This device will monitor the temperature inside a freezer or fridge and will send alerts via email when the temperature rises above a certain threshold. It will also track the length of time the temperature is in the 'danger zone' where harmful bacteria may grow faster. It should also be able to detect if the door is open to avoid food spoilage.
This device will be useful in areas with unreliable utility electricity or vacation homes that are vacant during the week.
There are many of these types of devices that can be found on the internet, so to stand out this device needs to have easy to use and useful features while minimizing cost.
The original idea is to monitor the temperature in a freezer, but with some extra mounting type and enclosure considerations this should be able to monitor temperature anywhere - basement, attic, outside, garage, car, etc.
I came up with a simple hardware block diagram and chose a few parts that will make up the main system components. I think I should remove low cost from the title because I don't think it will be as inexpensive as I was hoping.
I thought about various configurations that the system could be in and decided that I wanted to put everything in a single package. This may cause issues in the future as batteries do not run well in cold and I'm not sure how well wifi will penetrate the walls of a fridge. I will try to put everything in a single package for easy installation and if there are issues then I will try to work around it or change the design.
I've attached a basic hardware block diagram and I would like the system to following way:
Achieve lowest power possible by turning things off or entering sleep modes
Wake up periodically to test temperature and battery voltage and store data to memory. If there is a temperature issue then wake up on a higher frequency period.
When the door opens, I want the system to wake up and display the current temperature
Access to data through a phone app
phone app to change alarm thresholds for temperature and battery discharge level
I chose to use an RTD over a cheaper type of temperature sensor to have some decent accuracy since this is for food safety.
I have decided to use the ATwinc1500 wifi module over ESP8266 and its derivatives due to a lower current consumption in idle mode.
I will probably use the ATmega328pb MCU since I already have libraries made and I am comfortable with it. For a final product the MCU should probably be optimized for what the system actually needs.