MQTT refresher

The easiest way to observe TTN sensors is MQTT because it doesn’t requires any additional configuration. To check what your devices are sending, subscribe to the topic +/devices/+/up with a command-line tool like mosquitto_sub. The <Region> is the postfix of the Handler entry in your Application overview. <AppID> is the Application ID and <AppKey> is your access key. E.g., for a device called device01:

$ mosquitto_sub -v -h <Region>.thethings.network -t '+/devices/device01/up' -u '<AppID>' -P '<AppKey>'
{
	"app_id": "demo",
	"dev_id": "device01",
	"hardware_serial": "AJDJENDNHRBFBBT",
	"port": 1,
    [...]

The payload contains details about the device itself and the sensor data. The sensor data is stored in `payload_fields`. Depending on the device configuration it may contain a single value or multiple values.

Hardware setup

To connect your display to your development board, use some cables. A mini breadboard can help. There are many different displays and boards available and the labelling may vary. On a LoLin board the connections were made as shown in the table below:

Check the voltage of your display before connecting it. 

Software setup

Follow Micropython's instructions for installation and the setup. Make sure that you are able to use the WebREPL to upload files to your board. Use a boot.py file to enable the connection to your local WiFi network. 

The file main.py will contain all code to consume MQTT messages and show a value. As shown before you will need <Region>, <AppID>, <AppKey> and <DeviceID>. Also, set NAME to the key for the sensor data which is stored in payload_fields. The communication with the display is done over I2C. For further details see the documentation about umqtt and I2C.

import json
import machine
import ssd1306
import time

from umqtt.simple import MQTTClient

DEVICE_NAME = '<DeviceID>'

TTN_BROKER = '<Region>.thethings.network'
TTN_USERNAME = '<AppID>'
TTN_PASSWORD = '<AppKey>'
TTN_TOPIC = '+/devices/{}/up'.format(DEVICE_NAME)
NAME = 'Temperature'

i2c = machine.I2C(scl=machine.Pin(5), sda=machine.Pin(4))
oled = ssd1306.SSD1306_I2C(128, 32, i2c)

def sub_callback(topic, msg):
    oled.fill(0)
    oled.text(NAME, 25, 5)
    oled.text(str(json.loads(msg)['payload_fields'][NAME.lower()]), 25, 20)
    oled.show()

def main():
    c = MQTTClient(
        'ttn_client', TTN_BROKER, user=TTN_USERNAME, password=TTN_PASSWORD)
    c.set_callback(sub_callback)
    c.connect()
    c.subscribe(TTN_TOPIC)
    while True:
        if True:
            c.wait_msg()
        else:
            c.check_msg()
            time.sleep(10)

    c.disconnect()

if __name__ == "__main__":
    main()

Save this and upload the file. After a reboot of the ESP8266 it could take some time till you get the first MQTT message. But as soon you get one, it will be shown on the display.