🔔 Raspberry Pi LDR Blink Detector with MQTT (C# / Mono)

This project detects light pulses (e.g. from a blinking doorbell LED) using a light-dependent resistor (LDR) connected to a GPIO pin on a Raspberry Pi.
When a light pulse is detected, a message is sent via MQTT.

It runs as a background service using C# with Mono, as .NET (Core) isn't supported on Raspberry Pi 1 / Zero (armv6).

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⚙️ Requirements

• Raspberry Pi (Zero, Zero 2 W, 3, 4, etc.)
• Raspbian / Raspberry Pi OS (i am using dietpi)
• Mono (runtime for C#) and dependencies
• LDR (light-dependent resistor)
• MQTT broker (local or remote)

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📦 Installation (Mono Runtime)

Install Mono and libgpiod:

sudo apt update
sudo apt install mono-complete libgpiod2 libgpiod-dev

mono-complete includes the full Mono runtime and development tools.

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🔌 Hardware Wiring

LDR Pin	Raspberry Pi Pin
One side	GPIO18 (BCM) – Physical Pin 12
Other side	GND (e.g. Pin 6)

You can use the internal pull-up resistor in software — no extra hardware required.

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📁 Configuration (appsettings.json)

{
  "Gpio": {
    "LdrPin": 18
  },
  "MQTT": {
    "Server": "mqtt.example.com",
    "Port": 1883,
    "ClientId": "ldr-sensor-01",
    "User": "myMqttUser",
    "Value": "myMqttPassword",
    "Topic": "sensors/ldr/blink"
  }
}

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🧠 What It Does

• Initializes a GPIO pin as input with pull-up
• Monitors the pin for a light-triggered LOW signal
• When light is detected, sends an MQTT message to the configured topic

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🚀 How to Run (via Mono)

1. Build your C# project targeting .NET Framework 4.8 (or similar)
2. Copy it to the Raspberry Pi
3. Run the app:

sudo mono MyApp.exe

sudo is required for GPIO access.

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📌 Notes

• Make sure your user has access to /dev/gpiomem or run with sudo
• Debounce logic is included to avoid spamming messages
• You can expand this to detect longer pulses or multiple brightness levels

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📡 Home Assistant MQTT Auto-Discovery

This application supports Home Assistant MQTT auto-discovery, allowing it to register itself as a binary sensor and appear automatically in the Home Assistant UI.

🔧 Configuration Requirements

To enable auto-discovery, the following section must be included in your appsettings.json:

"MQTT": {
"AutoDiscovery": {
"Enabled": true,
"Topic": "homeassistant/binary_sensor/doorbell_ldr/config",
"ConfigPackage": {
"name": "Doorbell LDR",
"unique_id": "ldr_doorbell_sensor_01",
"state_topic": "sensors/doorbell/ldr",
"device_class": "light",
"payload_on": "ON",
"payload_off": "OFF",
"availability_topic": "sensors/doorbell/ldr/available",
"device": {
"identifiers": ["ldr_doorbell_device_01"],
"name": "Raspberry Pi LDR Sensor",
"manufacturer": "Custom",
"model": "Pi-LDR-MQTT",
"sw_version": "1.0.0"
}
}
}
}

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🚀 When is the discovery config sent?

The discovery config is sent:

• On MQTT connection or reconnection
• If AutoDiscovery:Enabled is true
• With the retain flag so Home Assistant keeps it even after restarts

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📬 Sensor State Updates

This service publishes to:

• sensors/doorbell/ldr with payload:
  • "ON" when a light impulse is detected
  • "OFF" when the light is no longer detected (optional or after timeout)

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🧰 Running as a systemd Service (Root)

To automatically start the LDR blink detector on boot and keep it running in the background, you can install it as a systemd service.

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✅ 1. Create the service file

Create a file called:

/etc/systemd/system/ldr-service.service

Paste the following content:

[Unit]
Description = Raspberry Pi LDR Blink Detector (.NET/Mono)
After = network.target

[Service]
Type = simple
ExecStart = /usr/bin/mono /root/RaspiLDR2MQTT/RaspiLDR2MQTT.exe
WorkingDirectory = /root/RaspiLDR2MQTT
Restart = on-failure
RestartSec = 5
User = root
Environment = DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1

[Install]
WantedBy = multi-user.target

Make sure the path to the .exe and working directory is correct.

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⚙️ 2. Enable and start the service

sudo systemctl daemon-reload
sudo systemctl enable ldr-service.service
sudo systemctl start ldr-service.service

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📄 3. Check service status and logs

sudo systemctl status ldr-service.service

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🧽 4. (Optional) Stop and disable

To stop and remove the service:

sudo systemctl stop ldr-service.service
sudo systemctl disable ldr-service.service

📄 License

MIT – free to use and modify.

🙏 Acknowledgements

• ChatGPT for the README.md generation
• Awesome Raspi Pinout at https://pinout.xyz/