Lesson-07 NOR Gate (IC 7402) – Inverted OR Logic

Introduction

In this lesson, we will learn about the NOR gate, an important logic gate in digital electronics.

A NOR gate is the inverse of the OR gate.
It gives an output HIGH (1) only when ALL inputs are LOW (0).
If any input becomes HIGH, the output turns LOW.

NOR gates are also called Universal Gates, just like NAND gates.

What Is a NOR Gate?

A NOR gate is a two-input logic gate.

Simple meaning:

Output is ON only when Input A NOR Input B are OFF.

Truth Table of NOR Gate

Output is HIGH only when both inputs are LOW.

IC 7402 – Quad 2-Input NOR Gate

IC 7402 contains four NOR gates inside one IC.

Key Features:

• 4 independent NOR gates
• Operates at 5V
• TTL logic IC (74xx family)
• Widely used in digital systems

IC 7402 Pin Configuration

Why Is NOR Gate Important?

NOR gates are used when:

• Output must stay HIGH only in idle state
• Active-LOW logic is required
• Control systems need inverted OR behavior

NOR is also a universal gate, meaning all logic gates can be built using NOR gates alone.

Components Required

Circuit Diagram / Wiring

1. Place IC 7402 on the breadboard
2. Connect Pin 14 to +5V
3. Connect Pin 7 to GND
4. Connect Push Button 1 to Pin 2 (Input A)
5. Connect Push Button 2 to Pin 3 (Input B)
6. Connect Pin 1 (Output) to LED through 220Ω resistor
7. Connect LED’s other end to GND

Working Principle

Case 1: No Button Pressed (0,0)

• Output = HIGH
• LED ON

Case 2: Any One Button Pressed (1,0 or 0,1)

• Output = LOW
• LED OFF

Case 3: Both Buttons Pressed (1,1)

• Output = LOW
• LED OFF

This confirms NOR gate behavior.

Observation Table

Common Mistakes & Troubleshooting

❌ LED always OFF
✔ Check if both inputs are LOW

❌ IC not working
✔ Verify VCC and GND pins

❌ Flickering LED
✔ Inputs may be floating

Real-Life Applications

• Control logic circuits
• Digital memory elements
• Alarm systems
• Industrial automation
• Signal processing