Smart Home Electronic Security System

A digital electronics project implementing a keypad-based security system using combinational logic circuits.

Overview

This project implements a Smart Home Electronic Security System designed as part of CS 1105 - Digital Electronics & Computer Architecture. The system uses digital logic components to create a secure access control mechanism for multiple rooms.

Student: Eden Gilbert Kisekka
Course: CS 1105 - Digital Electronics & Computer Architecture
Instructor: Joseph Okhuoya
Date: February 11, 2026

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Table of Contents

• System Architecture
• Components
  • Priority Encoder
  • Multiplexer
  • Logic Gates (Comparator)
  • Decoder
  • Demultiplexer
• How It Works
• Files
• Circuit Implementation
• Usage
• Theory
• References

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System Architecture

The security system is composed of four main functional blocks:

┌─────────────────┐     ┌──────────────┐     ┌──────────────┐
│  Data Input     │     │   Data       │     │  Processing  │
│  (User Interface)│────▶│  Routing     │────▶│ (Auth)       │
│  - Keypad       │     │  (Selection) │     │  - Comparator│
│  - Encoder      │     │  - MUX       │     └──────┬───────┘
└─────────────────┘     └──────────────┘            │
        │                    │                      ▼
        │                    │               ┌──────────────┐
        │                    │               │   Status     │
        │                    ▼               │  Indication  │
        │               ┌──────────────┐    │  (Feedback)  │
        │               │   Decoder    │    │  - LEDs      │
        │               │  (Visual)    │    │  - Locks     │
        │               └──────────────┘    └──────────────┘
        │
        ▼
┌─────────────────┐
│ Demultiplexer   │
│ (Door Control)  │
└─────────────────┘

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Components

1. Priority Encoder (Input Interface)

Component: 10-to-4 Priority Encoder (74LS147)

Function:

• Converts user's decimal button press (0-9) into 4-bit Binary Coded Decimal (BCD) output
• Ensures that if two keys are pressed simultaneously, only the highest value is registered
• Prevents invalid data errors through priority encoding

Why Priority?
A priority encoder ensures system reliability by handling multiple simultaneous key presses gracefully, registering only the highest priority (largest value) input.

2. Multiplexer

Component: Quad 4-to-1 Multiplexer (MUX)

Function:

• The system protects four different rooms (Room 0, Room 1, Room 2, Room 3)
• Each room has a unique, hard-wired 4-bit security code
• User selects the room using two toggle switches (S1, S0)
• The MUX takes the four hard-wired codes as inputs and outputs only the code corresponding to the selected room

Room Selection Logic:

S1	S0	Selected Room
0	0	Room 0
0	1	Room 1
1	0	Room 2
1	1	Room 3

3. Logic Gates (Comparator)

Components:

• Four 2-input XNOR gates
• One 4-input AND gate

Function: The comparator performs bit-by-bit comparison between the user's input code and the selected room's reference code:

• XNOR Gates: Act as "equivalence detectors" - output High (1) only when both inputs are identical
• 4-bit Comparison: Each XNOR gate compares one bit position
• AND Gate: Combines all XNOR outputs - outputs High only if ALL four bits match perfectly

Truth Table:

User Input:  1010
Reference:   1010
             ↓↓↓↓
XNOR Output: 1111
AND Output:  1 (Access Granted)

4. Decoder

Component: 2-to-4 Line Decoder

Function:

• Provides visual feedback to the user
• Takes the same two selection switches used by the Multiplexer
• Activates one of four output lines corresponding to the selected room
• Output lines can be connected to LEDs labeled "Bedroom," "Garage," "Kitchen," "Office," etc.
• Ensures the user knows exactly which door they are attempting to unlock

5. Demultiplexer (Enhancement)

Component: 1-to-4 Demultiplexer

Function: Routes the "Access Granted" signal to the specific door lock:

• Data Input: Single "Access Granted" output from the AND gate
• Selection Inputs: Same room selection switches (S1, S0)
• Operation: Routes the unlock signal exclusively to the output line corresponding to the selected room
• Benefit: Prevents entering the correct code for one room from accidentally unlocking another

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How It Works

Step-by-Step Operation

1. User Selection

   • User toggles selection switches (S1, S0) to choose a room
   • Example: Setting switches to 01 selects Room 1

2. Signal Routing

   • Switch signals travel simultaneously to:
     • Decoder: Activates LED for Room 1, confirming selection
     • Multiplexer: Selects "Code 1" (e.g., 1010) from hard-wired inputs

3. Data Entry

   • User enters a code on the keypad (e.g., presses '5')
   • Encoder converts decimal '5' into binary 0101

4. Authentication

   • Logic gates perform comparison:

     Input:     0101
     Reference: 1010
     XNOR:      0010 (bits don't match)
     AND:       0 (Access Denied)
     

   • If user enters correct code (1010):

     Input:     1010
     Reference: 1010
     XNOR:      1111 (all bits match)
     AND:       1 (Access Granted)
     

5. Door Unlock (with Demultiplexer)

   • When access is granted (AND output = 1):
   • Demultiplexer routes unlock signal to Room 1's solenoid lock only
   • Other rooms remain locked

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Files

File	Description
KeyPad.circ	Logisim-evolution circuit file containing the complete security system implementation
Keypad Design.png	Circuit diagram screenshot showing the complete system layout
Screenshots/	Directory containing additional screenshots of the circuit in operation
Keypad Project.lsebdl	Logisim project file

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Screenshots

Circuit Overview

Circuit in Operation

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Circuit Implementation

Opening the Circuit

1. Download and install Logisim-evolution
2. Open KeyPad.circ in Logisim-evolution
3. The circuit is organized into functional blocks:
   • Left side: Keypad buttons (0-9) feeding into Priority Encoder
   • Center: Multiplexer with hard-wired room codes
   • Right side: Comparator (XNOR + AND gates)
   • Top/Bottom: Decoder for visual feedback
   • Output: Demultiplexer for door control

Hard-Wired Security Codes

The system uses 4-bit codes for each room (example values):

• Room 0: 0000 (0)
• Room 1: 0011 (3)
• Room 2: 0101 (5)
• Room 3: 1001 (9)

Note: These values can be customized in the circuit by modifying the Constant components connected to the Multiplexer.

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Usage

Simulation Steps

1. Select Room

   • Toggle S0 and S1 switches to select desired room
   • Observe which LED lights up on the decoder output

2. Enter Code

   • Press the keypad button corresponding to the correct code for the selected room
   • The Priority Encoder converts the button press to BCD

3. Check Authentication

   • If code matches: AND gate outputs High (1), access granted
   • If code doesn't match: AND gate outputs Low (0), access denied

4. Door Control

   • With correct code: Demultiplexer routes unlock signal to selected room only

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Theory

Boolean Algebra Application

This system demonstrates practical application of Boolean algebra:

XNOR Operation (Equivalence):

A XNOR B = (A AND B) OR (NOT A AND NOT B)
Output is 1 when A = B

4-bit Comparison:

Access = (A0 XNOR B0) AND (A1 XNOR B1) AND (A2 XNOR B2) AND (A3 XNOR B3)

Where:

• A0-A3: Bits from user's keypad input (via Encoder)
• B0-B3: Bits from selected room's code (via MUX)

Combinational Logic

The entire system is a combinational circuit where:

• Outputs depend only on current inputs
• No memory elements (flip-flops) in the main logic path
• Immediate response to input changes

Priority Encoding

The Priority Encoder implements:

If multiple inputs active: Output = highest priority input
Priority: 9 > 8 > 7 > 6 > 5 > 4 > 3 > 2 > 1 > 0

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Technical Specifications

• Input: 10-digit keypad (0-9)
• Output: 4-bit BCD + Door unlock signal
• Logic Family: TTL (Transistor-Transistor Logic)
• Key ICs: 74LS147 (Encoder), 74LS153 (MUX), 74LS139 (Decoder)
• Simulation: Logisim-evolution v4.0.0

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References

Ndjountche, T. (2016). Digital electronics 1: Combinational logic circuits. John Wiley & Sons, Incorporated.

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Author

Eden Gilbert Kisekka