# Learning Electronics

Learn to build electronic circuits

# Fridge Door Alarm Circuit

Beeps if you leave open the door over 20 seconds

This circuit, enclosed into a small box, is placed in the fridge near the lamp (if any) or the opening. With the door closed the interior of the fridge is in the dark, the photo resistor R2 presents a high resistance (up to 200K) thus clamping IC1 by holding pin 12 high. When a beam of light enters from the opening, or the fridge lamp illuminates, the photo resistor lowers its resistance (less 2K), pin 12 goes low, IC1 starts counting and, after a preset delay (20 seconds in this case) the piezo sounder beeps for 20 sec. then stops for the same lapse of time and the cycle repeats until the fridge door closes. D2 connected to pin 6 of IC1 allows the piezo sounder beeping 3 times per second.

Circuit diagram:

Parts:

R1 = 10K
R2 = LDR
R3 = 100K
R4 = 100K
D1 = 1N4148
D2 = 1N4148
Q1 = BC337
C1 = 10nF-63V
C2 = 100uF-25V
B1 = 3V Battery
IC1 = 4060 Ripple Counter & Oscillator IC
BZ1 = Piezo Buzzer Incorporating 3KHZ Oscillator
SW1 = Miniature SPST Slider Switch

Notes:
• Connecting D1 to pin 2 of IC1 will halve the delay time.
• Delay time can be varied changing C1 and/or R3 values.
• Any photo resistor type should work.
• Quiescent current drawing is negligible, so SW1 can be omitted.
• Place the circuit near the lamp and take it away when defrosting, to avoid circuit damage due to excessive moisture.
• Do not put this device in the freezer.