A previous Idea for Design, “Alarm Sounds When Refrigerator Door Remains Open Too Long,” draws power for its operation by modifying the refrigerator bulb wiring. This may not be acceptable, considering safety as well as the warranty, if the fridge is still covered by the manufacturer.
This circuit uses a small 9-V battery for powering the alarm circuit instead (see the figure). The light-dependent resistor (LDR) is placed near the refrigerator’s light bulb. When the door opens, the bulb gets illuminated. The LDR resistance, which was very high when the door was closed, then drops. This provides sufficient bias current for the transistor T1 to turn the alarm on. The alarm circuit, which is in the collector load of T1, gets powered and operates as explained in the previous Idea for Design.