Design and Implementation of an 8-Bit CPU Using Discrete BJTs and Logic ICs

Abstract

In today’s world of embedded systems, microprocessors and microcontrollers are widely used in nearly every electronic application. While these devices deliver high performance and compact designs, their internal structure often remains unclear to students and beginners. This means that learners frequently do not understand how a processor executes instructions at the hardware level. To tackle this issue, this paper outlines the design and implementation of an 8-bit Central Processing Unit (CPU) created using discrete Bipolar Junction Transistors (BJTs) and standard Logic Gate ICs. The proposed CPU design includes key processing units such as the Arithmetic Logic Unit (ALU), Program Counter (PC), Accumulator Register, Control Unit, and Clock Generator, all built from basic digital components like logic gates, flip-flops, and transistor-based circuits. A 555 timer IC is used as the system clock, enabling the processor to run at a low frequency. This allows for clear observation of individual instruction cycles. The CPU operates on a standard fetch, decode, execute cycle, processing simple instructions such as LOAD, ADD, SUBTRACT, JUMP, and HALT. This project offers a complete view of data paths, control signals, and timing relationships, helping learners connect theoretical ideas with real-world implementation. The system is affordable, works at safe voltage levels, and acts as a useful educational tool for understanding computer architecture and the basics of digital electronics.

Introduction

The text describes the design and educational purpose of building an 8-bit CPU using discrete electronic components (BJTs and ICs) to help students understand how a processor actually works at the hardware level.

Modern processors used in embedded systems are highly integrated and efficient, but they hide internal operations such as instruction decoding, register usage, and control logic. While platforms like Arduino and Raspberry Pi help in programming, they do not expose the internal CPU structure, making core concepts too abstract for learners.

To address this, the project proposes constructing a simple 8-bit CPU using basic components like BC547 transistors and logic ICs. The goal is to visually demonstrate the full fetch–decode–execute cycle, allowing students to directly observe how instructions are processed at the circuit level.

The literature review shows that similar educational efforts using transistor-based computers and simplified CPUs have helped improve understanding of digital logic, switching behavior, and processor design, reinforcing the value of hardware-level learning.

The problem statement highlights that existing educational tools rely too much on software simulations or highly abstracted hardware, preventing learners from seeing real transistor-level behavior such as switching delays and logic formation. Therefore, a low-cost, transparent CPU design is needed for hands-on learning.

Key CPU components described include:

• Instruction Decoder: Built using BC547 transistors configured as NAND, NOT, and buffer circuits to decode ROM-based instructions into control signals.
• Program Counter: Implemented using CD4013 D flip-flops configured as a 4-bit asynchronous counter, with preset and clear functions used for jump instructions.

Conclusion

The proposed 8-bit CPU using discrete BJTs and logic ICs successfully demonstrates the fundamental working of a processor at the hardware level. The system effectively performs basic operations and follows the fetch–decode–execute cycle. It provides clear visualization of data flow and control signals, enhancing understanding of digital electronics and computer architecture. This project serves as a low-cost and effective educational model, bridging the gap between theory and practical implementation.

References

[1] Global Science Network, “How to Build a 4-Bit Computer on Breadboards Using Individual Transistors,” YouTube video. Available: https://youtu.be/_eo8l7HP-9U [2] FULXOR Dominic Boeuf, “How I Built a CPU with Minimal Transistors: Fully Exploring Computer Logic,” YouTube video. Available: https://youtu.be/xupra9UPzQ0 [3] Big Chaks, “I Built a Computer From Scratch!,” YouTube video. Available: https://youtu.be/vgeDAoJcGaM [4] Vocademy, “What is Saturation?,” YouTube video. Available: https://youtu.be/rkVDwUaXQ4Y

Copyright

Copyright © 2026 Mrs. J. R. Khandare, Mr. Ajinkya Sanas, Mr. Varad Sagade, Mr. Omkar Tathe, Mr. Tanmay Agrawal. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.