Alarm tells you to close the refrigerator door
A simple gadget that you place inside the refrigerator alerts you by beeping if the door stays open for 20 seconds or longer.
Boris Khaykin, TRW Automotive, Livonia, MI; Edited by Martin Rowe and Fran Granville -- EDN, June 25, 2009
The circuit in Figure 1 is a simpler and safer device than a similar one I recently read about (Reference 1). A few years ago, I built the circuit that this Design Idea describes, and the gadget still operates with the original 9V battery. The circuit operates by sensing a decrease in resistance of photocell PC1 that results from light in the refrigerator when its door is open. A counter is in a reset state when PC1 is in the dark, and its resistance is greater than 30 kΩ. Usually, the dark resistance is greater than 200 kΩ, and current consumption at this state is less than 40 µA. Oscillator-counter IC1 starts counting when PC1’s resistance is lower than 15 kΩ—that is, when the light bulb in the refrigerator is on. Then, in 20 seconds, it turns on a buzzer for 20 seconds or until someone closes the door. The current at this state is approximately 2.5 mA.
You can use almost any photocell, such as the Jameco 202403 CDS0018001 with 200-kΩ dark and 3-kΩ light resistance. This circuit uses a RadioShack 273-074 buzzer. You can use any similar piezoelectric buzzer with an operating dc voltage of 1.5 to 15V. V1 can be as low as 3V. The trade-off is that using a voltage this low gives you longer battery life but lower volume of sound.
Reference
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Babu, TA, “Alarm Sounds When Refrigerator Door Remains Open Too Long,” Electronic Design, March 26, 2009, pg 46.
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Great design!
Unfortunately, this still doesn't accomodate the occasions when the kids make a feeble attempt to close the door completely.....it closes enough to depress the switch to shut the light off, but not enough to seal the door.
GSN - 2009-29-6 03:46:00 PDT -
I like it, it reminds me of a rain sensor I built for my Mom when I was in school (25 years ago) from some junk box parts: telco wire wrapped about some perf board for the sensor, quad CMOS gate to sense rain (conductivity on sensor) and enable a multivibrator (2 more gates, what to do with the extra?) to drive an old transistor radio speaker thru via a transistor. Ran for years off the first 9V battery.
I'm going to take this as a challenge to design a micro controller version, crack that parts count down wards and run off a couple of CR2032's.
Ernie Murphy - 2009-26-6 13:39:00 PDT
