Computer • Chapter 04

Logic Gate

Logic Gate
Digital Circuits • Gate Types • Truth Tables • Applications • Exam Revision
Source: uploaded Computer PDF — Chapter 4, PDF pages 15–18. Gate rules and truth tables have been checked and corrected.
0 & 1
Binary Inputs
1 Output
One or more inputs
NAND / NOR
Universal Gates in source
7 Types
AND, OR, NOT, NAND, NOR, XOR, XNOR
⚙ Introduction to Logic Gates
  • Logic gates are the basic building blocks of digital circuits.
  • They perform logical operations based on binary inputs (0 & 1).
  • Used in computers, automation and electronic circuits.
  • A logic gate can take one or more inputs, but the output will be one.
  • The PDF states that a logic gate is used to allow or deny a digital signal.
  • The PDF states that logic gates are made up of diodes and transistors.
LOGIC GATESLogic Gate BASICAND • NOT • OR UNIVERSALNAND • NOR OTHEREX-OR • EX-NOR
Correct Boolean rules for the eight common logic gates
Correct Boolean rules for the eight common logic gates

Basic Logic Gates

ANDNOTOR

Universal Logic Gates

NANDNOR

Other Logic Gates

EX-OR / XOREX-NOR / XNOR
AND Gate
Source rule: output is 1 only if both inputs are 1.
A · B
OR Gate
OR gives 1 when at least one input is 1; it gives 0 only for 00. The + sign denotes Boolean OR.
A + B
NOT / Inverter
Page 15's inverter visual shows a single input changed to its opposite output.
0→1 • 1→0
NAND Gate
Source: opposite of AND; output is 0 only when both inputs are 1.
(A · B)̅
NOR Gate
Source: opposite of OR; output is 1 only if both inputs are 0.
(A + B)̅
XOR Gate
XOR (Exclusive OR) gives 1 when the two inputs differ, and 0 when they are the same.
A ⊕ B
XNOR Gate
Source rule on page 18: output is 1 if inputs are same.
(A ⊕ B)̅
REVISION NOTE
Correct gate rules and tables are shown below. NOT is a basic gate; NAND and NOR are universal gates. XOR means Exclusive OR; XNOR means Exclusive NOR.
Verified truth tables: AND, OR, NAND, NOR, XOR and XNOR
Verified truth tables: AND, OR, NAND, NOR, XOR and XNOR
AND — Truth Table
ABA·B
000
010
100
111
GATE RULE
Output is 1 only if both inputs are 1.
OR — Truth Table
ABA+B
000
011
101
111
EXAM TRAP
OR: at least one input 1 → output 1. Inverting a single input is the NOT operation.
NAND — Truth Table
AB(A·B)̅
001
011
101
110
GATE RULE
Opposite of AND; output is 0 only if both inputs are 1.
NOR — Correct Truth Tablediv>
AB(A+B)̅
001
010
100
110
EXAM TRAP
NOR gives 1 only for 00. For A=1, B=0, its output is 0.
XOR — Truth Table
ABA⊕B
000
011
101
110
EXAM TRAP
XOR = Exclusive OR: different inputs give 1. XNOR = Exclusive NOR: equal inputs give 1.
XNOR — Truth Table
AB(A⊕B)̅
001
010
100
111
GATE RULE
Output is 1 if inputs are same.
NOT / Inverter — Correct Truth TableTruth Table
NOT is a basic single-input gate, not a universal gate. It complements its input: 0 → 1 and 1 → 0.

ANOT A
01
10
d>0
NAND-only and NOR-only constructions of NOT, AND and OR
NAND-only and NOR-only constructions of NOT, AND and OR
🧩 Applications of Logic Gates
  • Used in computer processors & circuits.
  • Used in alarm systems & security devices.
  • Used in automatic gates & digital locks.
  • Used in artificial intelligence (AI) and robotics.
Digital Building Block

The chapter frames logic gates as the basic building blocks that perform logical operations on binary 0/1 inputs.

Input → Output Idea

One or more inputs can be accepted, while the output remains a single digital result.

⚡ Quick Recall
  • Logic gates: basic building blocks of digital circuits.
  • Inputs: binary 0 and 1.
  • Basic gates in source: AND, NOT, OR.
  • Universal gates in source: NAND, NOR.
  • Other gates: EX-OR and EX-NOR.
  • AND source table: only 11 gives 1.
  • NAND source table: only 11 gives 0.
  • XNOR source table: same inputs (00, 11) give 1.
EXAM TRAP / SOURCE ALERT
For input order 00, 01, 10, 11: AND = 0001; OR = 0111; NAND = 1110; NOR = 1000; XOR = 0110; XNOR = 1001. NOT complements a single input.