Building a Wireless Temperature Sensor Network Part 2

Welcome to part 2 of building a wireless temperature sensor network. In part 1 we learned the basics of how to use XBee and how to use it with Arduino. In this post we will start to build our sensor network and collect temperature data from multiple sensors. The parts list for this post can be found at the end of the part 1 post. Our hardware setup will be very similar to the setup in XBee Basics Lesson 4 video tutorial (found in part 1), except we will add a second XBee router with a temperature sensor and we will add a third temperature sensor to our Arduino board.

Let's get started by installing the proper firmware on our three XBee modules. We want to setup the XBee firmware the same way as in the XBee Basics Lesson 4 video except this time setup two routers instead of one. Be sure to use the same PAN ID for each. Once you have the proper firmware on the XBees let's put the hardware together as shown in the below figure.

Hardware setup for capturing temperature data from multiple XBees

As you can see in the figure, the two routers are tied to the same power supply. That is not very useful for creating a network, but this is just to get started. In part 3 we will look at options for powering each sensor so we can spread them out. You will need to solder a wire or pin to the AD3 hole on the XBee adapter board so you can connect it to the temperature sensor's output on the breadboard. Going forward I will refer to the Arduino and XBee coordinator as the "controller" since it gathers the temperature data from the router XBees, turns it into useful information, and communicates it to the user. Please note that I tied sensor 3 to one of the analog pins on the Arduino so we will use the Arduino to make the analog measurement for calculating the temperature from sensor 3. Having a temperature sensor on the controller is optional. Depending on how you choose to use your sensor network, you may or may not want to make temperature readings at the controller. Below are photos of my setup. The first photo shows the two routers on small red breadboards connected to the TMP36 or MCP9700 temperature sensors. The alligator clips you see in the picture are from the power supply, which is putting out 3.3 V since I am using the Sparkfun Xbee boards (no onboard regulator like the Adafruit versions).

The two routers each with a temperature sensor

XBee coordinator, temperature sensor, and Arduino Uno

The two main differences between this setup and the setup we saw in XBee Basics Lesson 4 is we have two routers sending the controller temperature data and we also have a temperature sensor tied to the controller. That means in the Arduino code we now need to read the address from the received frame of data from each router to determine which router sent it. We also need to read the temperature from sensor 3 and display it to the user. Below is the code for the Arduino with comments to explain what is happening in each line of code. When you go through the code you will want to have the the format or layout of an XBee API RX data frame on hand so you can understand what is happening in the code when it is handling incoming data from the routers. You can get this information from the XBee manual or from the XBee S2 Quick Reference Guide that was in lesson 3 and 4 of the XBee Basics video series. You can get the reference guide from the tunnelsup blog, if you do use it I encourage you to donate $1 to blog. Click here to go access the reference guide.

/*This program was written for the Arduino Uno. The Uno has an XBee Series 2 RF Module connected to it as a coordinator. The Uno uses the XBee coordinator to communicate with two router or end point XBees with temperature sensors. This program recieves the temperature readings from the two endpoint XBees and writes the data to the serial monitor */

/*Each Xbee has a unque 64 bit address. The first 32 bits are common to all XBee. The following four ints (each int holds an address byte) hold the unique 32 bits of the second half of the XBee address*/

 int addr1;

 int addr2;

 int addr3;

 int addr4;

 int sen3Counter = 0; //This counter variable is used print sensor 3 every 5 seconds

void setup()  { 

 Serial.begin(9600); //start the serial communication

}

void loop()  { 

  if (Serial.available() >= 21) { // Wait for coordinator to recieve full XBee frame 

    if (Serial.read() == 0x7E) { // Look for 7E because it is the start byte

      for (int i = 1; i<19; i++) { // Skip through the frame to get to the unique 32 bit address

        //get each byte of the XBee address

        if(i == 8) { addr1 = Serial.read(); }

        else if (i==9) { addr2 = Serial.read(); }

        else if (i==10) { addr3 = Serial.read(); }

        else if (i==11) { addr4 = Serial.read(); }

        else { byte discardByte = Serial.read(); } //else throwout byte we don't need it

      }

      int analogMSB = Serial.read(); // Read the first analog byte data

      int analogLSB = Serial.read(); // Read the second byte

      float volt = calculateXBeeVolt(analogMSB, analogLSB);//Convert analog values to voltage values

      Serial.println(indentifySensor(addr1,addr2,addr3,addr4)); //get identity of XBee and print it

      Serial.print("Temperature in F: ");

      Serial.println(calculateTempF(volt)); //calculate temperature value from voltage value

    }

  }

  delay(10); //delay to allow operations to complete

  //This if else statement is used to print sensor 3 value every 5 second to match the XBee routers

  //It uses the delay() function above to calculate 5 seconds

  if (sen3Counter < 500) { sen3Counter++; }

  else {

    Serial.println("Temperature from sensor 3:");//This is sensor 3

    Serial.print("Temperature in F: ");
    //the following line calculates voltage, then temperature, and then prints temp to serial monitor

    Serial.println(calculateTempF(calculateArduinoVolt(analogRead(A0))));

    sen3Counter = 0; //reset counter back to zero to start another 5 seconds

  }

}

//Function takes in the XBee address and returns the identity of the Xbee that sent the temperature data

String indentifySensor(int a1, int a2, int a3, int a4) {

  int rout1[] = {64, 176, 163, 166}; //Arrays are the 32 bit address of the two XBees routers

  int rout2[] = {64, 177, 63, 221}; 

  if(a1==rout1[0] && a2==rout1[1] && a3==rout1[2] && a4==rout1[3]) { //Check if Sensor 1

    return "Temperature from sensor 1:"; } //temp data is from XBee one

  else if(a1==rout2[0] && a2==rout2[1] && a3==rout2[2] && a4==rout2[3]) {//Check if Sensor 2

    return "Temperature from sensor 2:"; } //temp data is from XBee two

  else { return "I don't know this sensor"; }  //Data is from an unknown XBee

}

//this function calculates temp in F from temp sensor

float calculateTempF(float v1) { 

 float temp = 0;

 //calculate temp in C, .75 volts is 25 C. 10mV per degree

 if (v1 < .75) { temp = 25 - ((.75-v1)/.01); } //if below 25 C

 else if (v1 == .75) {temp = 25; }

 else { temp = 25 + ((v1 -.75)/.01); } //if above 25

 //convert to F

 temp =((temp*9)/5) + 32;

 return temp;

}

//This function takes an XBee analog pin reading and converts it to a voltage value

float calculateXBeeVolt(int analogMSB, int analogLSB) {

  int analogReading = analogLSB + (analogMSB * 256); //Turn the two bytes into an integer value

  float volt = ((float)analogReading / 1023)*1.23; //Convert the analog value to a voltage value

  return volt;

}

//This function takes an Arduino analog pin reading and converts it to a voltage value

float calculateArduinoVolt(int val) {

 float volt = (float)val * (5.0 / 1023.0); //convert ADC value to voltage

 return volt;

}

The next step is to take the code and upload it to the Arduino. Do not forget to disconnect the wire connecting the Arduino digital pin 0 to the XBee coordinator when you upload the code to the Arduino or else you will get an error. Once the code it uploaded, reconnect the wire to digital pin 0 and open the serial monitor. If you have everything setup correctly your serial monitor should look something like the one in the below figure. 

Serial monitor displaying data from each temperature sensor

It is a good idea to place all your sensors in the same area so you know that they should have a similar temperature reading. That way if one of the sensors is off you can easily spot it and investigate what the problem is. Note that these sensors have an accuracy tolerance of +/- 1 degree from the actual temperature so theoretically your sensor readings could differ by up to 2 degrees even if they are right next to each other. If your serial monitor does not look something like the figure above don't worry just go over instructions again to make sure everything is correctly setup, its easy to miss something! If you see the sensor 3 readings in your serial monitor, but you are missing sensor 1 or 2 or both that means one or both of your routers is not sending data to the coordinator. Check the power and communication lights on the XBee adapter boards to ensure they are getting power and are communicating with the coordinator. If they have power, but are not communicating check to make sure you loaded the correct configuration on them.

Well that is it for part 2. Normally I would share the list of parts you need for the next part of the project, but in part 3 and part 4 we will look at multiple options for powering the sensors in the project so read it first and then decide what works best for powering your temperature sensor network. If you have any questions on part 2 use the comment area below or feel free to email your question to me at forcetronics@gmail.com. Stay tuned for part 3!