Fridge Door Alarm Circuit: Robust Design for 1.3V Operation

Description

The primary goal of this design was to eliminate a minor flaw in the widely utilized Fridge Door Alarm circuit, available on this website since 1999 and constructed by numerous hobbyists. Unfortunately, this circuit ceases operation when the battery voltage drops below approximately 2.6 – 2.7 Volts. This issue stems from the 4060 CMOS IC utilized. Certain devices manufactured by some manufacturers (but not Motorola’s) fail to function even with a standard 3V supply voltage.

A straightforward solution to this shortcoming could involve substituting the original IC specified with a 74HC4060 chip; this should permit circuit operation down to 2V, though obtaining this IC can prove challenging. For this reason, an equivalent circuit, employing roughly the same components, was developed to ensure safe operation even when the battery voltage declines to around 1.3V.

74HC4060

The 74HC4060 is a versatile CMOS Timer IC, typically used in astable multivibrator circuits. It’s a popular choice for building oscillators and timing circuits. Its key characteristics include low power consumption, good switching speed, and a wide operating temperature range, making it suitable for various applications, including this fridge alarm circuit where it acts as the timing core.

Circuit operation:

The circuit, housed within a compact enclosure, should be positioned in the refrigerator near the lamp (if any) or close to the opening. With the door closed, the interior of the refrigerator is in darkness, causing the photo resistor R2 to present a high resistance (>200K), thereby stabilizing IC1 by maintaining C1 fully charged across R1 and D1. When a beam of light enters from the opening, or the refrigerator lamp illuminates, the photo resistor decreases its resistance (<2K), halting C1 charging current.

Consequently, IC1, configured as an astable multivibrator, initiates oscillations at a low frequency, and after a period of approximately 24 seconds, its output pin (#3) goes high, enabling IC2. This chip is also wired as an astable multivibrator, driving the Piezo sounder intermittently at roughly 5 times per second. The alarm is activated for around 17 seconds, then ceases for the same duration, and the cycle repeats until the refrigerator door closes.

Circuit diagram:

Parts:

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