Same buzzer, the same power supply but with a little ingenuity this gives an alarm when the water falls below a certain level.This circuit as shown in Schematic 7, which can be used to indicate when the level falls low in overhead tank, a sump or even in a bore well.Now you will find a 100K resistance between the trigger terminal and positive supply which pulls the trigger terminal to the positive potential. As in the previous case both wires from trigger and ground are kept in the water. Even though pulled up, trigger will be at ground potential as long as there is water across the probes and hence the piezo buzzer can not go off. Once the water level goes down, the trigger terminal loses its ground potential and reaches the positive potential because of the pull up resistor and the buzzer will be on.
Incidentally in digital electronics parlance this is known as going high or pulling up. If the voltage at any terminal is close to the ground potential as dictated by system requirement, it is known as going low or pulling down. Conversely if the voltage at any terminal is close to the positive potential as dictated by system requirement, it is known as going high or pulling up. Digital circuits work usually with 0 andl. Obviously one is high level and zero is low level, which are known as logic states. Noise in the digital circuits can easily be eliminated. Follow the logic states depicted here.
On the other hand analogue circuits process analogue signals which can output any value within a range of its power supply. Examples are audio, video amplifiers. Unlike digital circuits, noise here is difficult to control.Construction is similar to earlier project, except that the probes are now fixed at the required low level of water.
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