Custom Sensor
You can also monitor things that don’t have an exporter by writing it yourself. Here’s an example for an environmental DHT sensors.
DHT stands for Digital Humidity and Temperature. At less than $5 they are cheap and can be hooked to a Raspberry Pi easily. Add a Prometheus exporter to do at scale.
- Connect the Senor
- Provision and Install the Python Libraries
- Test the Libraries and the Sensor
- Install the Prometheus Exporter as a Service
- Create a Service Account
- Add to Prometheus
Connect The Sensor
These usually come as a breakout-board with three leads you can connect to the Raspberry PI GPIO pins as follows;
- Positive lead to pin 1 (power)
- Negative lead to pin 6 (ground)
- Middle or ‘out’ lead to pin 7 (that’s GPIO 4)
(From https://github.com/rnieva/Playing-with-Sensors---Raspberry-Pi)
Provision and Install Libraries
Use the Raspberry Pi Imager to Provision the Pi with Raspberry PI OS Lite 64 bit. Next, install the “adafruit_blinka” library as adapted from their instructions and test.
# General updates
sudo apt update
sudo apt -y upgrade
sudo apt -y autoremove
sudo reboot
# These python components may already be installed, but making sure
sudo apt -y install python3-pip
sudo apt -y install --upgrade python3-setuptools
sudo apt -y install python3-venv
# Make a virtual environment for the python process
sudo mkdir /usr/local/bin/sensor-dht
sudo python3 -m venv /usr/local/bin/sensor-dht --system-site-packages
cd /usr/local/bin/sensor-dht
sudo chown -R ${USER}:${USER} .
source bin/activate
# Build and install the library
pip3 install --upgrade adafruit-python-shell
wget https://raw.githubusercontent.com/adafruit/Raspberry-Pi-Installer-Scripts/master/raspi-blinka.py
sudo -E env PATH=$PATH python3 raspi-blinka.py
Test the Libraries and the Sensor
After logging back in, test the blinka lib.
cd /usr/local/bin/sensor-dht
source bin/activate
wget https://learn.adafruit.com/elements/2993427/download -O blinkatest.py
python3 blinkatest.py
Then install the DHT library from CircuitPython and create a script to test the sensor.
cd /usr/local/bin/sensor-dht
source bin/activate
pip3 install adafruit-circuitpython-dht
vi sensortest.py
import board
import adafruit_dht
dhtDevice = adafruit_dht.DHT11(board.D4)
temp = dhtDevice.temperature
humidity = dhtDevice.humidity
print(
"Temp: {:.1f} C Humidity: {}% ".format(temp, humidity)
)
dhtDevice.exit()
You can get occasional errors like RuntimeError: Checksum did not validate. Try again. that are safe to ignore. These DHTs are not 100% solid.
Create an Export Script
Add the Prometheus pips.
cd /usr/local/bin/sensor-dht
source bin/activate
pip3 install prometheus_client
And create a script like this.
nano sensor.py
import board
import adafruit_dht
import time
from prometheus_client import start_http_server, Gauge
dhtDevice = adafruit_dht.DHT11(board.D4)
temperature_gauge= Gauge('dht_temperature', 'Local temperature')
humidity_gauge = Gauge('dht_humidity', 'Local humidity')
start_http_server(8000)
while True:
try:
temperature = dhtDevice.temperature
temperature_gauge.set(temperature)
humidity = dhtDevice.humidity
humidity_gauge.set(humidity)
except:
# Errors happen fairly often as DHT's are hard to read. Just continue on.
continue
finally:
time.sleep(60)
Create a service
sudo nano /lib/systemd/system/sensor.service
[Unit]
Description=Temperature and Humidity Sensing Service
After=network.target
[Service]
Type=idle
Restart=on-failure
User=root
ExecStart=/bin/bash -c 'cd /usr/local/bin/sensor-dht && source bin/activate && python sensor.py'
[Install]
WantedBy=multi-user.target
Enable and start it
sudo systemctl enable --now sensor.service
curl http://localhost:8000/metrics
Create a Service Account
This service is running as root. You should consider creating a sensor account.
sudo useradd --home-dir /usr/local/bin/sensor-dht --system --shell /usr/sbin/nologin --comment "Sensor Service" sensor
sudo usermod -aG gpio sensor
sudo systemctl stop sensor.service
sudo chown -R sensor:sensor /usr/local/bin/sensor-dht
sudo sed -i 's/User=root/User=sensor/' /lib/systemd/system/sensor.service
sudo systemctl daemon-reload
sudo systemctl start sensor.service
Add to Prometheus
Adding it requires logging into your Prometheus server and adding a job like below.
sudo vi /etc/prometheus/prometheus.yml
...
...
- job_name: 'dht'
static_configs:
- targets:
- 192.168.1.45:8000
You will be able to find the node in your server at http://YOUR-SERVER:9090/targets?search=#pool-dht and data will show up with a leading dht_....
Sources
https://randomnerdtutorials.com/raspberry-pi-dht11-dht22-python/
You may want to raise errors to the log as in the above source.
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