Water level sensor circuit: projects for school students

I have posted wide varieties ofsmall electronic projects onwater level sensor in Circuits Gallery. This is another project for school students to detect the water level inside a water tank or any other water reservoir. We can call this as a liquid level sensor`, because this circuit is able to sense the presence and absence of liquid in a tank. Th
Water level sensor circuit: projects for school students - schematic

is school electronic project is built around BC548 transistors and a 7432 OR gate IC. This water alarm will produce a beep sound when the water goes below a specified level (h1) or when the water overflows. This is one of the home science experiments that can be implemented as science projects for high school students. You may also like to have a look at our other science experiments like automatic water tank level controller and numeric water level indicator. I have also published an article to detect overflow of water using alarm. The basic principle of this circuit is switching property of NPN transistors `. That is when a transistor is OFF` its collector voltage is Vcc (6V) and when it is in ON` state the collector voltage falls to 0. 02V (approx. 0V). When water level goes below h1` the wire from transistor Q3 disconnects from the power supply, hence Q3 becomes OFF`. Thus its collector voltage will be Vcc. This high` voltage is applied to the OR gate (IC7432)`. Hence its output becomes high (if any one of the input of OR` gate is high, output will be High`) and it leads to the turning ON` of the driver transistor Q4. Thus the buzzer produces a beep sound. When the water level goes above h2` Q1 becomes ON because its base terminal is energized by Vcc in the water, due to this Q2 will be in OFF` state. So the collector voltage of Q2 becomes High`. Again this High` voltage is given to the OR gate making its output High`...

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