Thermistor cold indicator....Use a thermistor to replace the lower central variable resistor. Then take this circuit to your fridge opened door. Now adjust the top central 100k ohm variable resistor until the lamp only just comes on. It is now set to come on whenever the temperature drops to that cold level. Practise here by altering the lower variable as though it were the thermistor....My thermistor activates this fridge lamp at approx 15 k ohms as in this diagram. You could alternatively use an LDR for light / dark instead of a thermistor hot / cold detector. Best use a regulated V supply for consistent results. Hang the thermister out of a window on a length of wires to let you know when to be aware of ice on roads. A buzzer or relay could replace the LED if reqd.
