We grow a lot of apples. We slice and dry them, we make applesauce, apple juice, eat them fresh and give them away. We do end up buying apples from the store when our own fresh ones run out so this year we filled an unused upright freezer with apples. The freezer is in an unheated shed. This works pretty good while the weather is cool but when it drops below 10 degrees F at night the old poorly insulate freezer goes below 29 degrees F which isn't good for stored apples. I could have hauled the apples into the house for the night but I have quite a few even colder nights coming. This Arduino project is pretty simple. It measures the current temperature once every couple of minutes and turns on a 15 watt bulb if it gets too cold. I can think of quite a few enhancements but this will do for now.
The completed project shows the one-wire temperature sensor to the bottom right side. Above that is a Arduino Duemilanove (version just before the Uno) with a Sensor Shield on top of it. The outlet box houses all the high voltage components.
This image shows the small home built relay board which normally positioned below the outlet in the metal box. It has a 5V coil and 1Amp @125 Volt power rating. The coil draws 90ma which is almost double the maximum output for an Arduino digital output pin to is controlled by an 2N3906 transistor.
The bottom of the relay board was coated with silicone sealant and a small piece of plastic was added to insulate the 120 Volt wires. This relay board is really ugly, took a fair amount of time to build and the relay alone cost more than a prebuilt relay "brick". The brick also has a relay with 5 times the amp rating, a LED indicator and nice terminals for both control wires and output wires. I did have several relay bricks that were being used on other projects. Building this one makes me appreciate the work others have done building shields and bricks for the Arduino.
The small copper tab between the two screws was removed so the top outlet could be switched leaving the bottom outlet alway powered for the 9 Volt power supply for the Arduino. The switched outlet may get used for a very small fan used to circulate air when the light is on heating the freezer.
These images show my economy standoffs used to mount the Arduino and temperature sensor brick to the plywood.
The standoffs are a one quarter inch piece of aquarium bubbler tubing with a #4 by 1/2 inch sheet metal screw. My small wire cutter did a great job of cutting each piece of tubing to just the right length. It gave a consistent length and did a good job of making square cuts.
The Arduino sketch is attached. It runs in version 0022 and requires the OneWire and DallasTemperature libraries.
The comments in the sketch should explain it pretty well.
Add a humidity sensor
Separate fan control for circulation
Outside temperature sensor.
Mechanism to open the door or a vent in the freezer
Wireless communication to report temperature, humidity and heater status.
Wireless/ethernet connection to get the weather report to allow for preemptive warming or cooling based on forecasts.
Here is the heater in the freezer. Only about one third of the storage apples were left in the freezer when this picture was taken. The white device on the top shelf is a thermometer and humidity sensor with maximum and minimum memory. Since I wasn't logging temperature this was a cheap and easy way to see what the temperature and humidity are in the freezer.
As of 2020 we have five freezers set up this way. The four newer ones control the defrost heater element and circulating fan to provide just enough heat. We store apples, beats, carrots, cabbage, onions, leaks, and potatoes. Contact me if you have interest in the newer freezer code.