Discrete component water level alarm

This water level alarm can sound an alarm in time when the water level in the water tank is full. The square wave signal generated by the transistor oscillator in the circuit is sent into the water through the probe. The probe has a long life because there is no electrolytic corrosion.

The whole circuit is composed of a transistor multivibrator, an amplifier and an alarm circuit.

The multivibrator outputs the audio frequency in the form of a square wave signal from the collector of T2, which is sent to one of the two probes on the top of the sink via C4. The other probe is connected to amplifier T3 via C5.

When the water tank is full, the audio frequency output by the oscillator spreads from the probe in the water tank and is received by the male probe. It is amplified by T3 through C5 and transmitted to the alarm part. T4 and T5 in the alarm part further amplify the signal and drive transistor T6, causing the buzzer to sound an alarm and lighting the indicator LED1 at the same time.

When the water tank is empty, the buzzer does not sound and the indicator LED1 does not light up.

In short, when the two probes are short-circuited by water or conductive liquid level, the buzzer will sound an alarm.

The circuit can be mounted on a common PCB and sealed in a suitable box. Two probes are made of thick wires and fixed on the top of the water tank so that the probes can touch the water surface when the water tank is filled with water.