Here is a simple circuit to control the temperature of a soldering iron. It is especially useful if the soldering iron is to be kept on for long since you can control the heat dissipation from the iron. When a soldering iron is switched on, the iron takes time to reach the solder’s melting point. Simply connect this circuit to the soldering iron as shown in the figure and the iron reaches the solder’s melting point quickly.
Triac BT136 is fired at different phase angles to get temperatures varying from zero to maximum. A diac is used to control the triac firing in both directions. Potentiometer VR1 is used for setting the temperature of the soldering iron.
The circuit can be housed in a box with the potentiometer fixed on the side such that its knob can be used from outside the box to adjust the soldering iron’s temperature.