When you have popped in your fridge alarm, getting peckish in the middle of the night and forgetting to close the door along with escalating electricity bills will be a thing of the past. Italian inventor Flavio Dellepiane has designed a 3V battery powered fridge alarm that beeps if you leave the door open for more than 20 seconds.
This circuit, enclosed into a small plastic box, is intended to be placed in the fridge near the inner lamp. With the door closed the interior of the fridge is in the dark and the photo resistor R2 presents a high resistance (>2M): therefore, R1 clamps IC1’s internal oscillator by holding pin 12 high.
When the fridge lamp illuminates, the photo resistor lowers its resistance (<2K), pin 12 goes low, IC1 starts counting and, after a preset delay (20 seconds in this case) the piezoelectric buzzer beeps for 20 sec, then stops for the same lapse of time and the cycle repeats until the fridge door closes
You can mount all the parts on a printed circuit board having the dimensions and layout as shown above in full size. Or you can use the suggested layout to accommodate the parts on a pre-drilled board of about the same size with holes spaced 2.54mm, as shown in the prototype photographs.
In this case, the connections of the components can be made on the underside of the board using pieces of wire (e.g. resistors, diodes and capacitors leads) cut to the right length and carefully soldered to the appropriate pins.
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