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You are here: Home / 555 IC Circuits / Make this Simple Refrigerator Door Open Alarm Circuit

Make this Simple Refrigerator Door Open Alarm Circuit

Last Updated on February 23, 2020 by Swagatam 15 Comments

caution electricity can be dangerous

This presented refrigerator door open alarm circuit which alerts you after a period of time whenever your refrigerator door is left open.

This circuit becomes very handy, because in case the door is left due to carelessness may cause significant increase in the consumption and affect the life of the fridge.

Circuit Operation

This circuit uses a photosensor LDR for detecting whether the door is open, or not. Whenever the sensor is illuminated by the light coming out from inside of the refrigerator, the circuit begins emitting an intermittent sound to alert you and bring the situation to your attention.

And as soon as the door is closed, and the fridge light goes off, the circuit and the alarm shuts off and stops emitting the sound.

For executing the entire operation a couple of timers ICs 555 are connected as shown in Figure.

When the LDR is not introduced to light the voltage on pin 2 (trigger) of the first IC 555 stays higher and its output (pin 3) is rendered low. Due to this the second IC 555 is rendered inhibited (low voltage level on pin 4) and the alarm is not allowed to activate.

When the LDR experiences an illumination (door is opened), the voltage level on pin 2 of the first 555 gets low causing the output (pin 3) to oscillate (square wave).

During the oscillation when output of the first 555 is at high level enables the second 555 to get triggered which also begins oscillating in tune with the first but at a much higher frequency.

A buzzer which may be seen connected with the output of IC2 now begins buzzing and alarming.

The circuit makes use of a PP3 9 volt battery, and should be placed as close as possible to the inner light of the refrigerator.

The circuit should be housed inside a box that may be waterproof and sealed to prevent moisture from affecting its operation.

Circuit Diagram

Parts list of the refrigerator door open alarm circuit

  • IC1 - IC2: 2 Timer 555
  • C1: 1uf 25V
  • C2: 100nF
  • R1: 10K 1 / 4W
  • R2: LDR (photoresistor)
  • R3: 2.2M 1 / 4W
  • R4: 1M 1 / 4W
  • D1:1N4148
  • Buzzer: Piezo type DC

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About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!

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  1. kris morris says

    July 1, 2019

    I am a mom with 4 kids who don’t shut the fridge properly, which freezes up the fridge. Your directions look like greek to me. Do you make these for people to purchase? Instructions for circuit boards are the only thing I have found on line.
    #NotAnEngineerJust(a)MOM

    Reply
    • Swagatam says

      July 1, 2019

      Thank you kris Morris! I am sorry I don’t sell readymade kits, however I can provide a much easier alternative with just a couple of parts, which you can buy and assemble yourself without going through any technical hassles, I’ll try to update the design soon in the above article…

      Reply
  2. Paul Kersey says

    March 30, 2019

    Would this be sensitive enough to be triggered by the tiniest amount of light from outside the fridge if the door was ajar ?

    Reply
    • Swagatam says

      March 30, 2019

      yes it can be done by increasing the value of R1 appropriately.

      Reply
  3. Ram says

    January 28, 2015

    Ok, thank you. Can you suggest anything to step up 6 v to 7 volt to use 2 led in series to save the energy and make circuit more efficient.

    Reply
    • Swagatam says

      January 29, 2015

      for stepping up the voltage you may have to employ a complex boost circuit, the easier option is to use single LEDs across the battery, all in parallel….by dropping the 6V to 3.5V through series diodes, or by using a emitter follower circuit as shown below:

      https://www.homemade-circuits.com/2012/08/simplest-dc-cell-phone-charger-circuit.html

      the 9V zener could be replaced with a 3.3V zener for dropping the voltage to the required limits

      Reply
  4. Abu-Hafss says

    January 28, 2015

    Hi Swagatam

    Sometimes, the inner light of the refrigerator goes bad and most of the consumers don't replace them. In that case, the above circuit won't work.

    I suggest, you can include in this article, another version of this circuit based on LM334, a thermistor or 1n4007 as temperature sensor. The sensor may be placed on the body of fridge just beneath the door. When the door is left opened, the cold air which is denser would escape from the lower part and come into contact with the sensor which will activate the buzzer after preset time. In this situation, it won't be required to place the circuit inside the fridge.

    Reply
    • Swagatam says

      January 29, 2015

      Hi Abu-Hafss,

      It's hard to imaging how somebody could keep the fridge without a light, but even in that case the external light entering the fridge would be sufficient to trigger the LDR whenever the door is opened.

      Another option is to use a hall effect sensor or a reed switch to sense the door's rubber seal magnet while it's closed or opened, and trigger the alarm.

      Reply
    • Abu-Hafss says

      January 29, 2015

      Swagatam

      I had thought about the external light but, at night it won't work.

      However, hall-effect or reed switch would be better option.

      Reply
    • Swagatam says

      January 29, 2015

      I don't think at pitch dark the user himself would be able to see the fridge materials…there has to be some light to see things.

      Reply
    • Abu-Hafss says

      January 29, 2015

      Suppose, the user opens his fridge (with inner light out of order) at night. He will use the room light for taking out the food. He closes the door and switches off the room light. But the door is not fully closed…………………………????

      Reply
    • Swagatam says

      January 29, 2015

      that's a 0.1% scenario, in such a terrible case this circuit won't help after all it's a straightforward, cheap and built on an extremely basic principle.

      because if the door is open by 1 or 2 mm then even the internal lights would be off disabling the LDR….and in a case where the door is left sufficiently open and the user completely ignores the buzzer, walks of switching off the room light then it's his problem not the circuit's problem.

      Reply
    • Swagatam says

      January 29, 2015

      actually the circuit can be even more simplified by using just one IC….this circuit was not designed by me.

      Reply
    • Abu-Hafss says

      January 29, 2015

      Actually, I misunderstood the circuit to warn when the door is not properly closed so that is not the circuits problem, it was my mistake 😀

      Reply
  5. Swagatam says

    January 28, 2015

    Ram, don't make it because it's not quite efficient, lot of power is wasted in the transformer. I have tested it

    It's better to connect the LEDs directly with the battery through a calculated resistor.

    Reply

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