A Homebrew Refrigerator Temperature Controller

I developed this Temperature Control back in 1988 to fill the need for a reliable, accurate way to control a refrigerator/freezer for fermenting and lagering. At the time there were no temperature control devices in any reasonable price range that would perform these functions. Even today, NONE of the currently available units come close to this one in terms of all of the available functions - max/min recording, adjustable dead band, Farenheit or Celsius readout, and the ability to control a heater as well as a compressor.

I built about six of these, including two for my own use and the rest for fellow TRASH members. If you follow these plans and know how to assemble electronic components you'll do well.

We spent dozens of hours trying to debug the design to work properly and be reasonably immune to electrical noise. Still, play it safe and a) don't connect more than one fridge/freezer to the same electrical circuit; and b) keep fluorescent lights off the same circuit as the controlled fridge.

To use it as a heater you can use a 100 ohm 225 watt ceramic power resistor as a heating element (about 1 foot long - try Ohmite model 270-225P.46). Be sure to also get mounting hardware. This can be connected directly across the 120 V ac line to act like a heater generating about 144 watts. Another option is to use a 100-200 watt red light bulb with one of those “bulb savers” in the bottom of the base to get rid of the turn-on surge. Don't use a white light bulb (white light can damage beer), or a red bulb without some kind of surge protection (it'll burn out in a short time).

Please note that IF YOU DON'T HAVE ANY EXPERIENCE WITH ELECTRONICS OR KITBUILDING, THIS IS NOT WHERE YOU WANT TO START!! Have an electronics-savvy friend or fellow brewclub member assemble it for you.

I've heard conflicting reports as to the availability of the 277-123 module that is the heart of this unit. Some say Radio Shack doesn't make it anymore. I'll leave this one up to you. I've also heard that RS now has another temperature module that can be used for this; again, this is for you to investigate. If it has been discontinued there's a chance some closeout vendor has a pile of them somewhere - those of you that know of such places might want to investigate.

Here's some updated (3/25/96) information on the Radio Shack temperature module from mtippin@swbell.net:

I did some more poking around and found out that the controller *is* still available from Radio Shack as a special-order part - it's now called part no. 1064-0316 and still goes for $19.99 + $2 shipping - it can be ordered directly from Radio Shack at 800-843-7422 or you can have any local store order it for you and it'll be shipped straight to your house.

If you use a freezer or an old single-door fridge from the 1950s, note that these can freeze a carboy solid if the override control fails (I made a great, but unintentional, “icebock” this way once). A good idea is to use the internal control as a failsafe - do a test with the internal control set to it's lowest position and measure how cold it gets. If it gets below 35 degrees and no colder than 28 it won't freeze up your carboy if the override control fails, but it'll still work OK to lager. Set to a setting that gives this temp range before connecting up your override control.

Any problems or questions: Send me e-mail at WALZENBREW@aol.com

Cheers and Happy Brewing,

Greg Walz

Parts List:

(These prices are current as of 4/30/96. Except for the temperature controller module, you can find the rest of the parts at any electronic supply store.)

Assembly Notes

1. You can save money by buying a cheaper metal minibox to house the unit. Look around at other electronic stores. Make sure the unit is big enough.
2. You must use wire capable of handling the current for the refrigerator. Cutting up an air conditioner extension cord is a cheap and dirty way of doing this. A hardware store will have cheaper items of this nature than Radio Shack.
3. Mount the temperature control in a square hole cut out in the box. Use the four phillips head screws in the temp control to mount it to the box. A nibbling tool (Radio Shack 64-823 $10.95) will make short work of the square cutout.
4. The Fahrenheit/Centigrade switch is optional, as is the Heat/Cool switch. Run wires directly across the desired position if switches are not desired.
5. Mount the other components on the perfboard, and then mount the perfboard on the standoffs. Mount the relay on its side with a tie-wrap (Radio Shack 278-1642 $2.99 for 30). Be extremely careful when soldering to the relay so that strands do not short. Best to use electrical tape to wrap each 120 VAC connection after it is made. Do not use smaller gauge wire or speaker cable for the 120 volt connections through the relay.
6. The wire wrap wire is the best to use to run the wires out of the temperature control unit to the switches and transistors. Use 22-24 gauge hookup wire for the other applications and interconnections, or simply bend the component leads over and tack solder together (being careful as to shorts).
7. The temperature controller comes apart by removing the four screws. When soldering to this, remember:
   a. Use a small wattage iron, NOT a big soldering gun.
   b. Ground yourself before touching this, especially if there is static electricity in the air (likewise the 4011 IC).
8. Run the wires out from the temperature controller to the switches. As most of the switches have a common connection to pin 16, tie them together and run one wire to pin 16 out from the controller.
9. DO NOT TIE THE SIGNAL GROUND TO THE EARTH GROUND unless you want to destroy the unit. Keep these separate. Tie the Earth ground to the metal chassis box, but keep the signal ground insulated from the chassis box. Tie Pin 1 of the Controller to the Signal ground only.
10. Test the unit with a light bulb before using the fridge.

Operating Instructions

1. Mount the unit on the side or the top of the refrigerator.
2. Insert the temperature probe into the refrigerator. The best method is between the hinges on the door. Tie it inside such that it is about 6” - 1 foot from the bottom of the box, and touching nothing.
3. Set the refrigerator's built-in temperature control to its coolest setting. Plug the refrigerator into the unit, and the unit into the wall.
4. TO SET THE UPPER TEMPERATURE LIMIT:
   Push the MAX button and note the display. To change the setting, push in the MAX button and hold, and push the SET button to increase the limit. Note that the limit only increases; to decrease, hold in until 120 degrees is reached and the unit wraps around to -40 degrees and increases. Release both buttons when the desired temperature is reached. The refrigerator will turn ON when the interior temperature increases beyond this set point.
5. TO SET THE LOWER TEMPERATURE LIMIT:
   Same as setting the upper limit, only you use the MIN button instead of the MAX. The refrigerator will turn OFF when the interior temperature decreases below this set point.
6. Note that the difference between the MAX setpoint and the MIN setpoint should be 3 degrees or greater. Anything less will damage the refrigerator by causing it to cycle too often.
7. TO ACTIVATE THE UPPER AND LOWER LIMITS:
   Push and hold the MAX button and push the ACTIVATE button. Release both buttons. The word “MAX” should be visible in the readout display. The upper limit is now activated.
   Push and hold the MIN button and push the ACTIVATE button. Release both buttons. The word “MIN” should be visible in the readout display. The lower limit is now activated.
   For the unit to work correctly, both MAX and MIN must be visible on the readout display.
8. To read the highest and lowest temperatures reached inside, push and release the MEM button. The word “MEM” will appear on the display. Push the MAX button to see the highest temperature reached; push the MIN button to see the lowest temperature reached. Push the MEM button again to return the display to that of the current interior temperature.

The Schematic (18KB GIF image)