The possible circuit of the poster's circuit board is as shown in the figure.

A small transformer and four diodes form a power supply circuit, which is used as the power supply of the circuit after filtering.

A unit of LM324 is used as a comparator, and a potentiometer (adjustable resistor) is used as a temperature setting resistor to adjust the operating voltage of the comparator. When the temperature is higher than the set value, the op amp outputs a high level, which is shaped by the second op amp and then expanded by a triode to drive the relay.

A possible problem with this circuit is that the power supply voltage connected to the comparator is unstable. Therefore, as the power supply voltage changes, the set temperature may not match the actual temperature. The output voltage of the comparator needs to be stabilized. You can use the unused op amp unit of LM324, add a Zener diode, and output a stable auxiliary power supply as the power supply of the comparator. The circuit will be more complete, such as the improved circuit in the attached figure.