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Comment by roland35 | Hacker News Reader

I am all for dumb appliances over the current crop of "smart" appliances, but one feature I would LOVE would be some kind of sensor log of my refrigerator! This would help to know if the power went off while I was gone - did the temperature rise too much? That is all I need...

The same would apply to other appliances and HVAC, just simple data to know everything is working correctly.

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The fridge is trickier, but for your freezer fill a small plastic cup with water and let it freeze. Then place a penny on top of it. If it gets warm enough for the water to melt the penny will drop.

Put a bottle water in the freezer until it's frozen. Turn it on it's side. If you notice the ice-air line isn't vertical, you had a warming event.

There are heat-sensitive stickers that change color if above some threshold.

It's on packaged meat in Norway I believe.

that would be a good single use solution, we generally only a long power outage once every few years so that could work.

You could do this today without any sort of "smart" appliance for about $1.00, $50 if you are being lazy. Just get a super simple micro-controller and radio on a board and throw it into your fridge. Have it record the temperature at a regular interval into a ring buffer and poll it periodically from a chron script. Get your power from the light switch in the fridge door. Alternatively, a Li-po or Li-ion battery can last an extremely lond time powering a little micro in a refrigerator. If you put a $0.02 photo diode on it, you could also track how often the door is open.

If you went the full on $50 route and bought a pre-integrated board with wifi and a full linux stack, you could even have a push notification to twitter or email or whatever when the temperature goes out of spec.

Or even simpler, put a cheap analog thermometer which records the min and max temperature in your fridge.

Maybe not exactly this one, but something like that:

https://images-na.ssl-images-amazon.com/images/I/61jmAUQ70pL...

I bought a digital thermometer with min/max from Walmart for $5, but the problem was the temperature would rise quite a bit (above 40F) if the door was left open even for ~10-30 seconds, so it was hard to use the "max" temperature reading.

It would have been more handy if the thermometer's min and max responded more slowly to reject these short fluctuations.

You could put it in something with substantial thermal mass as a buffer.

If all you want is to check for power outages, you could use a Raspberry Pi Zero that has a program that notifies you when it starts, and constantly logs a heartbeat to a file. If you run the program at startup, you'll be notified when the power comes back on.

I have a smart things hub which is pretty good for this purpose - but the main problem is to make sure the baby's milk stays good so I would like confirmation at the end of a power outage that the temperature is still acceptable.

Luckily the power is reliable here, but as they say, winter is coming!

I used these [1] with remote thermocouples to track temp mostly in curing epoxy for my biz, but also for freezers. They work great, and should for your freezer application too. Get the one with the display and you can view the current/hi/lo temps anytime, and grab the data for extended review. I've found that I can see the freezer's on-off cycles very well, pretty much the expected sawtooth wave...

[1] https://www.lascarelectronics.com/data-loggers/all?type=usb

edit: no commercial connection, just a happy customer

There are non reversible temperature and moisture sensitive labels.

Just shove in a $10 zigbee temperature monitor.

I can't believe I haven't thought of that ha! I'll have to check it out - hopefully I can get a zigbee signal through the fridge itself (I am not sure if it is a Faraday cage or not)

Why do you need a signal? You don't care about the temperature until you open your fridge to either consume to toss food, do you? Buy a digital thermometer for ten bucks, one that keeps a record of max/min, toss it in there, look at the max reading when you return from out of town.

If your power goes out you'll need aux power for whatever is reading the signal, otherwise it also won't work. You are making a huge project with all kinds of failure modes when this problem has been solved for ages. Kind of like the house with no mechanical light switches that you are responding to!

I mentioned it up earlier but I did try out a simple battery digital min/max thermometer, but every time you opened the door the air temperature would rise above 40F (highest recommended temp to store dairy), which made it hard to use for the intended purpose!

If the thermometer had more thermal mass attached to it (or simply had a low pass filter on the readout), it would have worked great.

I also had an analog thermometer at one point, but obviously the problem here is if the power goes out, then comes back on and returns to the proper temperature.

It should be a simple thing I agree!

"If the thermometer had more thermal mass"

Put it in some water?

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