This Custom Freezer Control Board Has an Interactive Screen and Several Innovative Features

The importance of refrigeration

Modern life is made possible by refrigeration, as it is how we store nearly all of our highly perishable/non shelf stable food. But what happens when that system of compression, evaporative cooling, and refrigerant chemicals breaks down? For Yaseen M. Twati, this became a reality when he discovered his freezer had stopped working. After investigating the temperature probe and compressor, the problematic component was narrowed down to a bad control board, which handles the interactions between the temperature readings, the target temperature, and when to turn on the compressor.

Building a replacement circuit

Once the original board had been sufficiently studied, Twati set to work finding the microcontroller board that would fit this project the best. In the end, he chose the Arduino Pro Micro due to its compact size. From here, a simple circuit was assembled on a breadboard consisting of the Pro Micro, a relay, a temperature sensor, and one circular screen to view the output in real-time.

Next, Twati made his schematic and board layout in KiCad before printing the exposed copper areas and etching the board with a solvent. A little soldering later and he had a fully functional circuit.

Closed loop controls

Instead of the traditional proportional, integral, and derivative (PID) loop that is commonly seen in systems that rely on readings and things that can affect those readings (such as a heater or DC motor), Twati went with a state machine. In each iteration, the current temperature is checked and compared with the currently set one. If it's within the range and enough time has elapsed, the compressor shuts off, otherwise, it continues to cool the freezer until either exceeding a time limit or reaching the target.

The interactive menu and logging

Wanting to go beyond the bare bones for this project, Twati incorporated a digital menu system, which lets users enter precise temperature set-points via a rotary encoder and button. Furthermore, he needed the ability to track the exact behavior of his freezer over time to find potential problems later on. By connecting an SD card reader and real-time clock (RTC) module, the Arduino Pro Micro can append a new line to a text file containing several useful data metrics for every entry.

Safety features

The last couple of features involved increasing the amount of redundancy and the number of checks to ensure the freezer would fail safely if a problem were to occur. First, the compressor is limited to a predetermined runtime so that it can't overheat from being continuously active for too long. And second, an additional temperature probe can be attached to the compressor that allows the Pro Micro to shut it down if it still manages to overheat.

To see more about Twati's project, you can watch his build video here on YouTube.