I have a fridge with two compartments, one for "fridge" temperature elements, and one for frozen foods.
As it's showing its age, I want to monitor the temperature in both parts to be sure everything is still edible.
For the fridge part, placing a zigbee sensor powered by a 2xAA or CR2032 battery is actually working fine, the battery is not too weakened by the 2°C temperature and the metallic body plates are not attenuating the signal too much.
For the frozen foods parts, it's a bit more complex as temperatures go as low as -25°C. On top of that, because the thermostat is is in the fridge part, it happened in the past that the compressor never turned on because the temperature in the garage where it is located reached 4°C, effectively within the thermostat set temperature.
As a result, I wanted to both have a quick way to check the current temperature, while having a history of said temperatures and to get this, I installed a zigbee sensor with a wired temperature probe like this:
This works fine but the probe cable is quite thick (3mm) leading to air leakage at the location where the cable is getting through the door seal.
Using a small scalpel, I was able to strip the outer insulation to get access to the three inner cables which are way thinner, hoping this would restore the seal effectiveness.
Sadly, while this is better, this is still not enough, ice is building up inside, starting along the three cables as can be seen here:
I had removed most of it two days before taking this picture and it's already back.
So now, I'm trying to figure out an alternative for this and NFC/RFID came to mind as this would allow to get a sensor without batteries getting frozen inside the compartment, thus not requiring any wire to get into it and thus no longer interfering with the seal function.
With the appropriate search terms, I stumbled across this reference design from TI: https://www.ti.com/tool/TIDM-RF430-TEMPSENSE
But this gets me even more questions:
- What cost for creating this board?
- What tool to program the MCU? and what cost for this tool?
- What to use to read back the temperature? I have RPI Pico W or ESP32 at hand for the wifi communication.
- Would reading work through the metal sides of the fridge?
- Would this work if the sensor is at 90° with respect to the reader?
For the last question, this stems from the fact that if question 4 answer is "no", then the RF waves would need to work through the 1/2 cm gap left by the compressed seal when the door is closed, as seen here:
From what I grasped reading various articles, the longer the distance between the RFID tag and the reader, the larger the reader's antenna would need to be. I even saw 60cm diameter antennas, which, well, won't fit here at all.
Do you think this RFID/NFC tag idea is worth exploring? Or would you suggest an alternative like cold resistant batteries if that exists?
Thanks for your insights.
