# Raspberry Pi 4 Temperature & Humidity Monitoring System (HTU21D)

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## Project Overview

The Temperature-Humidity-Sensor is the first module in a series of sensors for GNU/Linux intended for the construction of weather microstations that will be responsible for measuring air quality.
This module monitors temperature and humidity in real time using a Raspberry Pi 4 and an HTU21D sensor. The sensor reads the environmental data, sends it via I2C, processes it with a C program, and displays the results on a web dashboard using Nginx.

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## Objective

The objective is to learn about the development of drivers to establish communication between a sensor (hardware) responsible for collecting data and a set of programs (software) that process this data, and design a target compatible with a Raspberry Pi 4 to create weather microstations responsible for monitoring air quality.

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## System Architecture

The module is designed as a simple data channel where we connect the sensor HTU21D to a web interface for real-time data visualization.
As follows: 
    HTU21D Sensor → I2C → C Program → JSON File → Nginx → Web Browser

The sensor collects temperature and humidity data, sends it via the I2C Protocol to a Raspberry Pi 4, which reads and converts it into temperature and humidity values using a C program,then stores them in a JSON file. Nginx serves this file along with the web interface (HTML), where a JavaScript script fetches the data and updates the displayed values in real time.

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## Materials

### Hardware

* Raspberry Pi 4
* HTU21D sensor
* Jumper wires (female-to-female)
* MicroSD card with Raspberry Pi OS
* Power supply

### Software

* Raspberry Pi OS / Debian
* GCC compiler
* i2c-tools
* libi2c-dev
* Nginx
* Git

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## Hardware Connections

### Pin Reference Images

![Esquema de conexiones para la Raspberry Pi 4 y Sensor HTU21D](RaspberryPi4_Sensor_PinMap.jpg)

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### Wiring Table

| HTU21D | Raspberry Pi 4 |
| ------ | -------------- |
| VCC    | 3.3V           |
| GND    | GND            |
| SDA    | GPIO2 (SDA)    |
| SCL    | GPIO3 (SCL)    |

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## Deployment Guide (From Scratch)

### 1. Install Dependencies

```bash
sudo apt update
# Updates package list

sudo apt install gcc i2c-tools libi2c-dev nginx git
# Installs compiler, I2C tools, web server, and Git
```

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### 2. Clone the Repository

```bash
git clone https://gitea.itmorelia.com/Verano-Delfin-2026/Temperature-Humidity-Sensor.git
# Downloads the project from the server

cd Temperature-Humidity-Sensor
# Enters the project folder
```

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### 3. Enable I2C

```bash
sudo raspi-config
# Opens Raspberry Pi configuration menu
```

Go to:

```
Interfacing Options → I2C → Enable
```

```bash
sudo reboot
# Restarts system to apply changes
```

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### 4. Verify Sensor

```bash
i2cdetect -y 1
# Scans I2C bus for connected devices
```

Expected:

```
0x40
```

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### 5. Compile the Sensor Program

```bash
cd sensors/HTU21D
# Move to sensor source code

gcc main.c htu21d.c -o HTU21D -li2c
# Compiles the program and links I2C library
```

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### 6. Deploy Web Files

```bash
sudo cp -r ~/basic-ui-dashboard/* /var/www/html/
# Copies project files to Nginx directory
```

```bash
sudo mkdir -p /var/www/html/data
# Creates folder for JSON output
```

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### 7. Set Permissions

```bash
sudo chmod +x /var/www/html/sensors/HTU21D/HTU21D
# Allows execution of sensor program

sudo chown -R www-data:www-data /var/www/html
# Gives Nginx access to files
```

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### 8. Run Sensor Loop

```bash
cd /var/www/html
# Go to web directory

while true; do ./sensors/HTU21D/HTU21D > data/HTU21D.json; sleep 5; done
# Continuously updates JSON every 5 seconds
```

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### 9. Start Nginx

```bash
sudo systemctl start nginx
# Starts web server

sudo systemctl enable nginx
# Enables auto-start on boot

sudo systemctl status nginx
# Verifies server is running
```

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### 10. Get Raspberry Pi IP

```bash
hostname -I
# Displays local IP address
```

Example:

```
192.168.1.100
```

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### 11. Access from Another Computer

Open a browser and go to:

```
http://192.168.1.100
```

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### 12. Result

![Pagina resultante con las mediciones](PaginaResultado.png)

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