The goal of this Hackaton is to become familiar with several LinkSmart components and at the same time, create something useful.

In the following steps we will setup the DHT Adafruit Library and LinkSmart Device Gateway on a Raspberry Pi 3 or ZeroW in order to read measurements from a DHT22 sensor and publish them in SenML format to a MQTT broker.

Setup the DHT Library

1. Install the latest stable Docker for Raspbian / Raspberry Pi OS (armhf).

Follow the instructions here (Tip in 2020: use the convenience script).

2. Create and enter the directory structure

mkdir -p /home/pi/dgw
cd /home/pi/dgw

3. Download Adafruit_DHT_SenML.py.

This script reads temperature and humidity values from a DHT sensor and prints the measurements in SenML format. (You need appropriate drivers to run it)

wget https://raw.githubusercontent.com/linksmart/linksmart-iot-hackatons/master/iot-hackaton-2018-part1/Adafruit_DHT_SenML.py

4. Download an image with DHT drivers and runs it once.

The command tries to read from a DHT22 sensor with data pin connected to GPIO 4 and SenML basename bn/. Remove these arguments to see usage instructions.

docker run --privileged -v $(pwd):/home --entrypoint=python --rm farshidtz/adafruit_dht Adafruit_DHT_SenML.py 22 4 bn/

The SenML output should be similar to:

[{"bt": 1597584431.476398, "bn": "bn/", "v": 26.9, "n": "temp"}, {"v": 84.7, "n": "humi"}]

If you didn’t get an output similar to that, go back and figure out what went wrong.

Deploy Device Gateway

In the following steps, we will use these placeholders:

• <mqtt-broker-uri>: The URI of an accessible MQTT Broker
• <device-name>: A unique name for the device where you deploy the Device Gateway
• <hostname>: The hostname of the device. This could be same as device-name

We’ll use the DHT library container (from previous step) to run Device Gateway. The goal is to execute the Python script that reads the measurements and expose the SenML output over networking protocols.

1. Download Device Gateway

Download the latest v1.x.x device-gateway-linux-arm and make it executable. Deployment instructions on wiki.

TLDR? For v1.2.2 (latest stable at the time of writing):

cd /home/pi/dgw
wget https://github.com/linksmart/device-gateway/releases/download/v1.2.2/device-gateway-linux-arm
chmod +x device-gateway-linux-arm

2. Configure Device Gateway:

1. Configure the DGW service. Modify the following JSON object and place it in /home/pi/dgw/conf/device-gateway.json.

Replace with the device name. E.g.: linksmart-cyan

Replace with the broker endpoint. E.g.: ssl://demo.linksmart.eu:8883

{
  "id": "",
  "description": "Example Device Gateway",
  "publicEndpoint": "http://fqdn-of-the-host:8080",
  "http": {
    "bindAddr": "0.0.0.0",
    "bindPort": 8080
  },
  "protocols": {
    "REST": {
      "location": "/rest"
    },
    "MQTT": {
      "url": "<mqtt-broker-uri>",
      "prefix": "<device-name>-dgw-",
      "offlineBuffer": 100
    }
  }
}

1. Configure the device agent. Modify the following JSON object and place in /home/pi/dgw/conf/devices/dht22.json.

Replace all s with the device name. E.g.: linksmart-cyan

With the following configuration, Device Gateway executes the Python script every 120 seconds and exposes the resulting data over two protocols:

• MQTT: Publishes the sensor data to the given topic. The MQTT broker was configured in Device Gateway configuration step.

• REST: Exposes an HTTP endpoint to GET the latest collected data. The HTTP server was configured in Device Gateway configuration step. The endpoint will be constructed as <server-address>/<rest-location>/<sensor-device-name>/<resource-name>. In this example, it will be http://:8080/rest/dht22/measurements.

{
  "name": "dht22",
  "description": "This sensor measures Temperature and Humidity.",
  "resources": [
    {
      "type": "Resource",
      "name": "measurements",
      "agent": {
        "type": "timer",
        "interval": 120,
        "dir": null,
        "exec": "python /home/dgw/Adafruit_DHT_SenML.py 22 4 <device-name>/"
      },
      "protocols": [
        {
          "type": "MQTT",
          "methods": [
                "PUB"
          ],
          "pub_topic": "LS/v2/DGW/<device-name>/senml",
          "pub_retained": false
        },
        {
          "type": "REST",
          "methods": [
            "GET"
          ],
          "content-types": [
            "application/senml+json"
          ]
        }
      ]
    }
  ]
}

3. Run the container:

It should be in priviledged mode in order to access Raspberry Pi GPIO.

cd /home/pi/dgw
docker run --privileged -v $(pwd):/home/dgw --entrypoint=/home/dgw/device-gateway-linux-arm --rm farshidtz/adafruit_dht --conf /home/dgw/conf/device-gateway.json

If there were no errors, make a container that starts after boot and runs in detached mode (background):

docker run --privileged -v $(pwd):/home/dgw --entrypoint=/home/dgw/device-gateway-linux-arm -p 8080:8080 --name dgw_dht --restart=unless-stopped --log-opt max-size=10m -d farshidtz/adafruit_dht --conf /home/dgw/conf/device-gateway.json

Refer to docker run reference to understand the given arguments.

Try it out

• Subscribe to the correct topic at the broker with the endpoint configured in DGW configuration.
• Get latest measurement from the REST endpoint. The path comes from the names in device agent configuration. e.g. http://localhost:8080/rest/dht22/measurements

Continue with other hackathons

By Farshid Tavakolizadeh