This audible / visual alarm circuit will start a short time after light hits the LDR(light dependant resistor) . If the alarm is inside the refrigerator, it will be triggered by external light (when door is open) and by the internal lamp. It is a very common habit to leave the refrigerator door open for more time than necessary, and this wastes energy (money) and can also negatively affect the food in it. Simply knowing that the alarm will sound, will remind you to close the door as soon as possible. This simple alarm circuit has other useful applications: Place one inside your room closet or your desk´s drawer to preserve privacy and your belongings. Battery drain in the dark is very low (2uA) and has practically no influence on battery life.
Step 1: Circuit Overview
Step 2: Circuit Diagram & Main Parts List
IC1: CD4011 (cmos quad nand gate)
Q1: BC338 (or any other general purpose NPN transistor)
LDR: Light dependant resistor (5mm)
BUZZER: 5volt buzzer
C1:10microfarad (tantalum electrolytic capacitor)
Battery: 9volt alkaline
Step 3: LDR (light Dependent Resistor)
Step 4: C1 Charges Through R2
Step 5: Delay Before Sound
For longer or shorter delay, use the formula Time(seconds)=0.7 x R x C(R in megohms, C in microfarads)
Step 6: Multivibrator
Step 7: Buzzer Driver Transistor
Led can be omitted if you don't need visual alert.