Cheap and Simple IoT Home Sensor Network

Do you want to measure the quality of your property’s environment? There are lot of commercial products out there that’ll do it for you, [such as Netatmo or Sen.se Peanuts] but if you aren’t afraid of a little bit of DIY, then this is the guide for you. Using the power of the Internet of Things, we can easily monitor the conditions in each room of your house.

My aim is to show you that monitoring your environment doesn’t need to be expensive. In fact, the sensors can cost as little as £10-£25, depending on what you want to measure.

That said, this example is currently limited to a single node to gateway system due to ThingSpeak’s limitations, however, by removing the delay in the gateway, and changing to a different logging software (Such as MQTT), you can easily make it a multiple node to single gateway system.

What you need

The Hardware

Links to all the hardware mentioned in this post are available at the bottom of this article.

The Gateway

• ESP8266 module (this guide uses the NodeMCU v1.0, but the AdaFruit Huzzah ESP8266 Feather also works)
• NRF24L01+ radio Transceiver

That’s all. Really. The ESP8266 itself has a MCU with flash memory, and is programmable with the Arduino IDE, meaning that there is no need for a middleman Arduino between the two.

The Node(s)

• Arduino Nano Rev 3
• NRF24L01+ radio Transceiver
• DHT11*
• MH-Z19** (optional)
• RGB LED (optional)

* According to the tutorial linked below, there is an issue with the DHT22 and deep sleep, which may be present in the DHT11. An alternative is the DS18B20, but I haven’t had the chance to try this sensor out.
** If you don’t want to detect CO2, just delete the “getCO2()” method, as well as those variables marked as CO2.

The Software

This example uses ThingSpeak as a way of logging the data. ThingSpeak has a library within the Arduino IDE and is very easy to use.

Simply sign up, and create a “New Channel” with the below settings:

Now get your API key from the “API Keys” tab and insert the key into your gateway code:

ThingSpeak also has a mobile friendly web application, allowing for portable sensor tracking. There has also been a number of Mobile Apps developed for Android and iOS.

The Wiring

The Gateway

Your module may have its pins marked as “D#”, which means you can use the Digital Pin layout, or you may have to look for the GPIO pinout for your module and, therefore, follow the GPIO table.

The Node(s)

DHT11 Wiring

NRF24L01+ Wiring

MH-Z19

Status LED

The Code

The below code is a basic starting point if you were developing software for a home environment sensor system. There are a number of additions that would greatly improve the flexibility of the program, including: local web server to set up SSID/Passwords, deep sleep implementation for use with battery powered nodes, and moving to a MQTT server to allow for a smoother one to many relationship between nodes and a gateway.

To give each node and gateway it’s own unique address, I use a program to store a value in the EEPROM, which has a lifespan of around 100,000 erase/writes.

#include <EEPROM.h>

int deviceID = 0, curDeviceID;
void setup() {
  Serial.begin(9600);
  EEPROM.write(0, deviceID);
  curDeviceID = EEPROM.read(0);
  if (curDeviceID == deviceID) {
    Serial.print("Arduino ID successfully changed to: ");
    Serial.println(curDeviceID);
  }
}

void loop() {
  // put your main code here, to run repeatedly:

}

Run this code on your gateway and node, assigning a unique ID to each of them.

The Gateway Software

A lot of people, myself included, got stuck in a reboot loop, citing a Watchdog timeout error, or a “WDT”, which gives an error similar to:

ets Jan 8 2013,rst cause:4, boot mode:(1,6)

As far as I could tell, it was due to the wiring of the NRF24 using GPIO 15 /D8. GPIO 15 on the NodeMCU v1

To fix this, it requires a change in the initialisation of the radio object within the arduino program.

RH_NRF24 nrf24(2, 4);

This changes the CS and CE pins for the radio to unused GPIO pins 2 and 4, allowing the NRF24 to work correctly.

If you are not planning on using the CO2 sensor, delete the highlighted lines of code.

#include <SPI.h>
#include <RH_NRF24.h>
#include <Wire.h>
#include <EEPROM.h>
#include <ESP8266WiFi.h>
String apiKey = "APIKEY";
const char* ssid = "SSID";
const char* password = "PASSWORD";
WiFiClient client;
int gatewayID = EEPROM.read(0);

const char* server = "api.thingspeak.com";
// Singleton instance of the radio driver
//RH_NRF24 nrf24;
RH_NRF24 nrf24(2, 4); // use this for NodeMCU Amica/AdaFruit Huzzah ESP8266 Feather
// RH_NRF24 nrf24(8, 7); // use this to be electrically compatible with Mirf
// RH_NRF24 nrf24(8, 10);// For Leonardo, need explicit SS pin
// RH_NRF24 nrf24(8, 7); // For RFM73 on Anarduino Mini
int LED = 5;
void setup()
{
  Serial.begin(9600);
  Serial.print("Receiver Started, ID: ");
  Serial.print("Connecting to ");
  Serial.println(ssid);
  Serial.print(gatewayID);
  Serial.println();
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");
  nrf24.init();
  nrf24.setChannel(3);
  nrf24.setRF(RH_NRF24::DataRate2Mbps, RH_NRF24::TransmitPower0dBm);
  pinMode(LED, OUTPUT);
}

void loop()
{
  if (nrf24.available())
  {
    // Should be a message for us now
    uint8_t buf[RH_NRF24_MAX_MESSAGE_LEN];
    uint8_t len = sizeof(buf);
    if (nrf24.recv(buf, &len))
    {
      // new message, turn on LED
      digitalWrite(LED, HIGH);
      // Send a reply
      uint8_t sdata[] = "Data Received.";
      nrf24.send(sdata, sizeof(sdata));
      nrf24.waitPacketSent();

      int humidity = buf[0];
      int temperature = buf[1];
      int CO2 = (buf[3]) * 10;
      int deviceID = buf[2];

      Serial.println("--- Data retrieved from device ---");
      if (client.connect(server, 80)) { // "184.106.153.149" or api.thingspeak.com
        String postStr = apiKey;
        postStr += "&field1=";
        postStr += String(temperature);
        postStr += "&field2=";
        postStr += String(humidity);
        postStr += "&field3=";
        postStr += String(gatewayID);
        postStr += "&field4=";
        postStr += String(deviceID);
        postStr += "&field5=";
        postStr += String(CO2);
        postStr += "\r\n\r\n";
        
        client.print("POST /update HTTP/1.1\n");
        client.print("Host: api.thingspeak.com\n");
        client.print("Connection: close\n");
        client.print("X-THINGSPEAKAPIKEY: " + apiKey + "\n");
        client.print("Content-Type: application/x-www-form-urlencoded\n");
        client.print("Content-Length: ");
        client.print(postStr.length());
        client.print("\n\n");
        client.print(postStr);
        
        Serial.println("---- Data sent to Thingspeak ----");
        Serial.print("Device ID: ");
        Serial.print(deviceID);
        Serial.print(", Temperature:");
        Serial.print(temperature);
        Serial.print(", Humidity:");
        Serial.print(humidity);
        Serial.print(", CO2:");
        Serial.println(CO2);
      }
      client.stop();
    }
  } else {
    // no new message, turn off LED
    digitalWrite(LED, LOW);
  }
  // delay is due to ThingSpeak's limitations
  delay(20000);
}

The Node(s) Software

If you are not planning on using the CO2 sensor, again, delete the highlighted lines of code.

// Node code

#include <Arduino.h>
#include <SPI.h>
#include <RH_NRF24.h>
#include <idDHT11.h>
#include <EEPROM.h>
#define pwmPin 5

// Singleton instance of the radio driver
RH_NRF24 nrf24;
// RH_NRF24 nrf24(2, 4); // use this for NodeMCU Amica/AdaFruit Huzzah ESP8266 Feather
// RH_NRF24 nrf24(8, 7); // use this to be electrically compatible with Mirf
// RH_NRF24 nrf24(8, 10);// For Leonardo, need explicit SS pin
// RH_NRF24 nrf24(8, 7); // For RFM73 on Anarduino Mini
int TransmitLED = 3;
int ErrorLED = 4;

int idDHT11pin = 2; //Digital pin for comunications
int idDHT11intNumber = 0; //interrupt number (must be the one that use the previus defined pin (see table above)
int deviceID = EEPROM.read(0);

// CO2 measurement values
int prevVal = LOW;
long th, tl, h, l, ppm;

//declaration
void dht11_wrapper(); // must be declared before the lib initialization

// Lib instantiate
idDHT11 DHT11(idDHT11pin, idDHT11intNumber, dht11_wrapper);

void setup()
{
  Serial.begin(9600);
  pinMode(TransmitLED, OUTPUT);
  pinMode(ErrorLED, OUTPUT);
  while (!Serial)
    ; // wait for serial port to connect. Needed for Leonardo only
  if (!nrf24.init()) {
    Serial.println("init failed");
    digitalWrite(ErrorLED, HIGH);
  }
  // Defaults after init are 2.402 GHz (channel 2), 2Mbps, 0dBm
  if (!nrf24.setChannel(3)) {
    Serial.println("setChannel failed");
    digitalWrite(ErrorLED, HIGH);
  }
  if (!nrf24.setRF(RH_NRF24::DataRate2Mbps, RH_NRF24::TransmitPower0dBm)) {
    Serial.println("setRF failed");
    digitalWrite(ErrorLED, HIGH);
  }
  pinMode(pwmPin, INPUT);
  Serial.println("Transmitter started");
}
// This wrapper is in charge of calling
// mus be defined like this for the lib work
void dht11_wrapper() {
  DHT11.isrCallback();
}

void getCO2() {
  long tt = millis();
  int myVal = digitalRead(pwmPin);

  if (myVal == HIGH) {
    if (myVal != prevVal) {
      h = tt;
      tl = h - l;
      prevVal = myVal;
    }
  }  else {
    if (myVal != prevVal) {
      l = tt;
      th = l - h;
      prevVal = myVal;
      ppm = 5000 * (th - 2) / (th + tl - 4);
    }
  }
}

void loop() {
  getCO2();
  Serial.println("Sending to gateway");
  uint8_t data[4];
  int result = DHT11.acquireAndWait();
  switch (result)
  {
    case IDDHTLIB_OK:
      Serial.println("DHT11 Measurement OK");
      break;
    case IDDHTLIB_ERROR_CHECKSUM:
      Serial.println("Error\n\r\tChecksum error");
      break;
    case IDDHTLIB_ERROR_ISR_TIMEOUT:
      Serial.println("Error\n\r\tISR time out error");
      break;
    case IDDHTLIB_ERROR_RESPONSE_TIMEOUT:
      Serial.println("Error\n\r\tResponse time out error");
      break;
    case IDDHTLIB_ERROR_DATA_TIMEOUT:
      Serial.println("Error\n\r\tData time out error");
      break;
    case IDDHTLIB_ERROR_ACQUIRING:
      Serial.println("Error\n\r\tAcquiring");
      break;
    case IDDHTLIB_ERROR_DELTA:
      Serial.println("Error\n\r\tDelta time to small");
      break;
    case IDDHTLIB_ERROR_NOTSTARTED:
      Serial.println("Error\n\r\tNot started");
      break;
    default:
      Serial.println("Unknown error");
      break;
  }
  data[0] = DHT11.getHumidity();
  data[1] = DHT11.getCelsius();
  data[2] = deviceID;
  data[3] = ppm / 100;
  Serial.println("------------- Measurements -------------");
  Serial.print("Humidity: ");
  Serial.print(data[0]);
  Serial.print(", Temperature: ");
  Serial.print(data[1]);
  Serial.print(", PPM: ");
  Serial.print((data[3]) * 100);
  Serial.print(", ID: ");
  Serial.print(data[2]);
  Serial.println();

  digitalWrite(TransmitLED, HIGH);
  digitalWrite(ErrorLED, LOW);

  nrf24.send(data, sizeof(data));
  nrf24.waitPacketSent();
  // Now wait for a reply
  uint8_t buf[RH_NRF24_MAX_MESSAGE_LEN];
  uint8_t len = sizeof(buf);

  if (nrf24.waitAvailableTimeout(1000))
  {
    // Should be a reply message for us now
    if (nrf24.recv(buf, &len))
    {
      digitalWrite(TransmitLED, LOW);
      digitalWrite(ErrorLED, LOW);
      Serial.print("got reply: ");
      Serial.println((char*)buf);
    }
    else
    {
      digitalWrite(ErrorLED, HIGH);
      Serial.println("recv failed");
    }
  }
  else
  {
    Serial.println("No reply.");
    digitalWrite(ErrorLED, HIGH);
  }
  delay(2000);
}

Summary

To conclude, you will be surprised how cheap and easy it is to monitor your property using existing and easily accessible hardware out there.

Another option is to use the well developed library MySensors, which the wiring for the gateway will be compatible with.

Happy Coding!

Links

https://forum.mysensors.org/topic/3285/solved-esp8266-fails-to-boot-with-nrf24l01-connected/6

https://www.mysensors.org/build/esp8266_gateway

https://github.com/esp8266/Arduino/issues/1013

https://forum.mysensors.org/topic/3496/esp8266-gateway-where-are-spi-pins-defined/3

https://learn.sparkfun.com/tutorials/esp8266-thing-hookup-guide/example-sketch-goodnight-thing-sleep-mode

Examples used

Examples of using the NRF24 as a client/server
Radiohead’s NRF24_server and NRF24_client

This guide on the MH-Z19 (it’s in Russian, but easy enough to follow)
http://www.2150692.ru/faq/87-co2-mhz19-arduino

Arduino ThingSpeak guide

ThingSpeak

Hardware Links

Hardware Links
AliExpress links (expect 20-40 day delivery)

NodeMCU v1 (£3.13)
https://www.aliexpress.com/item/4M-4FLASH-NodeMcu-Lua-WIFI-Networking-development-board-Based-ESP8266/32551536028.html

MH-Z19* (£17.93)
https://www.aliexpress.com/item/1PCS-module-MH-Z19-infrared-co2-sensor-for-co2-monitor-Free-shipping-new-stock-best-quality/32643995676.html

NRF24L01+ x 2 (£0.61)
https://www.aliexpress.com/item/Free-Shipping-10PCS-NRF24L01-wireless-data-transmission-module-2-4G-the-NRF24L01-upgrade-version-We-are/32469366102.html

Arduino Nano 3 Clone (£1.80)
https://www.aliexpress.com/item/Nano-CH340-ATmega328P-MicroUSB-Compatible-for-Arduino-Nano-V3-0/32740641316.html