Who realizes this little project gets a device with the following functions:
• Displaying the current refrigerator temperature
• Monitoring the refrigerator temperature with alarm function
• Opening time monitoring with alarm function
• Programmable maximum allowed temperature and opening time

The circuit presented here is based on an ATtiny84 microcontroller. The microcontroller contains 10-bit AD converters, one of it is used to calculate the refrigerator temperature. As temperature sensor the KTX81-210 is used. To display the measured temperature a red 7-segment display is used. To deal very efficiently with the battery voltage the 7-segment display is driven in multiplexed mode on digit level. The procedure of this method was explained detailed in the elektor article 'Economical 7-segment Display' (7/8-2012). So we reach a maximum power consumption of only 5 mA. With a button we can set the highest allowed temperature and door opening time. If the allowed maximum temperature value exeeds a alarm tone will be triggered. After opening the refrigerator door a timer will start. When the maximum allowed opening time exeeds a siren sound gives you the advice to close the door and that you should no longer waste energy. When the door is closed the circuit is set to standby mode LDR controlled. In this mode the power consumption is reduced to 5 µA. And now the best: As case we use a Raspberry Pi box which is available for 5 €. The needed holes for switch, button, LDR, 7-segment display and speaker exist already in perfect arrangement.

Revision 1.1
• German documentation added
• Door open alarm countdown reduced to one minute as default
• Some comment corrections

Revision 1.2
• New english and german documentation
• Door open alarm countdown can be programmed now

Revision 1.3
• German version of Elektor.Post article 22 'Programmierbarer Kühlschrankwächter im Raspberry-Pi-Gehäuse' added