Project Overview
The "Photon-Link Transceiver" is a dual-node Free-Space Optical (FSO) communication system. While modern Wi-Fi uses radio waves, this project explores the principles of Li-Fi (Light Fidelity), using focused laser bursts to transmit ASCII-encoded text across a line-of-sight path. By implementing a custom Morse-to-ASCII translation engine, two Arduino-powered nodes can engage in a fully bi-directional (half-duplex) text conversation, demonstrating the fundamentals of Optical Modulation and Signal Demodulation.
Technical Deep-Dive
- The Transmission Layer (OOK Modulation):
- On-Off Keying (OOK): The laser diode is modulated using a binary protocol. To send data, the Arduino switches the laser between saturation (ON) and cutoff (OFF) at precise intervals.
- Timing Forensics: The system utilizes a tiered timing structure:
- Dot (32ms): The fundamental unit.
- Dash (64ms): Exactly double the duration of a dot.
- Underscore (96ms): Triple duration for extended character mapping.
- The Reception Logic (Analog Sensing):
- Photometric Detection: The receiver utilizes an LDR in a voltage divider. When the laser hits the sensor, its resistance drops, causing the voltage at the analog pin to spike above a software-defined "Brightness Threshold."
- Pulse Width Forensics: The firmware doesn't just check for light; it measures the duration of the light event. Using
millis()-based non-blocking logic, the receiver distinguishes between a transient flicker and a valid Morse element, incorporating a 5ms buffer to ignore atmospheric jitter.
- Protocol & Encoding:
- ASCII Mapping: The software features a unique 96-character array. By subtracting 32 from any incoming ASCII character code (the offset for printable characters starting at "Space"), the Arduino can instantly index into a Morse lookup table, allowing for 1:1 translation of complex sentences.
Engineering & Implementation
- Signal Integrity & SNR:
- Noise Floor Calibration: In an indoor environment, background lighting can interfere with the LDR. The project includes an "Alignment Phase" where the laser is held ON, allowing the user to view the real-time ADC value and set a dynamic threshold that maximizes the Signal-to-Noise Ratio (SNR).
- Half-Duplex Flow Control:
- To prevent both pagers from transmitting simultaneously (collision), the protocol utilizes a serial-to-laser buffer. The sender "locks" the line by transmitting their Username Strings, effectively establishing a temporary master-slave relationship for the duration of the message packet.
- Aerodynamic Alignment:
- FSO systems are highly sensitive to physical displacement. The engineering of the breadboard mount is critical; tethering the components ensures that the narrow beam of the 655nm laser remains centered on the 5mm LDR target even during slight table vibrations.
Conclusion
Photon-Link provides a tangible look at the future of secure, line-of-sight communication. By bypassing the congested RF spectrum, it offers a fascinating study in the physics of light-based data transmission.