Make Your Own Temperature Controller with an Arduino

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Raise the perfect bread, brew beautiful beer, and rear happy chicks with an Arduino temperature controller. If you live in a less than reliable climate like England, directions that tell you to keep something at a set temperature aren’t particularly helpful – we don’t have air conditioners, and raising the thermostat for the whole house isn’t practical for just making a loaf of bread. Even kept inside, chicks can die if the temperature drops at night; and getting them to hatch in the first place has an even stricter temperature range. But I need my bread, and the chicks need hatching – so instead of purchasing expensive equipment, we can cobble together a competent temperature controller with an Arduino and household bits.

The same is also true for keeping items cool – it can be wasteful to run a whole fridge just to make yoghurt – but with a temperature controller, the principle is the same. Instead of activating a heating element, you’ll be activating the plug on a mini-fridge or other cooling element, like a Peltier (thermoelectric cooler) – and of course, the logic will be reversed.

What You Will Need

This is an Arduino project – if you’ve never worked with Arduino before, our free beginner’s guide is a fantastic place to start.

  • Arduino
  • Temperature sensor – I’m using a TMP36, a cheap single package device that comes with the Oomlout (UK) / Sparkfun (US) beginner’s kit.
  • Relay or RC plug switches
  • Screw terminals
  • Box to trap the heat
  • Heating element or incandescent bulb and fixture (or both)

The last item has been left deliberately vague. If you have an incandescent bulb (the kind that gets hot, not an energy-saving bulb), or a hot lamp for sporting injuries and such, it’s probably the easiest to set up. I’m using a heating band – basically a band of rubber that gets warm when electricity is passed through, used on carboys and kegs for initial fermentation stages in wine or beer making – technically, this can be a fire risk when not wound around something, so please don’t do this, I’m only using it to test. You can also buy heating pads for the same purpose.

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For safety reasons, I’m using these RC plugs to switch AC devices, with a controller hacked apart detailed in this home automation article. It’s wireless, so at no point need I actually touch live wires.

Temperature Sensing

Let’s start by wiring up and testing the sensor. [Diagram from Adafruit]

adafruit tmp36   Make Your Own Temperature Controller with an Arduino

With the flat side toward you and legs face down, the TMP36 temperature sensor is wired up +, signal, GND in that order. The + goes to the 3.3 V output from Arduino; you also need another line going from the +3.3 V to the AREF pin – this tells the Arduino to use 3.3 V for analog input reference instead of 5 V. Connect the signal pin of the sensor to A1. In previous attempts, I had used the TMP36 directly on the 5 V line; it works, bit unfortunately when paired with a relay, there was a power drop whenever the relay was activated, resulting in highly fluctuating readings.

I used an old network cable as signal cable – very useful to have around, since there are 8 wires inside. The cable is quite thin though, so be sure to strengthen the other end with solder where it’ll be screwed into a terminal block.

tmp36 sensor cabling   Make Your Own Temperature Controller with an Arduino

The formula in the code assumes you’re using the tMP36 sensor; you should be able to find a code sample for other sensors. This sample code is from Adafruit – load it up and open the Serial console to examine the output.

tmp36 testing   Make Your Own Temperature Controller with an Arduino

Compare with a thermometer if possible. Readings not right?

  • Check the voltage being supplied is actually 3.3 V
  • Is the AREF connected to 3.3 V too?

Adding in Switch Logic

To control the heating element, I’m using these RC plug sockets from Maplin, and have taken apart the controller. Only the ground and control pin need be connected. I’ve modified the code to include the relevant libraries which you can download from here.

rc switches   Make Your Own Temperature Controller with an Arduino

At this point, I’m also going to remove all references to Farenheit and continue working with Celsius only. I’ve then defined a desired temperature to maintain, and added in a simple control structure like so:

if(temperatureC < desiredTempC){
    mySwitch.switchOn(1,1);
    Serial.println("Heater ON");
  }
  else{
    Serial.println("Heater OFF");
    mySwitch.switchOff(1,1);
  }

There’s nothing complex here that you won’t understand – just comparing the current temperature reading to the desired one, and turning on the switch if it’s lower; otherwise, turn it off.

The complete code can be found here, though you will need to adjust this if you’re using a relay (it’s not hard). Here’s the complete wiring diagram I used:

wiring 477x590   Make Your Own Temperature Controller with an Arduino

Putting It All Together

Tape the sensor inside the box you’re using, and place the heating element wherever is appropriate. Set the desired temperature, and turn it all on. If you keep your PC connected for now, you can use the Serial console to observe changes as your box heats up.

temperature controller box1   Make Your Own Temperature Controller with an Arduino

Further Work

  • To lessen the impact of any temperature fluctuations, you can try smoothing the results. Create an array to store 10 readings, and calculate an average on each loop.
  • To avoid rapid activation and deactivation of the heating element, create a variable to store a countdown. Each time you activate or deactivate, record the current time in the countdown, then before switching the state again check to see if X amount of time has elapsed since the last state change.
  • For a computer-less project, hook up a small LCD screen to display current temperature and allow you to see the current and desired temperature.

Putting It To The Test

Finally, what would this project be without a little test? I whipped up a batch of ready-mixed dough in the bread machine and split it into two loaves. The one leavened inside the box was mildy bigger, but then the ambient air temperature today is about 26 degrees Celsius anyway – this would be much more useful in winter. Regardless, I better go make some soup to accompany this lovely bread.

bread test   Make Your Own Temperature Controller with an Arduino

So, what would you make that requires a constant temperature?

Image credit: Ian Watkins/flickr

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3 Comments - Write a Comment

Reply

catprog

For a more accurate temperature look up a PID algorithm.

James B

Good advice, though I thought that would be a bit much for this article.

Reply

Taimur Khan

Here is my code using https://code.google.com/p/rc-switch/ and
http://learn.adafruit.com/tmp36-temperature-sensor/using-a-temp-sensor

I have added a bit of hysteresis, so the heater turns off when temperature more than 25, and then turns on when temperature less tan 20. Also made sure that it sends On or Off command every time as I have seen that the wireless transmitter /receiver can miss a transmission or two some times

___

/*
Example for different sending methods

http://code.google.com/p/rc-switch/

Need help? http://forum.ardumote.com
*/

#include

RCSwitch mySwitch = RCSwitch();
//TMP36 Pin Variables
int sensorPin = 0; //the analog pin the TMP36’s Vout (sense) pin is connected to
//the resolution is 10 mV / degree centigrade with a
//500 mV offset to allow for negative temperatures

int SwitchState=0;
void setup() {

Serial.begin(9600);

// Transmitter is connected to Arduino Pin #10
mySwitch.enableTransmit(10);

// Optional set pulse length.
// mySwitch.setPulseLength(320);

// Optional set protocol (default is 1, will work for most outlets)
// mySwitch.setProtocol(2);

// Optional set number of transmission repetitions.
// mySwitch.setRepeatTransmit(15);

}

void loop() {

//getting the voltage reading from the temperature sensor
int reading = analogRead(sensorPin);

// converting that reading to voltage, for 3.3v arduino use 3.3
float voltage = reading * 5.0;
voltage /= 1024.0;

// print out the voltage
Serial.print(voltage); Serial.println(” volts”);

// now print out the temperature
float temperatureC = (voltage – 0.5) * 100 ; //converting from 10 mv per degree wit 500 mV offset
//to degrees ((voltage – 500mV) times 100)
Serial.print(temperatureC); Serial.println(” degrees C”);

if (SwitchState ==1 ) {
mySwitch.switchOn(3,1);
if (temperatureC >25.0){
mySwitch.switchOff(3,1);
SwitchState=0;
}
}
else if (SwitchState ==0 ){
mySwitch.switchOff(3,1);
if (temperatureC<20){
mySwitch.switchOn(3,1);
SwitchState=1;
}
}
// Wait 10 second
delay(1000);

}

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