Lesson-09 XNOR Gate (IC 74266) – Equality Checker

Introduction

In this lesson, we will learn about the XNOR gate, which is the opposite of the XOR gate.

XNOR stands for Exclusive NOR.
An XNOR gate gives an output HIGH (1) when both inputs are the same.

Because it checks equality, it is commonly called an Equality Gate.

What Is an XNOR Gate?

An XNOR gate is a two-input logic gate.

Simple meaning:

Output is ON when Input A and Input B are equal.

• Both LOW → Output HIGH
• Both HIGH → Output HIGH

Truth Table of XNOR Gate

Output is HIGH only when A = B.

IC 74266 – Quad 2-Input XNOR Gate

IC 74266 contains four XNOR gates inside a single IC.

Key Features:

• 4 independent XNOR gates
• Operates at 5V
• TTL logic IC (74xx family)
• Suitable for comparison logic

IC 74266 Pin Configuration

Why Do We Use an XNOR Gate?

XNOR gates are used when:

• Equality check is required
• Data comparison is needed
• Verification logic is required

Real-life examples:

• Digital locks
• Data matching
• Error checking systems

Components Required

Circuit Diagram / Wiring

1. Place IC 74266 on the breadboard
2. Connect Pin 14 to +5V
3. Connect Pin 7 to GND
4. 10kΩ resistor from Pin 1 (Input A) → GND
5. 10kΩ resistor from Pin 2 (Input B) → GND
6. Connect Push Button A to Pin 1 (Input A)
7. Connect Push Button B to Pin 2 (Input B)
8. Connect Pin 3 (Output) to LED through 220Ω resistor
9. Connect LED’s other end to GND

Working Principle

Case 1: Both Buttons OFF (0,0)

• Output = HIGH
• LED ON

Case 2: Inputs Different (1,0 or 0,1)

• Output = LOW
• LED OFF

Case 3: Both Buttons ON (1,1)

• Output = HIGH
• LED ON

This confirms XNOR gate operation.

Observation Table

Common Mistakes & Troubleshooting

❌ LED not ON when both buttons pressed
✔ Check input connections

❌ IC not responding
✔ Verify power supply

❌ Output unstable
✔ Avoid floating inputs

Real-Life Applications

• Digital comparison circuits
• Equality checking logic
• Data verification
• Error detection
• Security systems