We used to put a marble and water in an empty camera film roll canister (transparant). Freeze in the freezer and flip over. If you come back and the Marble is frozen on the bottom of the canister: don't eat anything! #lowtech

I've always just put an ice cube in a ziplock bag. If it isn't square, that means it's melted & refrozen. The marble trick seems like it's probably more sensitive, but I also don't have marbles and film canisters laying around.

Low effort too: I just look at the ice in the bucket rose if it's frozen in a blob - or a pool of frozen water at the bottom. :)

This is a tip EVERYONE should know and I still do this! I use a half filled bottle of water, a small rock, and mark safe/not safe so anyone who's not me knows which way it should be.

Pretty sure it was my great-grandpa who taught me that.

I remember that my parents' old freezer had something like this built-in (using colored liquid instead of the marble, but the principle is the same). Seemed like such an easy and obvious feature.

I love low-tech solutions like this.

I dabbled in designing a cooler/freezer monitoring setup for a local farmer not too long ago.

The 2 biggest constraints I found were power and communication from within the freezer.

One promising approach was - a BLE enabled temp. sensor beacon with a coin lion battery - a nearby externally mounted unit (RaspberryPi) plugged into a wall socket as a relay - a small central 'black box' SSD/Wifi/router ground-station to collect, store and push the data to the internet

The biggest question was whether the beacon signal could get through the freezer walls. If not then I was going to look into the idea of some super flat cable to enable a wired sensor connection without compromising the freezer seal (like a meat thermometer in an oven)

Depending on the freezer, putting a hole somewhere and sealing it up really well is just as good as the walls and insulation that were in there before. I did this to a chest freezer that wasn't terribly expensive and I haven't ever noticed that the outside of the hole is any cooler than surrounding areas, nor did I notice that it had to work any harder to keep it cool.

That being said, I didn't exactly exhaustively detail the before and after performance of the freezer either. I used the smallest drill bit possible that would let me get a probe in there and everything else is external of the freezer down by the compressor motor where there is a lot of extra room- and input power to tap in to.

I was evaluating designs from a potential commercial opportunity perspective so keeping things non-invasive was considered a constraint for ease of use and to avoid warranty/liability risk.

Well then that makes a lot of sense for not wanting to put holes in it.

I keep approximately 5 minutes of temp readings (used a TMP36 sensor) at 1 minute intervals using an RTC (Real Time Clock) to more accurately time it. I made a small 'power tester' circuit that basically acts like a normally open button and closes when there's no power coming into the freezer. I may or may not have spliced into the wiring of the freezer for this and lied to my wife about it when she asked if I was going to burn the house down.

Side note: Is it just me or does it seem like you get asked this question frequently?

I have the 'button' circuit wired into an Arduino I/O pin just like you would an old normally open button. When the 'button' circuit closes the Arduino triggers a VERY audible siren I cut out of an old water leak detector. If I have 5 minutes above my temp threshold the siren goes off. I upgraded it over time to use a small LiOn battery and got a circuit to switch to it when incoming power is cut for whatever reason, and basically when in battery mode it's also siren time. I got through 10 minutes of siren battery testing before deciding it was good enough and I was tired of hearing it.

We've talked about doing something similar for the propane fridge and an activity sensor in my friend's cabin in remote central Oregon using a 3G breakout for communications but we've run into the issue that the 3G there is really spotty and we just couldn't reliably get a data connection out there. Also debugging electronics with no internet, running off generator and batteries, and in the dark because we were fishing all day doesn't lend itself to really getting the job done.

Since you have WiFi you may have what you need to do it more simply, although the 3G might be a safety net for the safety net once you get your solution done. My biggest problem at the cabin, besides the lack of cell coverage, is the lack of constant power. There's barely any solar power, and it's used to keep the batteries conditioned, and it's often weeks or months between people getting out there. I would think at your place you could pretty much just use my approach and add WiFi, or just use an ESP8266 instead of an Arduino and do everything else the same. The battery switchover stuff was the biggest pain in my butt, mostly just because I've not got a ton of LiOn experience.

EDIT: On the RTC and temp logging, it's really not necessary. I just did it to do it. You can use the oscillation ticks and a counter to get close enough. Hell, you can just bump the threshold up a little and raise the alarm as soon as you see it without logging anything and get a drastically simpler circuit.

Oops. "Nerf" - not "nerd". Although I think we'd both agree - both terms apply.