Building a Fridge Thermostat for Temperature Control

The fridge thermostat was designed in order to turn ON and OFF the compressor of the fridge depending on the temperature inside the fridge.

The temperature selection part utilizes a 10k NTC thermistor which is a type of resistor that changes resistance depending on the ambient temperature. To be able to give a varying voltage depending on the ambient temperature, a simple voltage divider network was created where the thermistor was used in conjunction with another resistor. Some IC would measure this voltage and be compared to a reference voltage. The software will compare the pin 7 voltage of thermistor/resistor network with the pin 2 voltage of potentiometer output.

The whole system is being powered with a 5V supply from a 7805 linear voltage regulator which takes its source from a battery pack. This is one of the features of the fridge thermostat making it a simple and small design for easy installation. Other features include being equipped with adjustable temperature setting and saving a lot of energy.

To minimize the effect of any ambient heat generated by the AVR and transistor, the thermistor has been mounted high off the stripboard.

Rest of the project