Receiver board

Project Lux: A Free-Space-Optical Audio Link

A laser-based audio communication system built for the Howard University IEEE H.O.P.E. (Hands-On PCB Engineering) Project (Fall 2025).

🎯 Project Overview

Project Lux is a free-space optical (FSO) communication system that transmits a high-fidelity audio signal over a laser beam.

This project demonstrates the core principles of optical communication. The system consists of two main components:

1. The Transmitter: Modulates an audio signal from a source (like a phone or laptop) onto a laser beam.
2. The Receiver: Intercepts the laser beam, converts the light back into an electrical signal, and amplifies it to drive a speaker.

This creates a simple, point-to-point wireless audio link.

Key Features

• Audio Modulation: An analog circuit modulates a 3.5mm audio input signal onto a laser diode.
• Optical Detection: A photodiode array (BPW34) detects the modulated laser beam and converts it back into an electrical signal.
• Custom PCB: All transmitter and receiver electronics are designed on custom PCBs using KiCad.

🛰️ Transmitter Board Design

The transmitter board is the "voice" of the system. Its job is to take a standard audio signal and convert it into a light-based signal by modulating the brightness of a laser.

1. Stage 1: Audio Input & Biasing

• The Problem: A standard audio (sine) wave is AC, meaning it has a positive and a negative "half." Our single-supply (+5V) circuit can't process negative voltages.
• The Solution: A voltage divider (R1, R2) creates a 2.5V "virtual ground." An AC-coupling capacitor (C1) lets the audio signal "ride" on top of this 2.5V bias, shifting the entire wave into the positive (1.5V to 3.5V) range.

2. Stage 2: Voltage-to-Current Conversion (Transconductance)

• Modulation: This is the core of the transmitter. It takes the voltage signal and converts it into a proportional current to drive the laser. This is a transconductance stage.
• The Circuit: An op-amp (U1A) buffers the signal, which then drives a transistor (Q1). The transistor acts as a "valve," controlling the current flowing through the laser.
• Idle Current: A variable resistor (R6_Variable) is used to set the "quiescent" or "idle" current. This keeps the laser on at "half-brightness" when no audio is playing, allowing the signal to modulate both brighter and dimmer.

3. Stage 3: Laser Driver

• Output: The modulated current from Q1 drives the laser diode (LD1), causing its brightness to flicker in perfect sync with the audio waveform.

📡 Receiver Board Design

The receiver board is the "ear" of the system, designed to convert the incoming light signal back into audible sound. The signal path is a three-stage process.

1. Stage 1: TIA (Current-to-Voltage)

• Detection: An array of BPW34 photodiodes captures the modulated laser light, generating a tiny electrical current proportional to the light's brightness.
• Conversion: The first op-amp (U2A) is configured as a Transimpedance Amplifier (TIA). It converts this weak input current into a usable voltage signal.

2. Stage 2: Voltage Pre-Amplification

• Gain: The voltage from the TIA is fed into the second op-amp (U2B), which is configured as an inverting gain stage to boost the signal.

3. Stage 3: Power Amplification

• Coupling: A 1uF capacitor (C2) blocks the DC component of the signal, passing only the clean AC audio to the final stage.
• Power Amp: A PAM8403D Class-D audio amplifier takes the line-level signal and provides the necessary power to drive a standard 4S1 speaker.

Key Components

• Photodiodes: BPW34 (x3)
• Op-Amp (TIA & Pre-Amp): LM358
• Power Amplifier: PAM8403D

Want more detail?

Check out the Full Circuit Walkthrough for a component-by-component explanation of the schematics.

⚙️ Tech Stack & Components

Hardware

• Audio: 3.5mm Audio Jacks, PAM8403D Audio Amplifier, Laser Diode
• Sensors: BPW34 Photodiodes
• Circuits: Op-Amps (LM358), Transistors (2N3904), Passive Components

Software & Design

• PCB Design: KiCad
• Version Control: Git & GitHub

📂 Repository Structure

This repository is organized to keep our hardware designs, firmware code, and documentation separate and clean.

/
├── hardware/
│   ├── Receiver board/     # KiCad project files for the receiver
│   └── Transmitter board/  # KiCad project files for the transmitter
├── firmware/
│   └── ...                 # Arduino C++ source code (coming soon)
├── media/
│   └── pictures
|         └─...                 # Schematics, 3D renders, and demo videos
├── docs/
│   └── Walkthrough.md               # Datasheets, presentations, etc.
├── .gitignore
└── README.md

📖 Project Status

This project is currently in progress.

• Phase 1: Research & Reference (Cornell ECE 4760)
• Phase 2: Initial Circuit Design & Component Sourcing
• Phase 3: Receiver PCB Layout
• Phase 4: Transmitter PCB Layout
• Phase 5: Final Integration & Testing

👥 Team

• Abhishek Rana - Hardware Design (Receiver Board, Transmitter Board)
• Aayush Dahal - Hardware Design (Transmitter Board)
• Diego McCullough - Hardware Design (Power Board)

📜 Reference & Inspiration

This project is heavily inspired by the "Laser Audio Transmitter" final project from Cornell University's ECE 4760 course.

• Link: https://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2009/dyz2_jl589/dyz2_jl589/index.html

📄 License

This project is licensed under the MIT License. See the LICENSE file for full details.