Have you ever left the refrigerator door open? You may deal with this slight carelessness with a simple circuit. Place it inside the refrigerator and it will inform you with an audible signal every time the door has been left opened.
The principle of operation of this simple device is based on the presence of light inside the refrigerator when its door is opened. The light is detected by the light dependent resistor FR1. The internal resistance of FR1 varies depending on the light it receives. In the dark, its resistance is about 1 Meg-Ohm while it is less than 1 K-Ohm in the presence of light.
The circuit is based on a single integrated circuit, the 4093 which contains four Schmitt trigger NAND gates internally. At the input of the first NAND gate (U1A), the voltage is at logic 1 in the dark and at logic zero when FR1 is illuminated. In the presence of light, a positive voltage (logic 1) appears at the output of U1A (terminal 3) which charges capacitor C2 through resistor R2. After about a minute, the voltage of C2 reaches a logic level of 1 and activates an oscillator consisting of the U1D gate, the C3 capacitor and the R3 resistor. U1D generate a square signal with a frequency of about 0.5 Hz. This signal drives U1C and activates/deactivates a second oscillator, consisting of the U1B gate, the R4 resistor and the C4. This second oscillator operates at about 4KHz and activates a piezoelectric transducer, the B1, which acts as a buzzer, producing an audible signal. The sound is like a “beep-beep”, because the 4KHz oscillator is modulated from the 0.5Hz signal of the first oscillator.
Due to the time delay from the R2-C2 circuit, the buzzer turns on about a minute after opening the refrigerator door. When the refrigerator door is closed, however, the buzzer switches off immediately because C2 is discharged immediately due to D1 diode and the oscillators are immediately cut off. The power consumption of the device is minimal and a 9 volt battery may last for several months.
The device can be easily assembled on the board of figure 2, according to the assembly guide of figure 3. To connect the battery on the circuit you must use a suitable clip for 9V batteries. This clip is not shown in the pictures. The piezoelectric converter is also connected to the board with two thin wires.
All the resistors we use in this project circuit are of 1/4W through-hole type. There are no critical values and you may experiment with different values in order to change the frequency of the oscillators or the time delay.
For best results, place the circuit near the refrigerator light bulb to avoid any shadows on the photoresistor.