Does your refrigerator door resist closing completely? Or are you just absent minded in closing the door at all?
We're going to make a timed alarm that will buzz if the fridge or freezer door is not completely closed within a certain time using basic electronics and some scrounged pieces.
Step 1: The Problem.
Our freezer door wants to stay just slightly ajar.
We try to push it shut, but it will fight. Especially when overloaded. Other times, it's the toddler that just realized she's big enough to open the magic food doors.
The freezer running all night will strain the whole appliance, build up frost, and in our case, cause water to get around, and make the GFI breaker trip randomly, which means spoiled food.
The challenge is to make something that will alert us when the fridge is even slightly open, even if the light is not on, and not run out of batteries with regular use.
Step 2: Wait for It...
We're going to need a circuit for a buzzer that waits a certain period of time after receiving power before it starts buzzing.
The most available I could find was a 555 Monostable Delay-on Circuit. It's not complicated, and there is a nice calculator here that lets you calculate the delay in seconds by changing the values of the capacitor and the resistor in the circuit.
I opted for about 53 seconds, and had to combine some resistor values to get that close. It's not exact, so you should breadboard it and time it to make sure it works as you expect.
Consider timing yourself (or others) when looking in the fridge. We likely underestimate the time we spend with the door open, and if it buzzes every single time, you're just going to turn it off.
Step 3: Open Fridge, Closed Circuit.
Since our issue is with the fridge being slightly open, we need an adjustable trigger that powers the circuit when the door is open, and cuts power when the door is closed.
This will be easier on the battery usage since there is zero power consumption while the door is closed.
The best plan I came up with was a clothes pin in this little gap at the top of the fridge. The closed door should pinch the clothespin, and open the jaws, breaking the connection.
But we'll need to modify the clothespin to fit our requirements.
Step 4: Pinchy Power
We need wires to be touching when the clothespin jaws are closed, so strip a length of multi-core wire, and wrap it around the jaws, keeping in mind the final position of the wire in relation to the clothespin's final position on the door.
Cut extra wire, because the more you can wrap the tighter it will be on the jaws, and the unstripped portions will let you set the buzzer and power back further from the front of the fridge. You may use heavier multicore wire for the jaws, and then a lighter wire for the connection to the buzzer, it will mean the clothespin's position isn't meeting resistance from stiff wire.
Flatten the multiple wires against the inside of the jaws so there's more flat surface area for contact. You can use a single core so long as the alignment works out so the wires on both jaws are touching when closed.
I used a free flat magnet to attach the clothespin to the refrigerator. This was not strong enough, and you may have to double them up, use adhesive, or embed stronger magnets. There will be some torque on the jaws side of the clothespin when the tail side is compressed by the refrigerator door, and that may detach your trigger from the desired position.
Step 5: Power Up, Build Up
I'm powering the circuit with 4.5 volts, though it's not ideal for the buzzer I'm using.
I used this 3xAA series battery holder I had from a flashlight I parted out.
It serves as a good base to mount the circuit board and attach the remote wire so the solder points aren't getting tugged on.
Step 6: Install and Adjust!
Use a short wood screw (or stacked hard materials) to extend the pinching area of the tail of the clothespin.
Using the screw means you can adjust it up or down to match the door's position when the clothespin is where you want it. Be sure to pre-drill the hole for the woodscrew so you don't split that half of the clothespin. I had to install it furthest from the hinge because that was the only place where the door was opened enough to really measure.