FlashNet

A simple, reliable system for transmitting digital data using screen flashing and photocell reception. Ideal for air-gapped communication, IoT sensor data collection, and educational demonstrations.

My product, the Halo Clock, uses this for configuration.

How It Works

FlashNet uses pulse width modulation over light signals to transmit binary data:

1. JavaScript transmitter flashes a screen element with precisely timed pulses
2. MicroPython receiver uses a photocell to detect light changes and decode data
3. Built-in error detection via checksum validation ensures data integrity

The protocol uses different pulse widths to represent sync signals, start markers, data bits (0/1), and transmission end markers.

Quick Start

Transmitter (JavaScript/Web)

// Flash "Hello" message to connected microcontroller
const data = [0x48, 0x65, 0x6C, 0x6C, 0x6F]; // ASCII bytes
const flashDiv = document.getElementById('transmitter');
flashnetSend(80, data, flashDiv); // 80ms pulse width

Receiver (MicroPython)

def handle_data(event, data):
    if event == 2:  # Valid packet received
        print("Received:", ''.join(chr(b) for b in data))

receiver = pc_receiver(handle_data)

# In main loop (call every ~10ms)
adc_value = photocell.read_u16()  # Read photocell
receiver.process(adc_value)

Use Cases

• Air-gapped communication - Send data to isolated systems
• IoT sensor networks - Collect data from battery-powered devices
• Educational projects - Demonstrate optical communication principles
• Backup data channels - Secondary communication path when RF is unavailable
• Screen-to-device configuration - Transfer settings/credentials via display

Technical Specifications

• Data rate: ~1 byte/second (depending on pulse timing)
• Range: Limited by photocell sensitivity and screen brightness (typically a few cm is ok)
• Error detection: 8-bit checksum validation
• Maximum packet: 70 bytes (configurable)
• Timing: 60-100ms pulses recommended for reliability

Hardware Requirements

Transmitter

• Any device with a web browser and display
• Smartphone, tablet, laptop, or desktop computer

Receiver

• Microcontroller with ADC input (Arduino, Raspberry Pi Pico, ESP32, etc.)
• Photocell/light-dependent resistor (LDR)
• Bias resistors (10kΩ typical)

Simple Wiring

  V++
   |
 10kΩ
   |
   +---- Photocell ---- + ---- ADC Pin
                        |
                       10kΩ
                        |
                       GND

Integration Guide

1. Set up hardware - Connect photocell to microcontroller ADC
2. Install receiver code - Copy pc_receiver.py to your MicroPython device
3. Create handler - Write function to process received data
4. Sample consistently - Call receiver.process() every 10ms with ADC readings
5. Add transmitter - Include JavaScript function in your web interface

Protocol Details

• Sync phase: 8 consistent pulses establish timing reference
• Start pulse: Longer pulse indicates data transmission begins
• Data encoding: Short pulse = 0 bit, medium pulse = 1 bit
• Checksum: Last byte validates entire packet
• Error handling: Invalid packets are flagged and can be retransmitted

Performance Tips

• Consistent timing - Sample photocell at regular intervals (timer interrupt recommended)
• Stable lighting - Minimize ambient light changes during transmission
• Signal strength - Position photocell close to display for best results
• Noise filtering - Built-in digital filtering handles minor interference

License

Apache 2.0 - Use freely in personal and commercial projects