2N4401 NPN transistors, which are working as switches. When the 100K thermistor will sense low temperature or coldness then its resistance will start increasing which will switch on the Transistor Q1. When the transistor Q1 will switch ON it will provide the voltage at the base of the transistor Q2 due to which transistor Q2 will also switch ON and activates the relay.
The 10K variable resistor is used to adjust the circuit to activate the relay on your desired low temperature. You can also use a potentiometer in the place of variable resistor it will make adjustment easy.
The circuit can be operated with 9 to 12 volt DC but you can also try low voltage like 5 or 6 volts and make sure to use the same voltage relay also.
The figure below shows a project of a very sensitive cold sensor switch circuit that can be used to sense coldness or low temperature. The circuit can be used to activate any device on your desired low temperature.
The circuit is using a 100K thermistor for sensing the temperature and two