This simple circuit will raise alarm when the water level in a tank goes below a certain level.
In the above circuit diagram, we can see that two probes are placed inside the water tank.
when water is touching these probes. or level of water is above these probes then the circuit will be in silent mode and when water is not touching probes i.e. level of water is below these probes then the circuit will activate buzzer and raise alarm.


When water is present between probes, Transistor T1 receives base current which causes current to flow through R3 and CE of T1 , and transistor T2 gets ON. In this condition current flowing through R4 is pulled toward the Gnd through T2 and no base current is provided to T3, thus T3 is OFF and buzzer remains inactive.

When there’s no water present between the two probes i.e. water is below the probes then T1 and T2 will be in Off state as both of them are not getting base current. As T2 is in Off state current flowing through R4 will be directed toward base of transistor T3, this will take transistor T3 to the saturation region i.e. T3 will be ON and Buzzer will be activated.

To avoid any False alarm due to the unwanted signals, a pull-down resistor R2 is added to the base of transistor T1.

About some Components:-

Buzzer:- There are different types of buzzers available on market for example:- 5V buzzer, 9V buzzer, 12v buzzer, 3v – 12v buzzer e.t.c and most of them will work with this circuit.

Power supply:- 3v to 12v Power supply can be used for this circuit. but it is also important to choose the right buzzer according to the Power supply or vice versa.
For example:- If 9v battery is used for this circuit then we cannot use a 5v buzzer else buzzer will get damaged due to the high voltage.
There are some buzzers that function in a wide voltage range, they are marked as “3v – 12v” or 3v – 24v” or similar i.e. they will work in these voltages range.

Transistor:- We can see three transistors; T1,T2, and T3 in the circuit, all of them are NPN transistor and we are using NPN transistor BC 547 in place of them. It is not that we can only use transistor BC547, we can also use other NPN transistor such as BC548, 2N3904, 2N2222 e.t.c.

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