Hello students! Welcome to today’s Computer Science lesson. In this video, we will cover Logic Gates, including their types, symbols, truth tables, and clear explanations, exactly according to the O-Level Computer Science 2210 syllabus. This is a high-scoring topic, so pay close attention.

What Are Logic Gates?

Logic gates are electronic circuits that take binary inputs, which are either 0 or 1, and produce a binary output. They work using Boolean logic, which allows computers to make decisions such as YES or NO and TRUE or FALSE. Every calculation, decision, or control operation inside a computer is based on the working of logic gates.

AND Gate

The AND gate checks whether all conditions are true. If even one input is false, meaning it is 0, the output also becomes 0. The output is only 1 when both inputs are 1. For example, a school may allow entry only if a student has an ID card and is wearing a uniform. According to its truth table, when both inputs are 0 the output is 0, when one input is 1 and the other is 0 the output is still 0, and only when both inputs are 1 does the output become 1.

OR Gate

The OR gate checks whether at least one condition is true. If any one of the inputs is 1, the output becomes 1. The output is 0 only when all inputs are 0. A real-life example of an OR gate is entering a building if you have a card or a passcode. From the truth table, when both inputs are 0 the output is 0, but if one or both inputs are 1, the output is 1.

NOT Gate

The NOT gate has only one input and it reverses the input value. If the input is 0, the output becomes 1, and if the input is 1, the output becomes 0. This is why it is also called an inverter. For example, if a rule says “NOT raining,” then rain means false. According to its truth table, an input of 0 gives an output of 1, and an input of 1 gives an output of 0.

NAND Gate

The NAND gate is a combination of an AND gate followed by a NOT gate. It produces an output of 0 only when both inputs are 1. In all other cases, the output is 1. This gate is very important because it is known as a universal gate, which means all other logic gates can be created using only NAND gates. From its truth table, the output is 1 for input combinations 00, 01, and 10, and it becomes 0 only for the input combination 11.

NOR Gate

The NOR gate is a combination of an OR gate followed by a NOT gate. It produces an output of 1 only when both inputs are 0. If any input is 1, the output becomes 0. Like NAND, the NOR gate is also a universal gate. According to the truth table, the output is 1 only for the input combination 00, while all other input combinations produce an output of 0.

XOR Gate

The XOR, or Exclusive OR, gate gives an output of 1 only when the two inputs are different. If both inputs are the same, whether 0 and 0 or 1 and 1, the output is 0. A common real-life example is a light that turns on when only one of two switches is pressed. The XOR gate is widely used in binary addition. From the truth table, the output is 1 for input combinations 01 and 10, and 0 for 00 and 11.

XNOR Gate

The XNOR, or Exclusive NOR, gate is the opposite of the XOR gate. It produces an output of 1 when both inputs are the same and an output of 0 when the inputs are different. In simple words, same inputs give output 1, while different inputs give output 0. A real-life example is a system that allows access only when both passwords match. The XNOR gate is commonly used in comparison circuits to check whether two values are equal.

Conclusion

Logic gates are the foundation of digital computers. If you clearly understand their working principles, symbols, and truth tables, many exam questions become very easy to solve. If this video helped you, like, share, and subscribe. See you in the next lesson—keep learning!