Calculator ver1

4-Bit Transistor Adder

A fully functional 4-bit binary adder built using discrete transistor-resistor logic and standard 74HC-series logic ICs. The project was developed from the initial logic design and transistor-level simulation to a custom two-layer PCB, PCB manufacturing, component assembly, and hardware testing. The final circuit accepts two 4-bit binary numbers, adds them together, converts the resulting 5-bit value into decimal digits, and displays the result on two 7-segment displays.

Overview

The main goal of this project was to design and build a physical digital calculator without relying on a microcontroller for the main arithmetic logic. The core 4-bit adder was developed using discrete transistor-resistor logic. The project includes:

• 4-bit binary addition
• Discrete transistor-based logic
• 5-bit binary result
• Binary-to-BCD conversion
• Two 7-segment displays
• CD4511 BCD-to-7-segment drivers
• Manual binary input using switches
• LED indicators for the binary result
• Custom two-layer PCB
• Complete KiCad schematic
• PCB design and Gerber manufacturing files
• Interactive Bill of Materials
• Falstad and Tinkercad simulations

Specifications

How It Works

Two 4-bit binary values are entered using eight switches:

A3 A2 A1 A0
B3 B2 B1 B0

The values are added together by the transistor-based logic, producing a 5-bit result:

S4 S3 S2 S1 S0

The maximum possible result is:

1111₂ + 1111₂ = 11110₂

which corresponds to:

30₁₀

The 5-bit result is then converted into BCD:

        5-bit Binary Result
                  │
                  ▼
          Binary-to-BCD
           Conversion
                  │
          ┌───────┴───────┐
          ▼               ▼
      Tens BCD         Ones BCD
          │               │
          ▼               ▼
       CD4511           CD4511
          │               │
          ▼               ▼
      7-Segment        7-Segment
       Display           Display

For example:

10101₂ = 21₁₀

The two displays therefore show:

21

Hardware

Final Board

PCB Design

The PCB was designed in KiCad as a two-layer board.

Front Side

Back Side

All Copper Layers

3D Model

Schematic

The complete schematic was designed in KiCad and contains the transistor-based adder, logic ICs, BCD conversion, display drivers, inputs, indicators, and power circuitry. A PDF version of the schematic is included in the schematic directory. Open Schematic PDF

Bill of Materials

An Interactive BOM generated from KiCad is included in the repository. It provides:

• Component references
• Component values
• Footprints
• Quantities
• Front and back PCB views
• Component locations Open Interactive BOM

Simulation

Simulation was an important part of the development process. Two different simulation environments were used for different stages of the project.

Falstad

Falstad was used to learn and develop the transistor-based logic from the ground up. The 4-bit adder was built step by step, starting from transistor-level digital logic and gradually combining the individual stages into a complete working 4-bit adder. A downloadable Falstad simulation file is also included in the repository. Open Simulation Documentation

Tinkercad

Tinkercad was used to verify the digital logic and binary-to-BCD conversion. The interactive Tinkercad simulation can be accessed through the simulation documentation.

Development Process

The project was developed through the following stages:

Concept
   ↓
Logic Design
   ↓
Transistor-Level Development
   ↓
Falstad Simulation
   ↓
Working 4-Bit Adder
   ↓
Tinkercad Simulation
   ↓
KiCad Schematic
   ↓
PCB Layout
   ↓
PCB Manufacturing
   ↓
Component Assembly
   ↓
Hardware Testing

The purpose of this workflow was to validate the logic progressively before moving to the final physical implementation.

Manufacturing Files

The repository contains the files required to reproduce the PCB. The pcb directory includes:

• KiCad PCB files
• Gerber manufacturing files The schematic directory contains the KiCad schematic and PDF documentation.

Repository Structure

4-Bit-Transistor-Adder/
│
├── 📁 bom/
│   └── Interactive BOM
│
├── 📁 images/
│   ├── Project photos
│   ├── PCB renders
│   ├── PCB layers
│   ├── Schematic images
│   └── Simulation screenshots
│
├── 📁 pcb/
│   ├── KiCad PCB files
│   └── Gerber files
│
├── 📁 schematic/
│   ├── KiCad schematic
│   └── Schematic PDF
│
├── 📁 simulation/
│   ├── README.md
│   └── Falstad simulation file
│
└── README.md

Tools Used

• KiCad — schematic and PCB design
• Falstad Circuit Simulator — transistor-level logic development
• Tinkercad Circuits — digital logic simulation