Don’t Lose Your Cool With This Fridge Buzzer

A composite picture with a 3D printed cylinder with a magnet at one end held in a 3D printed housing ring on the left composite picture and a fridge buzzer board with buzzer, CR2032 battery, MCP430 microcontroller and hall effect sensor slid into a 3D printed base on the right part of the composite picture

[CarrotIndustries] wanted to add an audible warning for when the refrigerator door was left open. The result is a fridge buzzer that attaches to the inside of a fridge door and starts buzzing if the door is left ajar for too long.

The main components of the fridge buzzer consist of an MSP430G2232 low-power MCU connected to a SI7201 hall sensor switch, along with a CR2032 battery holder, push button and buzzer. The MSP430’s sleep mode is used here, consuming less than 3 µA of current which [CarrotIndustries] estimates lasting 9 years on a 235 mAh CR2032 battery.

A 3D printed housing is created so that the board slides into a flat bed, which can then be glued onto to the fridge door. The other mechanical component consists of a cylinder with a slot dug out for a magnet, where the cylinder sits in a mounting ring that’s affixed to the side of the fridge wall that the end of the door closes on. The cylinder can be finely positioned so that when the refrigerator is closed, the magnet sits right over the hall sensor of the board, allowing for sensitivity that can detect even a partial close of the fridge door.

All source code is available on [CarrotIndustries] GitHub page, including the Horizon EDA schematics and board files, the Solvespace mechanical files, and source code for the MSP430. We’ve featured an IoT fridge alarm in the past but [CarrotIndustries]’ addition is a nice, self contained, alternative.

28 thoughts on “Don’t Lose Your Cool With This Fridge Buzzer

    1. Because this would need a more expensive solution than a simple thermostat. Modern fridges have a digital temperature control and display. And also a temperature drop based alarm.

      1. Nope, got a new simple freezer. (Don’t need an iot thing here) It has nothing. An led indicating it has power and that’s it!
        No door sensor, not even a buzzer on the board.

    2. Those type of thermostats are usually called “constant cut-in”, where the temperature that the refrigeration system comes on is unaffected by the dial setting. What you’re actually setting is how long the system stays on, or “how cold should the evaporator get” in other words. There’s some correlation between that and the temperature in the box, but not directly as you would intuitively think from your experience with household HVAC thermostats.

      1. Here ya go@Dan, the magic words are “555 monostable aka one-shot”, as demonstrated here:

        When S1 closes, the LED lights for 2.5 seconds (Pin 3 is high), then goes off. Increase R1 & C1 to get a longer period (eg 60 seconds).

        Now this triggers as soon as the door opens, move the LED so it’s between Vcc & pin 3, now it’ll be off until Pin 3 changes state (now sinking current). Replace the 555 with a MOSFET & buzzer, and you’re off to the races.

        And this beats the original circuit as its standby current is 0mA.

        As @David says, simple. I mean Google throws up plenty of circuits…

        The real old codgers will say “Why, all you need is an RC circuit and a few transistors”, and they’d be right too.

    1. Such a buzzer is included in my freezer and it works for sure without a microcontroller as it is more than 30 years old. It signals an open door or too hiv´gh temperature. You can disable it by pushing and twisting the switch button.
      If I would build such a device, I would start with a capacitive dropper PSU, something like a 40106 or 4093 CMOS IC for delay and beeper-oscillator and a few R and C. Then connect this contraption in parallel to the lamp.

    2. Especially ones that literally show you the inside temperature 24/7 on the front. You really needed to save $1 in parts to then prevent the user from paying far more than that in electrical bills or losing hundreds of dollars worth of food and possibly also damage things outside of the refrigerator or freezer?

  1. If I EVER get time to finish the project I did something for similar effect. As part of my home automation project I built and ESP8266 module that reads from 2 temperature sensors (one for the pantry refrigerator, the other for the pantry freezer *though I need to add another sensor as the refrigerator went on the blink and the replacement is a refrigerator/freezer combo*). The original worked fine, but I would get constant complaints from the spouse when I would call home to tell her the system ratted her out, and that the door was left open (as judged by the temperature going out of spec).

    So, Gen 2 (still on my work table) has a gentle british voice that tells those at home that the door is open. It will also include a huge “Shuddup” button that will mute the audio for 5 minutes while the temps stabilize. This is in addition to logging to home Pi automation system and sending e-mail and text alerts to me.

    Mind you, this has a simplistic charm about it.

    My son complains about the door auto closing feature. I told him “if you hadn’t left the freezer door open after getting ice cream a few months ago I would adjust it. As it stands the excuse ‘I thought I was going to put the ice cream back, but finished it instead’ doesn’t work with me”

    I really wish they’d build appliances with decent margins for operation. This “Going Green” crap is sending more appliances to the landfill. I don’t know how that’s Green.

  2. This “hack” falls into the category of coming up with the most elaborate possible solution for one of the world’s simplest problems. It might have been a fun project, but its the kind of thing that would get an engineer fired at a real company.

  3. yes, most refrigerators have a beeper now that alarms when the door is ajar. But it does not sound when the door is only a tiny bit open. On our LG the freezer door pushes just a little bit open when you close the main door a bit too fast. It’s the air pressure from the closing door that pushes the freezer door open. You won’t notice it until you see all the frost accumulated in the freezer compartment next time you open it and the door ajar alarm never comes on.. So I think this project has merit and the implementation seems simple enough.

  4. I built something like this years ago for my freezer door. Used an ATTiny85 and a normally-open reed switch. No need for coding a low power mode. The switch applied power whenever the freezer door was opened. Simple code starts the buzzer chirping 60 seconds after it gets power.

  5. Sometimes those adjustable feet are rusted in place, plastic crap, or the floor sags just enough to resort to shims under the front two feet. Genius, restored self closing door and convenience.

    Now bring back the foot pedal that opened the door on those early monitor-top style fridges that had a RV style latch on the closed door and you’re holding food in both hands to put inside.

  6. I prefer to use this solution for long battery life. See random ner tutorial for the latching circuit. Amazing. Search for “EXTREME POWER SAVING (0µA) with Microcontroller External Wake Up: Latching Power Circuit”. It latches to zero power until opened. I’m skeptical of nine years mentioned above, batteries self drain too.

  7. I use a cheap 433MHz Chinese temp/humid sensor inside the fridge. It’s interesting to see the temperature graph over time. When the temperature gets over threshold my home automation sends me a message over telegram.

  8. A processor, several lines of code, a dozen parts, a custom PCB. This is where the analog world beats the digital world hands down. 4 parts are needed to do this in analog form. 4. No CPU, no software, no custom PCB. Just a 555, a resister, capacitor and buzzer.

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