IoT temperature sensor with ESP8266 using LWM2M (Eclipse Wakaama)

The sensor uses an ESP8266 board with the protocol Lightweight M2M to communicate the temperature read by DS18B20 to the gateway. The sources of the project are available on GitHub: https://github.com/brusdev/smart-solar-heating-system/tree/master/sensor

ESP8266

The ESP8266 is a low-cost Wi-Fi microchip with full TCP/IP stack and microcontroller capability produced by Espressif Systems. The avalability of same SDK allows the chip to be programmed, removing the need for a separate microcontroller. To program the microchip i use the SDK Arduino core for ESP8266 WiFi chip and PlatformIO, that is an open source ecosystem for IoT development.

LwM2M

Lightweight M2M is a protocol from the Open Mobile Alliance for M2M or IoT device management and communication. It uses CoAP, a light and compact protocol with an efficient resource data model, for the application layer communication between LWM2M Servers and LWM2M Clients.

Each service on a constrained device/sensor/actor is modelled as an LWM2M object instance. An object has a unique identifier and may have instances. Some LWM2M object identifiers are standardised via the OMA Object & Resource Registry (http://technical.openmobilealliance.org/Technical/technical-information/omna/lightweight-m2m-lwm2m-object-registry).

libWakaamaEmb

I use libWakaamaEmb by David Gräff,  to enable LwM2M on the ESP8266, with a little change to fix the connection to a server without a bootstrap server. You can find the changes at following repository: https://github.com/brusdev/libWakaamaEmb/tree/patch-esp8266.

The library libWakaamaEmb is published to the PlatformIO library registry and Arduino library registry and available on Github. I use the PlatformIO and copy the library direcory under my project "lib" folder but i need to select lwip version 2. PlatformIO doesn't allow to select lwip version 2 so i need to change manually the file .platformio/packages/framework-arduinoespressif8266/tools/platformio-build.py replacing lwip with lwip2.

To expose the temperature i implement the object 3300 that rapresents a generic sensor.

/*
 * Implements a generic sensor
 *
 *                 Multiple
 * Object |  ID  | Instances | Mandatoty |
 *  Sensor| 3300 |    Yes    |    No     |
 *
 *  Ressources:
 *              Supported    Multiple
 *  Name | ID | Operations | Instances | Mandatory |  Type   | Range | Units |      Description      |
 *  value|5700|    R       |    No     |    Yes    |  Float  |       |       |  Sensor Value         |
 *  unit |5701|    R       |    No     |    Yes    | String  |       |       |  Sensor Units         |
 *  minv |5601|    R       |    No     |    Yes    |  Float  |       |       |  Min Measured Value   |
 *  maxv |5602|    R       |    No     |    Yes    |  Float  |       |       |  Max Measured Value   |
 *  minr |5603|    R       |    No     |    Yes    |  Float  |       |       |  Min Range Value      |
 *  maxr |5604|    R       |    No     |    Yes    |  Float  |       |       |  Max Range Value      |
 *  appt |5750|    R/W     |    No     |    Yes    | String  |       |       |  Application Type     |
 *  type |5751|    R       |    No     |    Yes    | String  |       |       |  Sensor Type          |
 *  reset|5605|    E       |    No     |    Yes    | Opaque  |       |       |  Reset Min and Max    |
 *
 */

To connect a server witouth the bootstrap server i need to add manually the server address:

#define LWM2M_SERVER_ADDR "coap://192.168.10.16"

lwm2m_add_server(123, LWM2M_SERVER_ADDR, 100, false, NULL, NULL, 0);

DS18B20

The DS18B20 digital thermometer provides 9-bit to 12-bit Celsius temperature measurements and has an alarm function with nonvolatile user-programmable upper and lower trigger points. The DS18B20 communicates over a 1-Wire bus that by definition requires only one data line (and ground) for communication with a central microprocessor. In addition, the DS18B20 can derive power directly from the data line ("parasite power"), eliminating the need for an external power supply.

I use the DallasTemperature library of arduino to read the temperature by DS18B20.

#include <DallasTemperature.h>

OneWire oneWire(ONE_WIRE_BUS);
DallasTemperature DS18B20(&oneWire);

DS18B20.begin();

numberOfDevices = DS18B20.getDeviceCount();
Serial.print( "Device count: " );
Serial.println( numberOfDevices );

lastTemp = millis();
DS18B20.requestTemperatures();

// Loop through each device, print out address
for(int i=0;i<numberOfDevices; i++){
  // Search the wire for address
  if( DS18B20.getAddress(devAddr[i], i) ){
    //devAddr[i] = tempDeviceAddress;
    Serial.print("Found device ");
    Serial.print(i, DEC);
    Serial.print(" with address: " + GetAddressToString(devAddr[i]));
    Serial.println();
  }else{
    Serial.print("Found ghost device at ");
    Serial.print(i, DEC);
    Serial.print(" but could not detect address. Check power and cabling");
  }

  //Get resolution of DS18b20
  Serial.print("Resolution: ");
  Serial.print(DS18B20.getResolution( devAddr[i] ));
  Serial.println();

  //Read temperature from DS18b20
  float tempC = DS18B20.getTempC( devAddr[i]);
  Serial.print("Temp C: ");
  Serial.println(tempC);
}