1 Refrigerator door alarm circuit

One way of conserving energy in refrigerator unit is making sure that that its door is always closed when not in use. Sometimes we forgot or we left the door open for several hours or even days, as a result is an increase in electricity consumption and bills. Using this simple circuit, it will remind us always to close the fridge door.
In the circuit is Light Dependent Resistor (LDR) that senses light. When the door is open, the light inside is up thus making the resistance of LDR very low. When this condition continues for several seconds (around 20 sec), the buzzer will sound.

fridge alarm schematic

Part List:

R1 - 12 kilo ohms 1/4W

R2 - 33 kilo ohms 1/4W

R3 - 100 kilo ohms 1/4W

LDR - any LDR

D1, D2 - 1N4148, 1N4001 or  similar diode

C1 - 10nF capacitor

C2 - 47uF electrolytic capacitor rated 16V

Q1 - 2N2222, CS9013 or similar NPN transistor

IC - HEF4060B or any 14-stage counter/divider and oscillator

Bz - piezoelectric buzzer

Bat - 3V to 6V battery 

4060 pins

2N2222 pins