Although only a small circuit, this smart alarm detects when your refrigerator door is open and lets you observe the internal temperature on a 7-segment display. Additional functionality indicates for how long the door is opened and sounds an audible alarm if this exceeds a predetermined time (or a predefined temperature value). The circuit uses only a few components, enabling it to fit inside a low-cost case, such as a Raspberry Pi box.
 
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The circuit presented here is based on an ATtiny84 microcontroller, garnished with a few other components. The microcontroller is provided with several 10-bit A/D converters, of which one is used to establish the current temperature inside the refrigerator.