What is LM339?

LM339 is a “Quad Differential Comparators”. In this quad means four hence the LM339 has four differential comparators in a single package.

Differential comparator is an operational amplifier configuration which compares two inputs and give an output according to the input voltage. Output of the differential comparator is always either high or low it doesn’t give a discrete or analog output. Differential comparators have two inputs, named inverting input and non-inverting input. Symbol of a comparator is shown in the below image.

Main Components

Lets see what are the required components for our project and what are the purpose of the components.

• LM339 Quad Differential Comparators
• BC547NPN Transistors
• LEDs (Green, Yellow and Red)
• 1MΩ Resistors
• 100kΩ Resistors
• 10kΩ Resistors
• 220Ω Resistors
• Bare copper wires (used as probes)

Let’s find out what are purposes of these main components.

Ø LM339 Quad Differential Comparators

As mentioned before this the component that sense the water level. To do that we need a voltage difference. We are going to use the resistivity difference of the water and air to get this done. This will be explained later in the tutorial.

Ø BC547 NPN Transistors

LM339 can only drive loads which draws current less than 20mA. But some LEDs can draw more than 20mA of current it can be harmful to LM339 IC to directly drive the LEDs. That is where the BC547 transistor comes in to play. It can drive higher current load using the signal from the LM339 IC’s differential comparator output.

Ø LEDs and Resistors.

LEDs are used to indicate the water level sensed by LM339 and resistors are used for current limiting and create voltage dividers in this circuit.