ColdSnap - Air Conditioner override controller
The ColdSnap is an air-conditioner override controller similar in purpose and basic function to the commercial "Coolbot" controller, but is simplified in that it does not have a display and input buttons; basic parameters are set in the program which is then downloaded to the Arduino ProMini microcontroller via a USB connected interface (FTDI Basic). A design option allows the adjustment control to vary the temperature setpoint without reprogramming; the range of the adjustment can be set when the Arduino program is compiled and downloaded (39F to 49F for instance). The idea was to keep it very simple, knowing that most people do not have a digital display in their refrigerator, and that you generally will know what temperature you want to hold and if it just simply works and is quite inexpensive it will be useful. The program includes a Deadband parameter that the user can specify - this allows the unit to minimize cycling - it remains off from the setpoint for the specified range above the setpoint; a 2 or 3 degree Deadband (differential) might be a typical choice.
There are many ways that one can accomplish the functionality of the air conditioner override. This project is given as a working and tested example of how it can be done, with specific parts, schematic, and code provided. It should be easy for people who work with Arduino to build this project, or for people to port this to another microcontroller they are comfortable with. The Pro Mini 328P unit is a very small microcontroller board and the USB interface is not normally needed in the final project, so it saves money and space to go this route. This project cost me less than $10 to build using an Arduino Pro Mini clone; using an authentic Arduino Pro Mini would cost only a little more. The FTDI Basic tool for downloading code to the Arduino Pro Mini (Atmega 328P, 5V) costs $7 to $15 (ebay/Sparkfun pricing), but the FTDI Basic is reusable for reprogramming or programming additional units.
Probably the most difficult part of the project is the construction of the temperature probes that are used. The probes are made with tiny and inexpensive chip thermistors that get soldered between 2 wires and then encapsulated with epoxy (and a protective brass sleeve for the FINS sensor that gets jammed into the coil fins); the chip thermistor used is very well characterized and so the probes are interchangeable with no calibration or changes to the code. But the thermistor chips are 0.5mm x 1.0mm and require some careful work to make, but they are small and will work with this design with no changes. It is perfectly possible to use other thermistor probes, but I had trouble finding ones sufficiently small or well-characterized like the Murata chip thermistors, and other probes will require the values at key temperatures to be determined and placed in the code. (This is actually not very hard to do since the finished unit, when connected to the computer with the FTDI Basic interface, can provide diagnostic messages to the Serial Monitor (Arduino interface) and if the probe is placed in a selected temperature, the "counts" are displayed so they can be tabulated and entered into the code as the control "counts" for the chosen temperature.
Please visit the ColdSnap web pages where I have more flexibility and space for images, including thermistor probe fabrication. http://people.umass.edu/~dac/projects/ColdSnap/ColdSnap.html
Finally, this is a DIY project - I can't build them for people. I wish that a 2-sensor-input controller board that conforms to this design was available from some one so non-builders could get an easy start. The CoolBot has some patent applications and so I'm not sure of the status of providing a fully functional (code-loaded) ColdSnap, but a generic 2-sensor-input controller board that the user then separately downloads the free, available, code onto would not infringe on a patent. I fully support anyone who would provide such a controller board, and will be working on a design myself until I see something else available. I have tried to get some vendors to provide this product/service but haven't had a taker (low volume expectations probably). I am also working on incorporating a Bluetooth interface for getting/setting parameters without reprogramming and without the need for front-panel displays and input keys. I am aware of one farmer who intends to incorporate Wi-Fi connectivity for data logging on ThingSpeak. There are many possibilites. Be open, share.