Level: Small project
GitHub Repository: -
What is "Freezer Controller"?
As the name implies, the Freezer Controller is something that controls a freezer.
Why "Freezer Controller"?
One day my freezer broke. It would turn on until it reached -30ºC and then would turn off.
I initially called support and they changed the main circuit of the fridge,
since it still wouldn't work, they then changed the refrigeration probe.
Since it still didn't work, I called them again, they did nothing and still charged me 70€ for the visit.
Since a new freezer is too expensive, and is difficult to fit into the current location of the frezzer in the kitchen, I decided to fix it.
Fixing process
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Tried different boards: Since at this point I had several boards, I tried all of them to reduce the probability of a broken board. None of them worked.
- Tried different probes: At this point I also had several probes, I even bought a new one to try out. None of them seemed to work either.
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Modelled the probe: The electrical resistance of the probe changes with temperature. I modelled the probe to visualize the resistance change with temperature.
By introducing it on another known good freezer, I got the evolution of the resistance with temperature from room temperature to -20ºC (left). I also got the oscillations that happen on stationary behavior (right).
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Controlled the probe using an Arduino UNO board: I connected the Arduino UNO to the relay that activates the freezer, and I monitored the value of the probe to see if behavior was nominal.
By removing the control over the relay from the board, I found that the behavior of the freezer was nominal, and that the problem was somewhere on the control board.
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Controlled the probe using a custom PCB: As the Arduino was too bulky and I needed it for other projects, I designed a PCB to substitute such a simple behavior.
This was achieved using an inverting comparator with hysteresis.
Following the steps described in video: https://www.youtube.com/watch?v=AOQOfp50KQM i managed to design a circuit that would fullfill my needs:
Since the simulation did not work for any comparator in Autodesk Fusion, I actually used Proteus to run the simulations. I also ended up purchasing another chip: LM311, as it was the only one available in my nearby hardware store. Only minor adjustments had to be made on the pin connections to the final PCB design.
Additionally, I rendered the final PCB and even added some images to make it look more like a fully built product.