I have an old fridge and sometimes the doors stay open wasting energy. So i created this simple Fridge's door alarm which will buzz when the door is open for x seconds.

Step 1: Materials

Here is what you will need for this project:

1x Plastic case
1x Small PCB
1x Attiny85 (or Arduino Uno/Nano/Pro.......)
1x Attiny85 "holder"
1x 1KΩ resistor
1x Lever interruptor
1x Buzzer
2x Electric terminals
1x Female jack (depending on your power supply)
1x 5V DC power supply

And tools as always:

Soldering iron
Soldering tin
Soldering vice
Lamp
Drill to make holes in the case
Glue gun or double sided scotch
Electric terminal pliers

If you choose to make the Attiny85 version, you will need an Arduino Uno (for example) to use as ISP to write the program in the Attiny85. You can find here a nice instructable from Plusea !

Step 2: Shematics

As you can see, nothing very difficult. The most difficult part is programming the Attiny85 if you have never done it before. If you don't want to, simply use an Arduino Nano.

How does it works ?

The Attiny85 test the interruptor on his pin 0. If it's HIGH, the fridge's door is open so X seconds after, it make the pin 1 HIGH and the buzzer buzz.

When you close the door, the Attiny85 detect it instantly and shut up the buzzer.

Step 3: Soldering

Let's go, take your favourite soldering iron and begin to give life to this project.

Don't forget to drill the case and to put wires in holes before soldering, and remember to use a Attiny85 holder too, don't solder the Attiny85 directly on PCB !

Once it's done, we have to program the Attiny85 (or Arduino Nano) before putting it on the PCB.

Step 4: The Program

Here is the sketch code that you can customize (time before buzzing etc...):

////////////////////////////////////////////////////////////////////////////////////////
// FRIDGE V1.0 ///////////// 01 september 2015 /////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////

/ Pin numbers:

// If Arduino Nano
//const int interrupteur = A5;
//const int buzzer =  13;

// If Attiny85
const int interrupteur = 0;
const int buzzer =  1;

// customizable variables
int tps_avant_activation = 25000;           // time before buzzing (25 seconds)
int tps_entre_activations = 3000;           // time between 2 buzz (3 seconds)
int tps_min_activation = 0;                 // minimum buzzing time duration 
int tps_max_activation = 300;               // maximum buzzing time duration (0.3 second)

// Do not touch variables
int statut_frigo = 0;                       // variable for reading the pushbutton status
int closed = 1;                             // 1 = door was closed
int open_counter = 0;                       // door's open counter 

void setup() {
  pinMode(buzzer, OUTPUT);
  pinMode(interrupteur, INPUT);
}

void loop() {
  statut_frigo = digitalRead(interrupteur);
  
  if (statut_frigo == HIGH) {
    if (closed == 1) {
        delay(tps_avant_activation);
        closed = 0;
      }
    digitalWrite(buzzer, HIGH);             // turn buzzer on:
    delay(tps_min_activation + open_counter);
    digitalWrite(buzzer, LOW);
    delay(tps_entre_activations);
    if (open_counter <= tps_max_activation) open_counter = open_counter + 25;
  } else {
    digitalWrite(buzzer, LOW);              // turn buzzer off:
    closed = 1;
    open_counter = 0;
  }
  delay(500);
}

Steps to program the Attiny85:

Upload the ArduinoISP example sketch to your Arduino Uno
Plug your Attiny85 shield programmer to your Arduino Uno, and the Attiny85 on it.
Choose the Attiny85 @ 8MHz board in tools > boards
In tools, choose the "Arduino as ISP" programmer
In tools, click "burn bootloader"
Now you can upload your sketch