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

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## Project Overview

This project implements a real-time temperature and humidity monitoring system using a **Raspberry Pi 4** and an **HTU21D sensor**. The system reads environmental data via I2C, processes it using a C program, and displays the results on a web dashboard using Nginx.

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## System Architecture

The system is structured as a simple data pipeline that connects the sensor, processing logic, and web interface. Data flows continuously from the physical environment to the user in a clear and modular way.

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### Hardware and Communication Layer

The HTU21D sensor measures temperature and humidity and communicates with the Raspberry Pi using the I2C protocol. This allows the Raspberry Pi to request and receive raw data from the sensor.

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### Data Processing Layer

A C program interacts with the I2C interface to read the sensor data. It converts the raw binary values into human-readable units such as degrees Celsius and percentage humidity, and formats the result as JSON.

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### Data Storage Layer

The JSON file stores the latest sensor readings. It acts as an intermediate layer, separating the data acquisition process from the web interface.

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### Web Server Layer

Nginx serves the JSON file and the web page over HTTP. It allows other devices on the network to access the system through a browser.

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### Presentation Layer

The web page uses JavaScript to periodically request the JSON file and update the displayed values dynamically, enabling near real-time visualization.

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### Overall Flow

Data is captured by the sensor, processed by the C program, stored as JSON, served by Nginx, and displayed in the browser. This structure keeps the system simple, modular, and easy to maintain.

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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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## Real-Time Updates

JavaScript fetch:

```javascript
fetch('./data/HTU21D.json?nocache=' + Date.now())
```

**What it does:**

* Prevents browser cache
* Forces fresh data request


## Conclusion

This project integrates:

* Embedded systems (sensor + Raspberry Pi)
* C programming
* Linux system configuration
* Web development (Nginx + JS)

It demonstrates a complete pipeline from hardware acquisition to real-time web visualization.

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