Junkbox Freezer Alarm Keeps Steaks Safe

A fully stocked freezer can be a blessing, but it’s also a disaster waiting to happen. Depending on your tastes, there could be hundreds of dollars worth of food in there, and the only thing between it and the landfill is an uninterrupted supply of electricity. Keep the freezer in an out-of-the-way spot and your food is at even greater risk.

Mitigating that risk is the job of this junkbox power failure alarm. [Derek]’s freezer is in the garage, where GFCI outlets are mandated by code. We’ve covered circuit protection before, including GFCIs, and while they can save a life, they can also trip accidentally and cost you your steaks. [Derek] whipped up a simple alarm based on current flow to the freezer. A home-brew current transformer made from a split ferrite core and some magnet wire is the sensor, and a couple of op-amps and a 555 timer make up the detection and alarm part. And it’s all junk bin stuff — get a load of that Mallory Sonalert from 1983!

Granted, loss of power on a branch circuit is probably one of the less likely failure modes for a freezer, but the principles are generally applicable and worth knowing. And hats off to [Derek] for eschewing the microcontroller and rolling this old school. Not that there’s anything wrong with IoT fridge and freezer alarms.


24 thoughts on “Junkbox Freezer Alarm Keeps Steaks Safe

  1. I like what he has come up with. It’s one of those situations that could be monitored in a number of different ways. The limitation I guess is what is in your junk box on a particular day.

    Another way could be to use a plug pack to supply what ever voltage and a battery powered circuit to monitor the voltage – when the mains fails the plug pack v drops, alarm goes off.

    A photo transistor etc. to monitor the power indicator on the freezer.

    1. Except that his monitoring only covers one specific scenario: loss of power to one outlet. And you need only three components to do this: a 120VAC relay, a battery, and a noise-making device of compatible voltage and current to said battery.

      It does not cover a wider power failure (such as the entire circuit, as opposed to one outlet.)

      It does not cover motor failure.

      It does not cover “door didn’t close” failure.

      And it only “detects” failure after a long period of time. A deep freezer might go a considerable amount of time between cycles, so when the alarm trips, the freezer is already well below its normal temperature range.

      At the end of the day, all you care about is “is the freezer cold”? Always monitor the condition you care about, not “things that could cause it”, unless you’re really, really sure you have ALL the causes nailed down.

      1. Why would it not cover ‘full circuit failure’?

        I would say that if the time period is set appropriately (for example, maximum noted time between compressor runs is 4 hours, so set the alarm to 6 hours), then the freezer would only get marginally warmer than if it was operating normally. The time to respond to the alarm would probably be longer (eg: at work, away for the weekend). But that’s the problem with most alarms – you have to actually be there to do something about it!
        The same problem *exactly* arises if you were monitoring temperature (it has to get warmer than the limit, which can’t trip under normal conditions), so I see no difference at all – except that a freezer can fail in ways other than a power loss. Personally, I’d do temperature.

  2. I went the same route as mythought62. My very first esp8266 project was a wifi thermometer that reports to nagios. Since then, i built a thermometer for every room, and a thermometer for the fridge and freezer in the kitchen. On my todo list is a set of thermometers for the breaker box and utility pole breaker. (Mobile home)

  3. Hmm, kind of useful actually. In my house builder’s infinite wisdom, the outdoor receptacles on the back of the house are protected by a GFCI in the garage (on the front of the house), where we have a freezer. My wife tripped the GFCI when working on the backyard landscaping, and it took us a good 24 hours to determine where the fault was. Luckily nobody opened the freezer and the steaks were safe :)

  4. And if the freezer itself fails, you’re still f*cked. Why not just stick a thermo couple in there and send an alarm if the temp gets too high – takes care of all possible scenarios.

    1. Not really. Say it’s winter, garage unheated, power or freezer itself fails at 9pm, but garage cold, low temp alarm doesn’t trip, you sleep sound in a false sense of security, leave for work, the sun rises and beats on the garage all day, and the temp alarm valiantly bleeps and bleeps it’s little heart out, problem, there’s a problem! but there’s nobody there… it’s last gasp, it’s dying beep, is half an hour before you walk in the door… you eat out of the fridge that night, or maybe order in, it’s not until you get home the next day you smell the rot…

      1. If it is winter and the garage is unheated, or like us, the freezer is out on the front porch, who cares if the power fails? The stuff will stay frozen anyway. You set the hi temp alarm for a temp that gives you time to act, and it’s all good, electronically or naturally cooled, it really does not matter, as long as it is kept frozen.

        When I first heard your design had a 555 in in it AI thought you had set the 555 to trip and sound the alarm every couple of hours and used the current sensor to reset the 555 before it times out.

  5. [Derek] says the device operates by measuring the current, and sounding the alarm when it detects the freezer isn’t drawing any. So what happens when the freezer’s own thermostat decides it’s cold enough and turns off? Shouldn’t it sound the alarm?

    Look closely. There’s no ballast resistor and no electrostatic shield on that ferrite core that’s pretending to be a current transformer. Since its output is connected directly to the high impedance op amp input, it’s really acting much more like a capacitively-coupled voltage sensor. It’s going to give an output more related to the voltage on the line than the current flowing through it.

    So, it’s a battery-powered line voltage sensor, not a current sensor. How often does that battery pack need recharging?

    After 3 iterations of the same problem at my house, and me building similar devices, I bought a radio-linked remote freezer sensor https://www.ambientweather.com/amws09c.html . One unit monitors both my fridge and freezer, the readout shows current temperature and alarm setpoint, and is conveniently in the kitchen, where I can hear the alarm. $35 with two remote sensors kinda stung a bit, but it saved me much more than that in time to build an equivalent, and it’s already paid for itself in saved food at least twice.

    1. Paul, thank you for the input and your interest in the project! To reply to each of your points…

      There are other electronics within the freezer which draw current while the compressor is off… so yes, there is a dropout, but still current draw. I didn’t mention this in the video to keep it simple. Still works. You bring up a good point though – if your freezer didn’t draw ANY current while the compressor is off, then it wouldn’t work properly.

      Nope, no ballast resistor and electrostatic shield. I’ve played with the coil/windings extensively and I can assure you that acts more like a transformer than a capacitively-coupled voltage sensor.

      The battery pack stays plugged in – which I also don’t show, but assumed that the average reader would realize this.

      The ambientweather.com device looks interesting, maybe I will end up using this for something myself. The purpose of my videos / YouTube channel is to show that hey, you can make something even if it already exists; Have some fun! Don’t be afraid to get your hands dirty, crack open a book, website try this stuff out yourself. Not as a reference for taking a product to market – there are other channels out there for that. If you dig that, please subscribe! If not that’s cool too. Thanks for your interest in the project either way.

      Best regards,

      1. It’s great you found a USB power pack that will power its output while being charged. All of mine, from four different manufacturers, refuse to produce any output voltage when they themselves are being charged. Annoying. I’ll have to check out that MyCharge brand.

        It is fortuitous you have a freezer that draws power when it’s nominally not actually running. Most chest freezers have no electronics all all, just a mechanical thermostat and, except for the few milliwatts for the pilot light if present, draw zero power when not running. See for example http://www.refrigeratordiagrams.com/wp-content/uploads/2011/02/chest-type-freezer-electrical.jpg My full-size upright self-defrosting one does have a defrost timer in it that draws a few milliamps all the time, but even that’s not enough to detect with an ordinary KillaWatt.

        Kudos to your design if it really is detecting the milliamp or less the pilot lamp draws. Boo to the freezer manufacturer if it’s drawing more than that when it’s not actually running.

      2. That is the hacker spirit! Just because something already exists, or can be bought for less, there is little fun in buying a solution. If you’re a lawyer or accountant you *have* to buy a solution. But a hacker can do it from scratch, and that is where the fun is.

  6. This device would have come in handy this past summer. We had a lightning strike take out several circuits in my house, including my network equipment, the GFCI circuit in my basement, and the GFCI circuit in my garage connected to a freezer. I didn’t realize the freezer was out until it was too late for the steaks. :(

    I like the simplicity of this. It could be made with more features – ESP8266 / ESP32 to send an email or text and temperature sensing like others have mentioned, but for the failure mode of not having mains voltage getting to the freezer while you are home this would work really well.

  7. Here in sweden, we have a even smarter solution for the fridge/freezer:

    A separate GFCI on just the fridge/freezer Circuit.

    I really love it, because if you trip the GFCI in the rest of the house, it wont make your steaks bad, but if an electrical fault happens IN the fridge/freezer, and current goes through the casing and then the ground wire, you WANT the GFCI to trip because else you can get a shock by touching the metallic casing/handle of the fridge/freezer.
    (Your, or your wife’s Life is more Worth than a tousands of dollars of steaks in the freezer)

  8. My mom had an extension cord socket fail when they were on vacation, was not obvious until my brother stopped by to pick something up and smelled something wrong – so he stopped by where I was working and told me about the smell and how he did not have time to “investigate” was a full size standup freezer – took me some time to get all the dripping meat and stuff to the garbage cans and clean things up –

  9. Keep extra space in freezer filled with mostly filled water bottles-jugs-2liters. Helps ride out shorter outages and insures stable temps with funky thermostats. If you don’t have a backup plan, keep plenty of charcoal on hand and a big barbecue grille. That’s what happened in Houston when the floods came, a Texas sized barbecue.

    1. Happened here too. Whole neighborhood broke out into an everyone cooking/everyone invited BBQ on day 3 of power fail. Most fun ever had! Those that were a bit more prepared got a few days further out. Folks with large freezers covered them with blankets and got through it fine. Just plan ahead is all it takes.

  10. Fill a water bottle halfway, freeze it, then place it upside down. Check it every time you get something out of the freezer. If the ice is still at the top, you’re good to go. If it’s at the bottom, throw away the meat.

  11. Tried to post this comment yesterday but something wasn’t working right so I’m trying again…

    It was my understanding that the National Electrical Code allowed one non-GFCI outlet in the garage for the express purpose of being able to plug in a freezer without nuisance tripping shutting down the freezer. But a google search for a source on this seems to show that might no longer be the case. I found pages going both ways, some saying that all garage outlets must be GFCI protected, others saying that you don’t have to have GFCI protection on appliance outlets.

    I also found a page that says the NEC does not prevent a homeowner from removing a GFCI outlet and replacing it with a non-GFCI one. But I would only recommend that if the outlet the freezer is plugged into is the only one on the circuit. You really want to keep GFCI protection on all the other outlets in your garage. Thing is, if the freezer is already on its own circuit you won’t have tripping when plugging other things into other outlets anyway.

    If you’re worried about such things I’d recommend, as others have said, some sort of device for indication that your freezer has thawed and been refrozen. If you go on vacation for a week and the power is off for two days early in the week, when you return everything may be frozen and it won’t be obvious that it’s been thawed for a while and might not be safe.

  12. Freeze a cup of water. Put a coin on top. If next time you look at it the coin is just under the surface of the ice the freezer was off for a bit and restarted. If the coin is halfway down or at the bottom everything thawed and refroze. If you replace the coin with a magnet and put a magnetic reed switch to sound a buzzer you have a simple freezer alarm which would go off even if the freezer did not fail but was only partially closed. Something which happens way more often in my experience. Something shifts and blocks the door a bit usually.

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