The purpose of this circuit is to keep the soldering iron at a constant temperature, thus saving energy. The control has a thermocouple temperature sensor that generates a voltage of the order of millivolts which is applied to an operational amplifier. The thermocouple signal is compared with the signal; given by the R3 setting consisting of a 1M ohms trimpot. When the thermocouple signal is greater than the reference voltage given by the trimpot, the signal generated by the operator takes the BC548 transistor to cut and the relay shuts off the iron supply. With the temperature drop, when a voltage lower than the reference is obtained, the relay closes its contacts by feeding the iron.
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