Raspberry Pi Server¶

The Raspberry Pi acts as the central hub for the sensor network. It:

Broadcasts a WiFi access point (GasMonitor) that the sensors connect to
Provides time synchronisation to sensors on connection
Receives sensor readings via HTTP POST
Stores readings in a local InfluxDB instance
Forwards readings to Grafana Cloud

Hardware¶

Raspberry Pi 5 (2GB RAM tested; 4GB recommended for comfort)
microSD card for Pi OS (16GB minimum)
Ethernet cable (for network connection)
Power supply (official Pi 5 USB-C 27W recommended)

OS Setup¶

Flash Raspberry Pi OS Lite (64-bit) using Raspberry Pi Imager.

In the Imager settings before flashing:

Hostname: gasmonitor.local
Enable SSH
Set username and password

Boot the Pi and SSH in:

ssh pi@gasmonitor.local

Update:

sudo apt update && sudo apt upgrade -y

WiFi Access Point Setup¶

The Pi broadcasts GasMonitor on its wireless interface while using ethernet for university network connectivity.

⚠️ For most university networks it is not permitted to operate the Pi as an access point to the university. IPv4 forwarding must be disabled.

Disable IPv4 forwarding¶

Disable immediately:

sudo sysctl -w net.ipv4.ip_forward=0

Make persistent:

sudo nano /etc/sysctl.conf

Add:

net.ipv4.ip_forward=0

Apply:

sudo sysctl -p

Install packages¶

sudo apt install -y hostapd dnsmasq
sudo systemctl stop hostapd dnsmasq

Static IP for wlan0¶

sudo nano /etc/dhcpcd.conf

Add at the bottom:

interface wlan0
    static ip_address=192.168.50.1/24
    nohook wpa_supplicant

DHCP server (dnsmasq)¶

sudo mv /etc/dnsmasq.conf /etc/dnsmasq.conf.orig
sudo nano /etc/dnsmasq.conf

interface=wlan0
dhcp-range=192.168.50.10,192.168.50.50,255.255.255.0,24h
domain=local
address=/gasmonitor.local/192.168.50.1

Access point (hostapd)¶

sudo nano /etc/hostapd/hostapd.conf

interface=wlan0
driver=nl80211
ssid=GasMonitor
hw_mode=g
channel=7
wpa=2
wpa_passphrase=your set password here
wpa_key_mgmt=WPA-PSK
rsn_pairwise=CCMP

sudo nano /etc/default/hostapd
# Change: DAEMON_CONF="" → DAEMON_CONF="/etc/hostapd/hostapd.conf"

Enable and reboot¶

sudo systemctl unmask hostapd
sudo systemctl enable hostapd dnsmasq
sudo reboot

After reboot, GasMonitor WiFi should be visible. Connect your laptop to it and verify SSH still works at 192.168.50.1.

InfluxDB¶

wget -q https://repos.influxdata.com/influxdata-archive_compat.key
echo '393e8779c89ac8d958f81f942f9ad7fb82a25e133faddaf92e15b16e6ac9ce4c influxdata-archive_compat.key' | sha256sum -c
cat influxdata-archive_compat.key | gpg --dearmor | sudo tee /etc/apt/trusted.gpg.d/influxdata-archive_compat.gpg > /dev/null
echo 'deb [signed-by=/etc/apt/trusted.gpg.d/influxdata-archive_compat.gpg] https://repos.influxdata.com/debian stable main' | sudo tee /etc/apt/sources.list.d/influxdata.list
sudo apt update && sudo apt install -y influxdb2
sudo systemctl enable --now influxdb

Open http://192.168.50.1:8086 in a browser connected to GasMonitor WiFi and complete the setup wizard:

Organisation: lab
Bucket: sensors
Generate an All Access API token and save it — you'll need it below

Flask Receiver¶

Install dependencies¶

sudo apt install -y python3-flask
pip3 install influxdb-client requests --break-system-packages

Install server script¶

mkdir ~/gas_project
cp server.py ~/gas_project/
nano ~/gas_project/server.py
# Edit: paste your InfluxDB token into LOCAL_TOKEN
# Edit: paste your Grafana Cloud credentials into CLOUD_USER and CLOUD_TOKEN

Test¶

python3 ~/gas_project/server.py

From another terminal or device on the GasMonitor network:

curl -X POST http://192.168.50.1:5000/sensor-data \
  -H "Content-Type: application/json" \
  -d '{"sensor_id":"test","location":"test","co2":500}'

You should see {"status":"success"}.

Run as a system service¶

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

[Unit]
Description=Gas Sensor Data Receiver
After=network.target influxdb.service
Requires=influxdb.service

[Service]
ExecStart=/usr/bin/python3 /home/pi/gas_project/server.py
WorkingDirectory=/home/pi/gas_project
Restart=always
RestartSec=5
User=pi

[Install]
WantedBy=multi-user.target

sudo systemctl enable --now gasreceiver
sudo journalctl -u gasreceiver -f   # watch logs

Grafana¶

sudo mkdir -p /etc/apt/keyrings/
wget -q -O - https://apt.grafana.com/gpg.key | gpg --dearmor | sudo tee /etc/apt/keyrings/grafana.gpg > /dev/null
echo "deb [signed-by=/etc/apt/keyrings/grafana.gpg] https://apt.grafana.com stable main" | sudo tee /etc/apt/sources.list.d/grafana.list
sudo apt update && sudo apt install -y grafana
sudo systemctl enable --now grafana-server

Access local Grafana at http://192.168.50.1:3000 (default login: admin / admin).

Add InfluxDB as a data source:

Type: InfluxDB
Query language: Flux
URL: http://localhost:8086
Organisation: lab
Token: paste your InfluxDB token
Default bucket: sensors

Grafana Cloud (optional)¶

For remote access without a VPN, the server script forwards readings to Grafana Cloud using the InfluxDB line protocol over HTTPS. Configure CLOUD_URL, CLOUD_USER, and CLOUD_TOKEN in server.py with credentials from your Grafana Cloud stack.

API Endpoints¶

The receiver exposes the following HTTP endpoints:

Endpoint	Method	Description
`/sensor-data`	POST	Receive a reading from a sensor
`/time`	GET	Return current Unix timestamp (used by sensors for time sync)
`/health`	GET	Health check
`/sensors`	GET	List last reading from each sensor

POST /sensor-data payload¶

{
  "sensor_id": "chamber1",
  "location": "Growth Room A",
  "co2": 850,
  "o2": 20.91,
  "uptime": 3600,
  "timestamp": 1746540130
}

Fields o2, uptime, and timestamp are optional. If timestamp is provided and valid (>1700000000), it is used as the InfluxDB write time; otherwise the Pi's system time is used.

Troubleshooting¶

Problem	Action
`GasMonitor` WiFi not appearing	`sudo systemctl status hostapd` — check for errors
Sensors not connecting to Pi	Verify sensor WiFi credentials match `GasMonitor` / `your set password`
No data in InfluxDB	Check `sudo journalctl -u gasreceiver -f` for POST errors
Timestamps wrong in Grafana	Run `timedatectl` on Pi — NTP must be active and synced
Pi can't reach Grafana Cloud	Check ethernet connection: `curl https://grafana.net`