Improving fridge thermostat

Since some time my fridge’s thermostat started to work erratically, so I decided to replace it. But, you know, do it in a normal way: go to the spare store, buy a new one, and replace it (2 hours top) it’s not an option for a geek.  So choose the hard (and long way): make a new one. Course it should be much better than the old one, so I designed it with some features I want it:

  • Configurable “on temperature”, and “off temperature” (hysteresis)
  • A configurable minimum off time, to allow defrost.
  • A configurable maximum on time, to  allow defrost also, and to cool down the compressor if something happens (a door left open)
  • An automatic backup if something fails (a mechanical thermostat is not prone to fail like an electronic)

Thermo

My thermostat while I was testing it

So with all this a made a simple program in a PIC16F88, a use a LM335 for temperature sensing and two relay: one for the thermostat and another one (using an NC contact) to put in parallel the old thermostat in case of power fail (my PIC power source) or a cut in the sensor cable (implemented by software).

As I don’t wanted to deal with buttons, displays and so, I use a piece of code from other project: a home network over RS485. I’ll write some day about it, but basically I designed (from the ground) a communication protocol between several slaves (pic’s) and one host (a PC). I plan to use it to automate my future home, but so far I have three devices in my network, one of them: my fridge.  So, with commands, from my PC I can change temperatures , times and turn on and off the fridge. And everything without any button or display. But also I can log the temperature over the time.

ThermGraph

Temperature over time. The two anomalies where the door opening (dinner 10pm, a coke 1.30am)

I plan to use that (objective) information to make some investigations regarding how the fridge performance is related to: refrigerant pressure, insulation, distance from the wall, room temperature, etc. And also, to find the right mass to the sensor, because it’s not a good idea to leave the sensor naked to the air, because if you do it that way when you open the door the temperature will raise (temporary) and the fridge compressor will start. But what is important is the food temperature. So I think that the mass of the sensor should be equal to the less massive peace of food that is sensitive to temperature. But that’s theory… I’m not sure yet.

I have a really long wish list (It’s incredible how many things I would like to add to my fridge if I had the time) But so far I ‘m very happy with my fridge with RS485 interface ;)

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