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Raspberry Pi 4 Temperature & Humidity Monitoring System (HTU21D)

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.

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.

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.

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

Hardware Connections

Pin Reference Images

Wiring Table

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

Deployment Guide (From Scratch)

1. Install Dependencies

sudo apt update
# Updates package list

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

2. Clone the Repository

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

3. Enable I2C

sudo raspi-config
# Opens Raspberry Pi configuration menu

Go to:

Interfacing Options → I2C → Enable

sudo reboot
# Restarts system to apply changes

4. Verify Sensor

i2cdetect -y 1
# Scans I2C bus for connected devices

Expected:

0x40

5. Compile the Sensor Program

cd sensors/HTU21D
# Move to sensor source code

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

6. Deploy Web Files

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

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

7. Set Permissions

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

8. Run Sensor Loop

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

9. Start Nginx

sudo systemctl start nginx
# Starts web server

sudo systemctl enable nginx
# Enables auto-start on boot

sudo systemctl status nginx
# Verifies server is running

10. Get Raspberry Pi IP

hostname -I
# Displays local IP address

Example:

192.168.1.100

11. Access from Another Computer

Open a browser and go to:

http://192.168.1.100

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