Temperature Control for Freezer or Refrigerator

This instructable covers the integration of a Johnson Controls temperature control into an extension cord with switch and electrical outlet for controlling a freezer. For fermenting beer, a chest freezer is a fantastic platform but the factory controls are far too cold. This controller project effectively overrides those by cutting power to the freezer once it chills to the desired temperature. The controller in this project is further enhanced with a standard electrical outlet and light switch controller.

When the switch is on and the temperature is higher than the thermostat setting, the outlet is powerered. When the switch is turned off, or the temperature drops below the thermostat setting, the outlet is off.

Required tools:
1) Wire stripper
2) Phillips head screwdriver
3) Electric Drill with small drill bit may be required for attaching the controller to the metal electrical box.

The required parts for this project are:
1) Johnson Controls Thermostat control. This particular one is a model A19ABA. This was purchased from ebay for about $19.00.
2) Extension cord. Make sure this cord can carry the full amp load listed on the freezer or device you are attaching to it. Mine is a 12gauge "heavy duty" and can easily handle the current draw of this freezer.
3) A 2 gang metal electrical box. Available at a home center for about $2.00
4) Misc. electrical wiring. Everything in this project is 12-2 residential wiring or ROMEX. You will only need a few short or scrap pieces for this project. You can also clip down a 10" or so piece of the extension cord if you are frugal or don't have ROMEX scrap lying around. If you go that route, then use this piece whenever this instructable mentions your ROMEX wire.
5) 1 electrical outlet and 1 switch. These are available at a home center for $0.50-$1 a piece. You don't need anything fancy here.
6) Switchplate to cover the face. Optional, but better than getting shocked. Get the 2 gang where one side is switch plate and the other is for an outlet like in the picture.

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Step 1: Prepare the Extension Cord

Clip off the female end of the extension cord. You may want to save this for another project. Then cut the insulation back a bit and strip the three component wires back about 1".

While you've got the wire strippers out, cut back some of the ROMEX electrical wire the same way.

Step 2: Attach the Control Box

Using a few metal screws, attach the Johnson Control unit to the side of the electrical box to make it easier to handle. I had to use an electric drill and small drill bit to get through the metal electrical box. This may or may not be required in your case.

Step 3: Pull the Electrical Wires

Pull the electrical wires from the extension cord and from the ROMEX 12-2 into the electrical box. I pulled these through the same port and I did not use "plugs" to fill the hole in later. This should not be a problem for this application.

Step 4: Connect the Switch

Connect the black wire from the extension cord and from the ROMEX to the electrical switch. The order is not important here.

If you've never done this before, loosen the screw terminal and form a small hook at the end of the wire you want to connect. Pull the hook down until it is as close as possible to the post of the screw and hold it with one hand while you tighten the screw with the other.

Step 5: Connect the Johnson Controller

Pull the other end of the ROMEX through the hole in the base of the Johnson Controls. If you've got extra wire here, clip it back to only what's needed. Strip away the outside (yellow) insulation and connect the white and black wires to the screw terminals on the controller. Then take the bare copper ground wire and connect it to the green terminal at the base of the controller.

Be very careful with the controller when the cover is off. The temperature apparatus at the top is sensitive.

Step 6: Finishing the Wiring

There are a few steps left to finish up the wiring.

You should attach a short (5") bare copper wire to the ground terminal of both the electrical outlet and the switch. These should be twisted together with the bare copper that you attached to the case of the Johnson Controller and the green wire from the extension cord. Attach a wire nut to the top to keep them together.

The other electrical connections should be made with the 2 white wires (one from the ROMEX and one from the extension cord.) Attach these to the hot and the neutral terminals on the side of your electrical outlet.

The second picture shows the electrical outlet connected and the wire nut attached to the bundle of the 4 grounding wires.

Once the connections are made, screw down the switch and the outlet into the metal box. I typically wrap the outside of outlets and switches with electrical tape when working with metal boxes for safety.

Notice that the extra ground bundle of wires is tucked neatly behind the outlet here.

I attached a rough schematic of the wiring for this project. The cyan is used to represent white wires and green for ground. Ground is sometimes just the bare copper wire and has no coating. Note that where the green ground wires crosses the black at the bottom right of the switch, this is NOT an electrical connection, just a bad schematic.

Step 7: Finish and Testing

Attach the switch plate and the cover to the Johnson Controller.

Now it's time to test it. In the picture below I set the thermostat on the Johnson Control to 60F. As you can see, with the switch on, and a lamp plugged in, the electrical outlet is powered. To test, we'll drop the temperature bulb in a cup of ice water.

Within a few seconds the bulb has cooled to 60F and the controller breaks the connection turning the light off.

When using this with your freezer, you should drop the copper temperature sensor bulb in the freezer and plug it in. Set your freezer on high and set the Johnson Controls unit to the desired temperature.

As always, be careful when working with household current. If you don't understand the steps of this project or feel uncomfortable with them, consult a qualified electrician.

Good luck and happy brewing.

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    14 Discussions


    4 years ago on Introduction

    There are many more things to consider here. I purchased a Johnson touch pad controller from Northern Brewer for about $100. I still had to go over to an electrical engineer friend's home to decipher the directions. But we adjusted and tested it out on his counter top with a cup of ice water, in a 72 degree room. The Johnson sensor probe is dead nuts accurate. However there's a slippage which can bite you with a lager fermenting. You should add three degrees F. to your lower setting for the cooling. In the winter, you change one jumper in the box and then it can be reset to heat your fermentor while its inside the turned off fridge. He said to play with it and then he can look to put a small soldered switch in the plastic case to cycle the heat cold without changing the jumpers. There doesn't seem to be any reason to use a jumper to lock the screen, as you have to go to menu and then select a field, before any adjusting will occur. We gave it a ten minute time delay to take any pressures off of the older 14 Cu.Ft. refrigerator's compressor. The max delay is twelve minutes, and five would have been plenty for a new fridge.


    11 years ago on Introduction

    I have a slightly different johnson control - 3 terminals instead of 2.
    The diagram shows terminals red, blue, and yellow:
    red+blue = open on temp increase...
    yellow+red = closed on temp increase...
    i'm pretty sure this control will work, but i'm not sure how this translates to your schematic. any thoughts?

    4 replies

    Reply 11 years ago on Introduction

    answering my own question through experimentation:

    red+yellow = cooling wires. these complete the circuit if temperature surpasses a certain level.

    red+blue = heating wires. (not applicable in this situation. would complete the circuit if temperature falls below a certain level.)


    Reply 10 years ago on Introduction

    If you live in a cold climate and your fridge/freezer lives in a garage you might want to hook up a reptile heater or other type of heater to keep it from getting to cold. I wish I had one of these.

    Good point. I've seen a lot of different controllers for applications similar to this. Mine would be refrigeration only so the circuit is closed when temp goes over the set limit but there are some that work the other way around. Yours apparently is capable of both. Make sure you get the right controller for your project!

    Lots of folks have mentioned finding the Johnson Controls on Ebay for around $15 -$20. The cheapest I can find one for it about $40. Am I looking at the wrong model? Also, there seems to be a TON of different models -- will they all pretty much work? Thanks! Evan

    2 replies

    You can "roll your own" control with a thermocouple and arduino for about $20.

    This instructable shows you how to do it for $60, but you can get a much cheaper arduino (RBB) and I counted the cost of the relay, outlet and other stuff that's already part of this project.


    It might be the kind of thing where you just need to keep an eye out for a good deal. I would caution when looking for different models since some are designed for different purposes other than refrigeration controls. This model was an A19ABA.


    9 years ago on Step 6

    Thank you for this hack . . it will save me a good $30! One question -- do I need to use the switch? The alternative would be just unplugging the extension cord when I'm not using the fridge, right? Because the Johnson Controls has an internal swich? Thanks much, Evan

    1 reply

    Correct, the switch is a complete override for the Johnson Control and definitely not a requirement. Should simplify the wiring a bit as well.


    10 years ago on Introduction

    Don't run wire out of a hole in a metal box like that, at least use a grommet, those punch out holes are very sharp. Stranded wires like those found on extension cords do not hold as well as solid wires. I would definitely recommend a wire clamp, it will help keep the cord from being pulled out.


    11 years ago on Introduction

    Is ROMEX flattish?if so you may get some at Grundfos.I saw some flat mains cable being strapped to a wellpipe for the sump pump down there,it came in a crate marked GRUNDFOS


    11 years ago on Introduction

    This hack also works if you're not brewing beer and just want another refrigerator that is extra-efficient and cheap to run. Nice job laying out the process for those who are not familiar with it!