Sensors DHT

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 if you want 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)

Image of DHT Connection

(From https://github.com/rnieva/Playing-with-Sensors---Raspberry-Pi)

Provision and Install

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.

Install the Prometheus Exporter as a Service

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.