EnvMonDev Continuous Power

Environmental Monitoring Device

This project is an open source environmental monitoring device designed to measure and record CO₂ concentration, temperature and relative humidity. It uses a Sensirion SCD4x sensor and a Raspberry Pi Zero W to collect and process the data. This data is then stored locally on the device and can optionally be transmitted via MQTT.

Hardware Requirements

• Raspberry Pi Zero W
• Sensirion SCD4x sensor (SCD40 or SCD41)
• Tactile button
• Common cathode RGB LED

Wiring

The following connections are required for the environmental monitoring device to work. These can be made directly using a Raspberry Pi and the required components, without replicating the PCB.

The button uses the Raspberry Pi's internal pull-up resistor.

The RGB LED colour mapping is based on the actual LED connections rather than the internal pin order of the LED:

• GPIO14 (Pin 8) → Blue
• GPIO15 (Pin 10) → Red
• GPIO17 (Pin 11) → Green

PCB

A custom PCB was designed for this project to integrate the Raspberry Pi, SCD4x sensor, RTC, power management circuit, button and RGB LED. The PCB design files and schematic are included in the repository.

The custom PCB is not required to reproduce the device. If building the circuit without the PCB, follow the schematic for the required connections and components.

Installation

1. Clone the repository

git clone https://github.com/msbavaresco/Environmental-Monitoring-Device.git
cd Environmental-Monitoring-Device

2. Install required packages

sudo apt update
sudo apt full-upgrade
sudo apt install -y \
    build-essential \
    gcc \
    g++ \
    cmake \
    libgpiod-dev \
    i2c-tools \
    util-linux-extra \
    libpaho-mqttpp-dev \
    libpaho-mqtt-dev \
    nlohmann-json3-dev

3. Enable I2C on the Raspberry Pi

Run:

sudo raspi-config

Then go to:

Interface Options -> I2C -> Enable

Reboot:

sudo reboot

4. Set up the MCP7940N RTC

Check that hwclock is available:

command -v hwclock

You should see:

/usr/sbin/hwclock

Check that the RTC was detected:

sudo i2cdetect -y 1

The address for the MCP7940N should show up as:

0x6f

Enable the RTC kernel overlay by editing the Raspberry Pi boot configuration:

sudo nano /boot/firmware/config.txt

Add the following line at the end:

dtoverlay=i2c-rtc,mcp7940x

Save and exit nano:

Ctrl + O
Enter
Ctrl + X

Reboot:

sudo reboot

Confirm the RTC device exists:

ls /dev/rtc*

You should see:

/dev/rtc0

If the Raspberry Pi is connected to the internet, the system time should normally be set automatically using network time synchronisation:

date

If the date is wrong, set it manually (example):

sudo date -s "2026-09-10 09:50:17"

Write the system time to the RTC:

sudo hwclock -w

Check the RTC time is correct:

sudo hwclock -r

5. Create the configuration directory

sudo mkdir -p /var/lib/thisdevice

6. Copy the example configuration file

sudo cp Source/config/config.example.json /var/lib/thisdevice/config.json

7. Edit the configuration file

sudo nano /var/lib/thisdevice/config.json

Example:

{
  "mqtt": {
    "host": "localhost",
    "port": 1883,
    "username": "",
    "password": "",
    "base_topic": "envmon"
  },
  "device": {
    "location": "Office",
    "id": "Sensor1",
    "interval_minutes": 15
  }
}

8. Build the project

mkdir build
cd build
cmake ..
make

You should get:

envmonitor

9. Run the program

To be able to shut down the Pi when pressing the button, the program needs to be executed using sudo:

sudo ./envmonitor

Hosting an MQTT Server on the Raspberry Pi

If you want the MQTT server to be hosted on the same device, install the Mosquitto broker and clients:

sudo apt install mosquitto mosquitto-clients

Make Mosquitto start running when the Pi boots:

sudo systemctl enable --now mosquitto

Check the status:

sudo systemctl status mosquitto

It should show:

Active: active (running)

To verify it is working, open two command prompts. First, the subscriber:

mosquitto_sub -h localhost -t test

Then in another command prompt window, the publisher:

mosquitto_pub -h localhost -t test -m "Hello World"

To subscribe to the environmental monitoring device:

mosquitto_sub -h localhost -t "envmon/#"

To subscribe to all MQTT topics:

mosquitto_sub -h localhost -t "#"

Creating a systemd service

By creating a systemd service, the program runs in the background and starts running automatically once the Raspberry Pi boots.

After building the program, copy the executable to /usr/local/bin/:

sudo cp envmonitor /usr/local/bin/envmonitor

Create a new systemd service file:

sudo nano /etc/systemd/system/envmonitor.service

Add the following:

[Unit]
Description=Environmental Monitoring Device
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
ExecStart=/usr/local/bin/envmonitor

Restart=on-failure
RestartSec=5

User=root

[Install]
WantedBy=multi-user.target

Reload service configurations:

sudo systemctl daemon-reload

Enable the service so that it starts automatically when the Raspberry Pi boots:

sudo systemctl enable envmonitor.service

Start the program without needing to reboot:

sudo systemctl start envmonitor.service

Check if the program is running:

sudo systemctl status envmonitor.service

The service should show:

Active: active (running)

Reboot the Raspberry Pi:

sudo reboot

After the Pi starts, check the service status:

sudo systemctl status envmonitor.service

The service should show:

Active: active (running)

To view the program output via the system journal:

sudo journalctl -u envmonitor.service

Useful commands

Notes

• The software may need to be adapted if a different LED configuration or GPIO mapping is used.
• The configuration file must exist at /var/lib/ThisDevice/config.json.
• I²C must be enabled for the SCD4x sensor to work.
• Update the MQTT settings and device information in config.json before running the program.
• The envmonitor service runs as root to allow access to the required hardware interfaces and to perform system shutdown.