Sometimes the original thermostat of the fridge (refrigerator) fails or its accuracy is no longer sufficient.
Older refrigerators used a mechanical thermostat sensing temperature using liquid or gas filled capillary.
Through capillary the pressure is transmitted to the diaphragm (bellows) and thus the contacts of thermostat, switching the compressor.
However, this system has less accuracy and can wear out.
Food storage temperature should be kept between +2 ... +8 °C. Nominal temperature of the refrigerator is 5 °C. Thermostat is characterized by two parameters: start and stop temperature (or average temperature and hysteresis). Hysteresis is required to prevent the compressor to turn on an of too often. I chose the mean temperature of 5 °C and 2 °C hysteresis, so thermostat turns on when temperature rises to 6 °C and off when temperature falls to 4 °C. This setting is sufficient to maintain the ideal storage temperature, while providing a sufficiently long interval of switching to prevent excessive wear on the compressor. This is especially important for older refrigerators that use thermal relays for starting the motor. Newer refrigerators tend to have a start-up PTC thermistors.
Electronic thermostat is the suitable replacement of the original thermostat. Electronic thermostat reads the temperature using a sensor that is changing its electrical characteristics depending on the temperature. Often a thermistor (NTC) is used, but the problem is its low accuracy and the need to be calibrated. To ensure accurate temperature setting and avoid the calibration, consisting of many hours sitting in front refrigerator, I chose an integrated sensor circuit LM35. It is calibrated in degrees Celsius with a coefficient of 10 mV / °C. Due to the fact that the threshold temperature is close to zero, the relative change in the output voltage is relatively large. The output can therefore be monitored using a simple circuit with two transistors. Since the output voltage is too low to open the B-E junction of the bipolar transistor, the circuit LM35 (IO1) is operated as a current source. Its output is loaded by the resistor R1 and the power terminals behave as a current source proportional to temperature. This current induces drop at R3. The drop is monitored by T1. If the drop exceeds the threshold voltage of the B-E junction, the T1 and T2 are subsequently turned on. Then the relay Re1 turns on, whose contacts are connected instead of the original thermostat contacts. R5 acts as a positive feedback. It injects a small current into R3, which shifts the threshold and thus provides hysteresis. The relay coil is 5 or 6V. Contacts must be rated to sufficient voltage and current. The sensor (IO1) is located inside of the refrigerator to a suitable point. R1 is located directly to the sensor, allowing the probe to have only only 2 wires. To the sensor cable may bring the radio interference into the circuit. Therefore the C2 is used to suppress it. The circuit runs from a power supply of about 5 to 6V. The current consumption depends mainly on the coil of the relay used. The power supply must be safely isolated from the network, otherwise the entire circuit including sensor is under mains voltage. You can use a ready-made power supply.
The great advantage of this circuit is that it works on the first start without calibration and setting. Just build it with values of components according to the schematic and it is ready to use. If it was necessary to slightly change the temperature setting, Adjust R1 or R3. The hysteresis is determined by R5.