29-12-2012, 06:59 PM
Fridge Door Alarm
1Fridge Door.pdf (Size: 81.48 KB / Downloads: 88)
Circuit operation:
This circuit, enclosed in 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 has a high resistance (>200K) thus
clamping IC1 by holding pin 12 high. When a beam of
light enters from the opening, or the fridge lamp lights,
the photo resistor lowers its resistance (<2K), pin 12
goes low, IC1 start counting and, after a presetted 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 makes the piezo sounder beeps 3 times per
second.