How to build a two stage thermostat for a Fermentation Fridge

Controlling fermentation temperature can be difficult in the home. There are many different approaches to keep temperatures in the yeast's ideal zone.

If you have the space using a temperature controller with  freezer is going to be one of the best options. 

Many of the commercially available controllers are single stage (cool only) which should be fine if you are fermenting in your house. But if you ferment in your basement or garage you may need to heat as well.  You can get two stage controllers that have outlets for heating and cooling elements.

With the correct equipment a Two stage controller can be set up and operational in a matter of a few hours work.

You can buy the controller off eBay or Amazon  I bought most of the other bits at Lowe's.  At Radio Shack you can buy a small project box,

Below you can see the technical details for the temperature controller that I bought.

Below is a picture of the device. Be careful to make sure it is the dual stage sensor that you order

There are many similar Blogs and posts that cover this kind of device and how to build a fermentation fridge. I made a few tweaks and added a couple of LED's in to the box to let me know when the cooling or heating relays were energized. this helps trouble shooting

Here are the base components.  

Ceramic light fitting with a 3meter extension lead connected to the back. This plugs in to the heater circuit

 

Project box from Radio Shack. Two way outlet socket. Additional extension lead. and a face plate for the outlet socket all from Lowe's or Home Depot. You must remember to cut the little connection (bridge) between the live terminals on the socket or else both heating and cooling circuits will come on together all the time.

 

Some blogs say a light bulb is sufficient to heat up  the fridge. I was a bit concerned that with a large freezer it would not be sufficient and that moisture might cause the bulb to shatter. I came across this nifty little 120 watt heating element designed for an  amphibian tank. It works an absolute treat.

 

Sorry the picture is upside down but in this packet are a couple of 120v neon lamps also from Radio Shack.

First thing to do is to mark out the cut needed for the sockets and the temperature controller. You will also need some appropriate size holes for the sensor probe and the mains in lead.

The best way to cut the holes out is to drill a small pilot hole and then use a coping saw to follow the pencil lines as best you can. Don't worry about getting it too neat as you can hide most of the rough edges with the socket plate and there is a nice flange around the sensor housing. Additionally you can use a small file to tidy up any real  nasty cuts.

The rear has the holes cut out for the sockets.

This front  side I have drilled two holes for the LED's

Here is a picture of the wiring diagram that I did in powerpoint.  It's not as intimidating as it looks and is pretty simple.

After all of the cuts were made, all of the components were wired up.  If you can solder all of the connections that this is the best method. In my case I used a connection block which can be bought from Lowes (the connections to the STC-1000 use set screws so no need to solder).  The outlets were designed to be clipped in but the project box was too thick for the clips so I trimmed the clips and glued the outlets in place. 

Below you can see the heating element mounted in the fridge and side by side with some beer :-)

 

Once all the connections are in place then you can plug in the mains and program the timer.

My settings are as follows.

F1 20.5 Temperature that you want
F2 0.5 Temperature variation
F3 5 cooling delay start in minutes
F4 0 calibration adjustment

You can then test the circuits by connecting xmas lights and holding/cooling the temperature probe. I just went for it and connected the freezer to the cooling circuit and the heating element to the heating Circuit. I used the LED's as a guide to test that each circuit was activating appropriately.

below are some pictures of the finished item

Here is a video of the sensor in operation.