Simple liquid level alarm circuit

This simple circuit is used for liquid level alarm. It consists of two BC547 transistors (T1 and T2) and two 555 timer chips (IC1 and IC2). Both IC1 and IC2 are connected in multivibrator mode, the difference is that IC1 generates low frequency and IC2 generates high frequency. As a result, when the liquid level rises and short-circuits the two probes, the circuit emits a beeping alarm.

Initially, the liquid level is very low, and transistor T1 is not conducting. T2 is conducting, and pin ④ of IC1 is at a low potential, which prohibits IC1 from working, IC1 does not oscillate, and its output pin ③ is also at a low potential, and IC2 is also prohibited from working. As a result, the speaker does not make any sound.

But when the tank is full, the base of T1 is positively charged and conducts. Then T2 is cut off, and the ④ pin of Ic1 becomes high potential, and Ic1 starts working, generating low-frequency output at the ③ pin. This low frequency modulates the ④ pin of Ic2, making IC4 emit intermittent high frequency during the negative half cycle of each oscillation wave of IC3, so that the sound heard by the speaker is a beep. VR1 can control the volume.

The circuit is powered by a 9V battery, or from a 220V AC mains using a 9v AC adapter.

The circuit can be installed on a common PCB board and sealed in a suitable box. Use two bare metal wires as probes, one at the bottom of the tank and the other at the highest point of the liquid level at the top of the tank, and then use soft wires to connect the probes to the circuit.