DIY refrigerator door alarm

Today I will teach you how to make an excellent energy saving refrigerator door alarm for less than 2 USD.

It happens to everyone in every home around the World. You are about to open the fridge door and you notice that it was left open, often overnight!

When the fridge door is not shut tight the cold escapes and your fridge thermostat will make the motor (actually it's a compressor) run non-stop to try to maintain the cold temperature. Refrigerators are power hungry appliances and any unnecessary energy consumption is a waste that adds to your electrical bill and harms the planet!

Remember that about 2/3’s of all electrical energy is generated by heavily polluting sources like coal, gas and oil. So, any waste should be avoided to help preserve our planet!

This problem affected our home, so I looked for a solution. I searched for refrigerator or fridge alarms but, surprisingly I could not find an effective refrigerator alarm. I did found an alarm that replaces the fridge light bulb, but there are two potential problems with this solution. Since the light bulb replacement alarm is placed inside the fridge it may not be always be heard and (equally bad) the alarm may not be activated if the door is only partially opened, which is what usually happens. This lead me to consider developing a fridge door alarm that would be reliable, easy to build and cheap. But then I remembered I had some cheap window/door alarms in my “Made in China” gadget bin. Why reinvent the wheel when a solution is already available? Surely they could be hacked to work as a refrigerator alarm, right? Well, as a matter of fact they can!

A DIY REFRIGERATOR ALARM FOR UNDER 2 USD

After some experimentation I found that “standard” window/door alarms could be used as refrigerator door alarms with a simple “mod” or “hack”. These alarms are very small and come with double sided tape for easy attachment. They are loud and durable. I have made this hack a few years ago and it still works reliably. With the suggested hack the batteries (3x AG13) can last for years without needing replacement.

Generic Window/Door Alarm

HOW IT WORKS

These alarms use a magnet to switch the alarm on or off. Inside the alarm there is a reed switch which consists of two flexing metal contacts inside a small glass container. If the magnet is near enough the alarm, the reed switch contacts touch each other and the alarm will not be triggered. As soon the magnet is removed, the reed switch contacts open, the alarm is triggered and a loud chirp will sound immediately.

1. Inside a window/door alarm.

THE WINDOW/DOOR ALARM HACK

My hack consists in introducing a delay in the alarm triggering circuit, so that the alarm will not be triggered immediately, but only after a certain amount of time. 

2. Alarm trigger delay circuit.

This delay is determined by the values of capacitor C1 and resistor R1. Point P1 is at low logic level (ground). When the magnet is near the reed switch, the contacts are closed and point P2 is also placed at a low logic level state. When the magnet is distanced from the reed switch, the reed switch contacts open, point P2 is no longer grounded and C1 begins to charge. When C1 reaches a certain minimum charge level, P2 goes into high logic level state and the alarm is triggered. I determined that about 1 minute was the ideal alarm delay time, but you can make the alarm sound earlier or later than 1 minute by using a capacitor with a different value. A bigger capacitance value will increase the alarm delay time. A smaller capacitance value will decrease the delay time.

Parts:
Windows/Door entry alarm (< 2 USD)
C1 22µF 10V electrolytic capacitor (for 60s delay)
R1 2.2K 1/4 W resistor
R2 100K 1/4 W resistor

To make this “mod” or “hack” you will need a low power soldering iron. A multimeter or voltmeter is recommended but not essential.

Notes:

• The capacitor can have any voltage rating above 6V, but above 25V it may not fit inside the alarm plastic enclosure.
• The resistor values are not critical, you may use approximate values.
• You can use salvaged resistors and capacitors from old or faulty electronic devices (radios, computer peripherals, etc).

Step 1 – Buy a window/door alarm

These small alarms are very cheap and can be found on many places. On the Internet you can buy them on eBay, Banggood, Dealextreme, and many other sites and with free shipping. Locally you can try Pound shops (in the UK), 99 cents stores (US) and many other stores that sell cheap Chinese electrical and electronic items. You may support this blog by buying from DX or Banggood. Your support is appreciated.

Step 2 – Test the alarm

Place the magnet next to the alarm and align the by the markings on the alarm and the magnet. Turn the small on/off switch to the on position. The alarm should remain silent. Now move the magnet away from the alarm. The alarm should be triggered and sound very loud. Turn the alarm off.

Step 3 – Open the alarm enclosure

Open the battery compartment and unscrew the small screw located bellow the battery compartment.

Step 4 – Make the following modifications to the alarm circuit

3. Hacked window/door alarm solder points.

1 - Perform a continuity test (with a multimeter) between the battery compartment negative contact (blue wire) and points 1 and 2. One of these points should show a short to the battery negative contact. Take note of which point (P1 or P2) is connected to the battery negative contact.  

2 - Solder one lead wire of a 22 µF electrolytic capacitor to one of the points (1 or 2), making sure to respect capacitor polarity. For example: if point 1 is connected to battery negative, then solder the capacitor negative lead wire to point 1 or the capacitor positive lead wire to point 2. The polarity of electrolytic capacitors is marked with a stripe or a ( - sign) along the negative lead.

3 - Solder one lead wire of a 2.2K Ohm resistor to the other point. Trim excess lead wires and solder the resistor to the other capacitor lead wire. When soldering is finished it should look like image 4.

As it is the alarm should now work with a delay of about 1 minute. Test it.

The alarm is now ready to be installed on your fridge. But, if you find the alarm too load you can add a resistor (R2) between the speaker (piezo buzzer) and the circuit board.

4. Hacked window/door alarm for refrigerator door.

To reduce the loudness of the alarm:

Take note where the two buzzer wires are soldered on the circuit board.
De-solder one of the wires from the circuit board.
Solder a resistor (R2) to the point on the board where the speaker wire was connected before.
Solder the other lead of R2 to the disconnected speaker wire (point 5).

The suggested resistance value for R2 should lower the alarm sound volume considerably, but still make it clearly heard. However, you can try different values until you get the volume you want.

FRIDGE DOOR ALARM INSTALLATION

You can place the alarm anywhere alongside the refrigerator door. However, it is best to place it high on the door. This way you can easily reach the manual switch to switch the alarm off, in case you need to clean your fridge.

 

These alarms come with double sided tape attached. Remove the protective paper from the tape and press the alarm aligned with the edge of the fridge cabinet.

Now you need to attach the magnet to the fridge door. Here we must be creative, since the distance from the fridge door to the fridge (and the alarm) is too big. You can use a piece of clear acetate film and attach the magnet to one side and attach the opposite side to the fridge door. You can hold the acetate film in place by putting it between the door handle and the door or using double sided tape. Before attaching to the fridge door, test the correct distance for alarm activation and de-activation. The switch can be heard clicking as you approach and distance the magnet from the alarm. The alarm should be triggered with the door barely open. Since the magnet is not going to be screwed you can cut both ends of its plastic housing. I did not bother since it is out of sight.

Hacked window/door alarm on refrigerator door.

Tips:

• If your refrigerator as two doors you can use two alarms or attach the alarm to one door and the magnet to the other. If either of the refrigerator doors is opened, the alarm will be triggered.
• If your refrigerator is not white but silver, just spray paint it with silver colour paint.

That's it. Your refrigerator is now protected against accidental door openings, saving you money on your electrical bill and at the same time helping to save the planet!

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Buy the parts
Buy the Window and Door alarm from Banggood
Buy the Window and Door alarm from Dealextreme
Buy 12 Values 120pcs Electrolytic Capacitor Pack from Banggood
Buy 12 Values 120pcs Electrolytic Capacitor Pack from Dealextreme
Buy 30 values resistor kit from Banggood
Buy 40 values resistor kit from Dealextreme