Internet of Things: LoRa DHT22 Temperature & Humidity Sensor on Breadboard

Continuing with my mission to convert my ESP8266 based sensors into LoRaWAN enabled sensors I pushed on with getting a DHT22 on breadboard and getting the code written to submit temperature and humidity over LoRa.

If you’d like to replicate this mini project you’ll need the following:

  • Arduino Uno or equivalent
  • SX1272 / SX1276 LoRa module
  • DHT22
  • Breadboard for hooking everything up
  • Miscellaneous electronics stuff such as jumper cables, stuff you’d find in your tool box.
  • IBM’s LMIC & Adafruit’s DHT22 Arduino Libraries (both can be installed via manage libraries in Arduino IDE)

First I hooked up the SX1276 module to my Arduino Uno as per my previous post, and connected the DHT22 to 5v VCC, ground and pin 7 on the relevant DHT22 pins. The pins on the DHT22 can be identified by looking at the meshed plastic side of the device, the pin on the left is VCC followed by data, not connected and ground.

Next I moved onto the code, I started by merging the DHT22 and LMIC samples together however upon uploading the sketch it became apparent the sketch was too big for the 32kb of the ATMega328-PU, so I continued by shrinking the code by removing some of the serial output and removing unused remnants of the LMIC example, this allowed me to fit the sketch on the Arduino (just!) but also made debugging super hard so I turned to TheThingsNetwork Slack channel to discuss an alternative strategy.

As a result Matthijs Kooijman came forward with his fork of LMIC which included the ability to use a more lightweight AES encryption framework which although slower was 8kb smaller. To replace the original LMIC from IBM installed by the Arduino IDE download Matthijs’s fork from and copy and overwrite the files in your Arduino library’s LMIC folder. Now when compiling the sketch it came to a much more acceptable 23kb leaving plenty of room to add back in the debug serial prints.

Here is the results of my efforts… A fully working DHT22 and LMIC combo which periodically transmits temperature and humidity readings over LoRa, remember before using this code to set your network key, application key and device id prior to uploading to your board.

Download DHT22 & LMIC Arduino Sketch

Now to see it in action! With everything connected up I placed the sensor in range of a TTN LoRa Gateway and powered it up. Now to check TheThingsNetwork and see what kind of data is coming from the device...

Ok, so you are probably thinking wtf is that payload. As you know with LoRa you need to keep the payload as small as possible to reduce airtime and hence reduce duty cycle (a legal restriction in the EU), I decided upon having only one decimal place within my measurements so what you see here is the hex of the temperature and humidty float appended to each other, 31 34 2E 30 is the temperature and 35 34 2E 37 is the humidity.

Lets convert these into something human readable using Python:

Python 3.5.2 (v3.5.2:4def2a2901a5, Jun 26 2016, 10:47:25)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import binascii
>>> binascii.unhexlify('31342e30')
>>> binascii.unhexlify('35342e37')

OK looks like the temperature is 14°C and relative humidity is 54.7%, sounds about right considering the device in next to an open window in the shade and given the current weather today.

Next steps will involve moving the electronics on to a more permanent board (probably veroboard / stripboard), mounting the device in a small enclosure and subscribing to the TTN’s MQTT and publishing the results to a fancy dashboard.

By @Robert Putt in
Tags : #internet, #iot, #electronics, #technology,