Hi Becky!
I haven’t fully fleshed out the idea yet, but I want to make a little shelf attachment for my fridge door, that can read if my vegetables are still edible.
I’ve been looking at a combination of pH sensors and ethylene sensors (rather unsuccessfully).
The pH sensor is quite expensive (https://www.dfrobot.com/index.php?route=product/product&product_id=1025), so I will look at making my own, following these instructions: http://overskill.alexshu.com/cheap-ph-meter-hack-for-arduino/
The ethylene sensor seems to be rather expensive/inaccessible as well: http://www.winsen-sensor.com/products/4-series-electrochemical-toxic-gas-sensor/me3-c2h4.html
So now I’m looking at this methane sensor: https://www.sparkfun.com/products/9404, as decaying food will also emit methane.
EDIT 1:
Sketch of vegetable freshness monitor, using pH sensor:
The box would sit inside a shelf in my fridge door. The sensor would read the pH level of the water that the leafy green vegetables are sitting in (+ other veggies that sit in water), and translate it to a freshness level out of four bars: the fewer bars that are lit up, the less fresh the veggie is!
EDIT 2:
I was considering that the methane sensor wouldn’t make a lot of sense in combination with veggies, but perhaps I could translate it to work with spoiled meat:
So the idea is generally the same (in the sketch I am playing with the idea of red-yellow-green lights instead of bars), except that the meat sits in separate, closed compartments such that the air can saturate with methane.
I will update this post throughout the day with more details as I come across them—just wanted to make sure that I got it up on time!