One Second: Temperature Controller with an Arduino

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.

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]

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.

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.

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, andhave 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 candownload from here.

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:

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.

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.