LightBox — Intelligent Power Changeover System

An automated multi-source power management system for environments with unreliable mains electricity. LightBox manages three power sources (mains/NEPA, inverter, and generator), automatically switching between them based on availability and voltage quality, with remote monitoring and control via a mobile app.

System Architecture

LightBox uses a multi-node, multi-MCU architecture with three physically separate hardware nodes communicating wirelessly:

┌─────────────────────────┐     Serial2      ┌──────────────┐    ESP-NOW     ┌──────────────────┐
│   CHANGEOVER UNIT       │    (9600 baud)    │  ESP8266      │  (wireless)   │  GENERATOR NODE  │
│   Arduino Mega 2560     │◄────────────────►│  Bridge       │◄────────────►│  ESP8266          │
│                         │                   │               │               │                   │
│  • 3-ch AC voltage      │     Serial1       │  MAC: 32:AE:  │               │  • Servo (choke)  │
│  • CT clamp current     │    (9600 baud)    │  A4:07:0D:65  │               │  • 2x relay       │
│  • 3x MOSFET relays     │                   └──────────────┘               │    (ignition +    │
│  • SSD1306 OLED         │                                                   │     starter)      │
│  • Buzzer (fault alarm) │                                                   │  • AC presence    │
│                         │                                                   │  • Battery monitor│
│  ┌─────────────┐        │                                                   │                   │
│  │  SIM800L    │        │                                                   │  MAC: 32:AE:     │
│  │  GSM Modem  │        │                                                   │  A4:07:0D:66     │
│  └──────┬──────┘        │                                                   └──────────────────┘
└─────────┼───────────────┘
          │ GPRS
          ▼
   ┌──────────────┐         WebSocket        ┌──────────────┐
   │  HiveMQ      │◄──────────────────────►│  React Native │
   │  MQTT Broker │        (port 8000)       │  Mobile App   │
   │  (port 1883) │                          └──────────────┘
   └──────────────┘

Node 1: Changeover Controller (Arduino Mega 2560)

The central brain of the system. Reads AC voltages from all three power sources, measures current draw, controls relay switching, displays status on an OLED, and communicates with the GSM modem and ESP8266 bridge.

Pin	Function
A0	Mains (NEPA) AC voltage sense
A1	Inverter AC voltage sense
A7	Generator AC voltage sense
A2	Battery voltage (R1=10k, R2=965Ω)
A6	CT clamp current sensor
A3	Buzzer output (fault alarm)
D4	Inverter relay MOSFET
D5	Mains relay MOSFET
D6	Generator relay MOSFET
D8	Mode toggle button
D2	Auto/Manual interrupt
D3	Internet on/off interrupt

Node 2: ESP8266 Communication Bridge

Serial-to-ESP-NOW bridge. Receives commands from the Mega via UART and forwards them wirelessly to the generator node. Returns battery level and generator status data.

Node 3: Generator Starter Controller (ESP8266)

Physically controls the generator with a servo motor (choke), two relays (ignition + starter motor), and monitors AC presence and battery voltage.

Generator Start Sequence:

1. Servo → 120° (engage choke)
2. Ignition relay → HIGH
3. Starter relay → HIGH for 2–3s (crank)
4. Starter relay → LOW
5. Servo → 0° (disengage choke)
6. Check AC sensor — if LOW = success, else retry

Up to 4 retries with alternating choke positions and crank durations.

Communication Protocols

Protocol	Path	Details
Serial	Mega ↔ ESP bridge	Serial2, 9600 baud. "1" = gen on, "0" = gen off
Serial	Mega ↔ SIM800L	Serial1, 9600 baud. AT commands via TinyGSM
ESP-NOW	Bridge ↔ Generator node	Peer-to-peer WiFi, ~3.5s interval
MQTT	Device → Cloud	Publish to dan/in, subscribe to dan/out
GPRS	SIM800L → Internet	Airtel / MTN Nigeria APNs

MQTT Telemetry Payload (device → cloud)

{
  "Main": 220,
  "Inv": 215,
  "Gen": 0,
  "Curr": 2.5,
  "Mode": 1,
  "OpMode": 1,
  "BatV": 85,
  "GenV": 70
}

MQTT Command Payload (app → device)

{
  "phmode": 1,
  "phout": 2,
  "pshtdwn": 0
}

• phmode: 1 = auto, 0 = manual
• phout: 1 = mains, 2 = inverter, 3 = generator, 4 = off
• pshtdwn: 1 = shutdown

Sensing & Measurement

AC Voltage (3 channels)

Resistive voltage divider → ADC → calibration factor:

AcVoltOut = (analogRead(pin) * (4.9 / 1023)) * calibrationFactor;
// calibrationFactor: 93.67 (mains/inverter), 570.67–790.67 (generator)

• Available: ≥ 140V
• Safe range: 130V – 255V

Current

CT clamp via EmonLib:

emon1.current(6, 48.0);          // pin A6, calibration 48.0
Irms = emon1.calcIrms(1480);     // 1480 samples per reading
wattage = Irms * AcVoltage;

Battery Voltage

Resistive dividers on both nodes:

// Changeover unit (A2): R1=10k, R2=965Ω → 12V battery
input_voltage = (analogRead(A2) * 4.8 / 1023.0) / (r2 / (r1 + r2));

// Generator node (A0): R1=10k, R2=1060Ω → 20-sample moving average

Operating Modes

Auto Mode

Evaluates all 8 possible power source combinations (3 binary inputs) with priority logic:

Mains > Inverter > Generator

If no sources available → sends genOn command to start the generator.

Manual Mode

Long-press button cycles through: MAINS → INV → GEN → OFF (2-second hold intervals).

Remote Mode (MQTT)

Mobile app can override both modes via JSON commands.

Fault Detection & Safety

Fault	Threshold	Action
Under-voltage	< 130V	Isolate source, switch to alternate, buzzer alarm
Over-voltage	> 255V	Isolate source, switch to alternate, buzzer alarm
Overload	> 16,001W	All relays off (emergency shutdown), buzzer alarm
Low battery	< 11.5V	Save state to EEPROM, prepare for power loss
Gen low battery	< 3.2V	Cut power to generator node to preserve battery

Faults auto-clear when voltage returns to the safe range.

OLED Display Layout (128×64)

┌────────────────────────────────┐
│  MAIN:220V  INV:215V  GEN:0V  │  ← Active source inverted
│                                │
│       OUT: 1500W               │  ← Or "MAIN: FAULT" / "OVERLOAD!!"
│                                │
│  MODE: AUTO    BAT: 85%       │
│  GEN: 70%      G+  16dB      │  ← G=GPRS, G+=MQTT, NC=offline
└────────────────────────────────┘

State Persistence (EEPROM)

Address	Data
0	Output mode (1=Mains, 2=Inv, 3=Gen, 4=Off)
2	Operating mode (1=Auto, 2=Manual)

Read on startup via lastStateCheck() to restore previous state after power loss.

Mobile App (React Native)

• Real-time voltage, current, and wattage display
• Battery and generator fuel level indicators
• Auto/Manual mode toggle
• Source selection: Main, Inv, Gen, Idle, Shutdown
• MQTT over WebSocket (HiveMQ, port 8000)

Libraries

Library	Purpose
EmonLib	AC current measurement via CT clamp
Adafruit_SSD1306	128×64 I2C OLED display driver
Adafruit_GFX	Graphics primitives for OLED rendering
TinyGsmClient	GSM modem abstraction (SIM800L AT commands)
PubSubClient	MQTT publish/subscribe client
ArduinoJson	JSON serialization/deserialization
ESP8266WiFi + espnow	ESP-NOW peer-to-peer wireless protocol
Servo	Generator choke control
EEPROM	Non-volatile state persistence
react_native_mqtt	Paho MQTT client for mobile app

Repository Structure

├── Final_code/
│   ├── Thelightbox_final.ino    # Final changeover controller firmware
│   └── genStarter.ino           # Final generator starter firmware
├── CHANGE OVER V2/              # Changeover firmware iterations
├── GEN STARTER V2/              # Generator starter iterations
├── display/                     # OLED display + MQTT integration
├── espBridge/                   # ESP-NOW bridge firmware
├── app/                         # React Native mobile app
├── acVoltTest.ino               # AC voltage calibration test
├── currentTest.ino              # CT clamp calibration test
├── communicationTest.ino        # ESP-NOW protocol test
├── GSMCode.ino                  # SIM800L connectivity test
└── sudocode.txt                 # System design pseudocode

Video Demo