Water Level Indicator


A water level indicator may be defined as a system by which we can get the information of any water reservoir. Water level indicator system is quite useful to reduce wastage of water from any reservoir while filling such reservoir.

Whenever the tank gets filled, we get alerts on particular level by this simple transistor based Water Level Indicator circuit. Here we have created four levels (low, medium, high and full). We have added three LEDs to indicate the initial three levels (A, B, C) and one buzzer to indicate the full level (D). When the tank gets filled completely, we get beep sound from buzzer.


Sl. No. Components Quantity
1 Transistor (BC548) 4
2 Resistor (220Ω) 6
3 LEDs (R,Y,G,) 3
4 Buzzer 1
5 Battery (9V) 1


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We can consider this whole circuit as four small circuits, each one for indicating, when a particular level (A, B, C or D) of water has been reached. When water level reaches to point A, the circuit with RED LED and transistor Q1 gets completed and RED LED glows. Similarly, when the water level reaches to point B, the circuit with YELLOW LED and transistor Q2 gets completed and YELLOW LED glows. Likewise, when the water level reaches to point C, the circuit with GREEN LED and transistor Q3 gets completed and GREEN LED glows. And finally, when the tanks gets full (point D), circuit with Buzzer gets completed and Buzzer starts beeping.


Here, we are using transistor (NPN) as a switch. Initially, there is no voltage applied to the base of the transistor Q1 and the transistor is in OFF state and no current is flowing through the collector to emitter, and LED is OFF.

When the water reaches to point A in the tank, the positive side of the battery gets connected to the base of the transistor Q1 through the water. So when the positive voltage has been applied to the base of the transistor Q1, it gets into ON state and current starts flowing from collector to emitter and RED LED glows. Resistors (R4, R5, R6) are the base of the each transistor are used to limit the maximum base current. Generally, a transistor gets its ON state fully when a voltage of 0.7V is applied to the base. There are also resistors (R1, R2, R3) with each of the LEDs to drop the voltage across LEDs, otherwise LED may blow up. Same phenomenon happens when water level reaches to point B. As soon as water level reaches to point B, a positive voltage gets applied to the transistor Q2, it gets ON and current starts flowing through YELLOW LED and LED glows. With the same principle, GREEN LED glows when water reaches to point C. and finally, Buzzer beeps when water level reaches to point D.

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Demonstration of Water Level Indicator circuit

Date of the project: 15/09/2016
(Won 2nd prize in “Technical Exhibition” at Engineer’s Day’2016, Jorhat Institute of Science and Technology )