Disaster-Resilient LiFi Communication System

Off-grid emergency mesh network using solar street lamps and infrared communication. Built for CBSE Regional Science Exhibition 2025-26.

Problem

Power grids and mobile networks fail simultaneously during natural disasters. Communities lose contact with rescue teams when they need it most.

Solution

Retrofit solar street lamps with IR transceivers to create a disaster-resilient mesh network. Each lamp forwards emergency messages hop-by-hop using light, operates entirely off-grid, and routes SOS alerts to headquarters using gradient-based routing.

Protocol: LiFi (IEEE 802.11bgn standard, includes IR). IR chosen for mesh backbone due to lower power, no visible flicker, mature TSOP receivers, and built-in sunlight filtering via 38kHz AC modulation.

How It Works

Gradient Routing

The network uses distance gradients instead of routing tables. HQ sends INIT packets that propagate outward, building a hop-count map. Messages then flow "downhill" toward HQ using decreasing hop counts.

Example: Lamp4 (hop=5) → Lamp3 (hop=4) → Lamp2 (hop=3) → HQ (hop=0)

For detailed routing logic, topology examples, and protocol specifications, see project_report.pdf.

Message Types

Type	Direction	Purpose	Header Format
0	HQ → All	Build gradient map	[src][id][hop][0] (9 chars)
1	HQ → All	Broadcast alert	[src][dst][1][hash] (13 chars)
2	HQ → One	Targeted alert	[src][dst][2][hash] (13 chars)
3	Lamp → HQ	SOS emergency	[src][dst][3][hop] (11 chars)
4	Node → HQ	Status message	[src][dst][4][hash][hop] (15 chars)

Types 3 & 4 use gradient routing. Types 1 & 2 flood the network.

Reliability

• Deduplication cache: Prevents forwarding loops using (source, hash) pairs
• Automatic retransmission: 2-3 redundant sends in first minute, no ACKs needed
• Four-directional broadcast: Coverage regardless of lamp orientation

Hardware

IR Transmitter Circuit (per direction)

NodeMCU GPIO (D2/D3/D0/D7) 
    → 2.2kΩ resistor 
    → NPN transistor base (2N2222)
    → Collector connects to:
        → 2-3x IR LEDs (940nm) in series
        → 100Ω current-limiting resistor
        → 5V supply
    → Emitter to GND

Why 4 separate pins? Prevents exceeding single GPIO current limit (~12mA). Hardware can use omnidirectional setup with single amplifier.

IR Receiver Circuit

TSOP38238 IR Receiver
    VCC → 3.3V
    GND → GND
    OUT → NodeMCU GPIO (D5)

Why 38kHz? Low cost, faster data rates, readily available receivers, AC modulation filters DC sunlight interference.

SOS Button (Lamp nodes only)

Push button between GPIO (D6) and GND
Internal pull-up resistor enabled (INPUT_PULLUP)

Complete Node BOM

• NodeMCU ESP8266
• TSOP38238 IR receiver (38kHz)
• 2N2222 NPN transistor (×4 for 4 directions)
• IR LEDs 940nm (×8-12, 2-3 per direction)
• 100Ω resistor (×4 for LED current limiting)
• 2.2kΩ resistor (×4 for transistor base)
• 220Ω resistor (×1 for status LED)
• Push button (×1 for SOS)
• Breadboard + jumper wires
• Power: USB or 3.7V battery (solar in production)

Performance

• Demo range: ~20cm (tabletop exhibition)
• Production range: 50m+ with amplifiers and focusing lenses
• Latency: 1-2 seconds per hop
• Tested: 5-hop chains, stable indoor operation
• Protocol: NEC IR via IRremote library

Building It

1. Flash Node Firmware

# Install Arduino IDE + ESP8266 board support
# Install IRremote library

# Edit config.h for each node
#define NODE_ID "102a"        # Unique 4-char ID
#define HQ_ID "000h"          # Headquarters address

# Upload to ESP8266
arduino --upload main.ino --port /dev/ttyUSB0

2. Wire the Hardware

Follow circuit diagrams above. Use separate GPIO pins for each IR transmitter direction.

3. Start HQ Dashboard

cd src/hq/lifi\ hq/code
pip install flask flask-socketio pyserial
python app.py

Open http://localhost:5000

4. Run 3-Node Demo

1. Flash 3 ESP8266s with IDs: 000h (HQ), 101a, 102a
2. Connect HQ to computer via USB
3. Click "Connect Arduino" in dashboard
4. Send INIT|01 to build gradient map (wait 5-10s)
5. Press SOS button on 102a
6. Verify "SOS from 102a" in dashboard

Expected: Single hop 1-2s, 5-hop chain 5-10s, zero duplicates.

Current Status

• 3-5 node mesh with stable forwarding
• Gradient routing with automatic path selection
• SOS alerts route to HQ
• Real-time dashboard monitoring
• Four-directional IR broadcast

Protocol Limitations

• No encryption (add for production deployment)
• Basic hash integrity (not cryptographically secure)
• Manual INIT required to rebuild gradient after topology changes
• Sequential transmission (not concurrent)
• 5-node maximum tested

Code Structure

├── src/
│   ├── hq/                      # Headquarters node
│   │   ├── arduino/main/        # ESP8266 firmware
│   │   └── code/                # Flask dashboard
│   └── lamp/                    # Lamp node firmware
└── structure/v3/upg/            # Latest protocol version
    ├── config.h                 # Pin assignments, timing, node ID
    ├── ir.h                     # IR communication layer
    ├── lifi.h                   # Routing and protocol logic
    └── main.ino                 # Main event loop

config.h → Change node ID, pins, timing constants, protocol parameters
ir.h → Handles NEC IR protocol (swap this file to use LoRa, RF, or other physical layers)
lifi.h → Gradient routing, message processing, caching
main.ino → Button handling, message forwarding, retransmission

To use different communication technology: Replace ir.h with your implementation. Keep function signatures: irInit(), irSendString(), irReceiveString(). Protocol layer stays the same.

Full Documentation

See project_report.pdf for complete technical documentation, circuit details, testing methodology, and references.