the maximum water level in the tank. Connect sensor and circuit with cable topic.