Cheap Digital Temperature Controllers

Just recently a fellow blogging homebrewer posted an entry about his use of a Lux outlet thermostat as a temperature controller--he's using his to turn on a heater when it gets too cold to ferment.  These outlet thermostats work by turning on power to your heating or cooling device when the temperature is below or above what you have the thermostat set to.   You plug the thermostat into the wall outlet, and your heating or cooling device into the thermostat. They cost about half of what the Johnson controls that many homebrewers use for temperature control.  I have been using  two of these for a couple of years now, to control both my fermentation fridge, and my converted freezer chest kegerator/lagering chamber.  I'm surprised I haven't already written about them, but since Mark Taylor at the Backyard Brewer started the ball rolling with his post, I thought I would follow up with one of my own.

I modified both of them, because out-of-the-box, they didn't do exactly what I wanted them to.  First of all, as you can see from the photo above, the sensor wire built into them is only a few inches long, meaning you can only sense the temperature right next to the outlet.  Since I wanted to use them to control temps inside a fridge or freezer, this meant that I had to run an extension cord to the thermostat, which I would then place inside the fridge, and then plug the fridge cord into the thermostat.  The fridge cord would have to run to the outside of the fridge, where it was hard-wired in.  You could use it this way, which I did for awhile, but I realized it wasn't optimal to have a heavy duty extension cord running into the fridge, and also to have the thermostat outlet inside the fridge, operating in a cold, damp environment. 

Before I explain how I modified these thermostats, be aware that any modification will void the warranty.  Also, be sure to make sure that thermostat is unplugged, and the battery is removed before doing any work on the thermostat.  Señor Brew™ is not suggesting that you make any changes to your thermostat, nor is he responsible if you break your thermostat, or electrocute yourself or the cat or set your house on fire, or any other unfortunate event that may occur during or because of modifications you may make to your outlet thermostat.

So I added a length of cord to the sensor wire.  This was easily accomplished by cutting the sensor cord, attaching some lamp wire, and reattaching the sensor to the other end of the lamp wire.  Now, I could eliminate the extension cord, and just run the sensor wire from the outlet thermostat into the fridge.

Thermostat with sensor wires cut.

Sensor cut off

Lamp cord ends stripped

Heat shrink connectors at thermostat

Heat shrink connectors at sensor

Secondly, I wanted to use the modified freezer chest to lager my lagers, at least when I wasn't using it to serve beers.  Ideally, I wanted to set it to just above freezing, to lager my beers as cold as possible.  The problem here was that the outlet thermostats lower range was 45F.  I did find that there was an adjustment factor you could program in, adjusting the temperature plus or minus 5F.  Meaning, if you adjusted it down all the way, the thermostat would read 45F when it was actually 40F.  Of course this still wasn't good enough for me, so I decided to swap out the sensor.

I contacted an engineer at Lux via email to ask them what type of sensor was in the thermostat currently.  It turns out that it's a thermistor.  I don't have the original email, but I remember that it was an NTC thermistor, and I believe it was 50k ohms.  NTC stands for Negative Temperature Coefficient, meaning that resistance decreased at higher temperatures.  (This is opposite of how metals react, which is why computers run better when they are cold, there is less resistance in the copper circuits within them).   I did the math, and realized that if I swapped in a 30k ohm thermistor, my thermostat would think it was 20 degrees Fahrenheit warmer than it truly was.  Meaning that I could set the thermostat to 53 degrees Fahrenheit, and it would continue to run the freezer until it was actually 33F.  (Incidentally, if you wanted to use this controller to heat something outside its upper range of 95F, you could swap in a higher resistance thermistor, so the thermostat would read a temperature lower than the true temperature).

Instead of cutting the sensor cord, as I did with the other thermostat, I simply opened the thermostat, removed the sensor cord and all, and swapped in another lamp cord with my 30k ohm thermistor soldered into the other end of it.

Lamp cord connected inside thermostat

30k ohm thermistor purchased from Allied Electric

New thermistor soldered in and covered with rubber