This circuit was found in an old GE manual. It uses a thermocouple as a sensor. The SCR must be in accordance with the power of the controlled load.
This circuit, which uses germanium transistors, is from a 1965 English magazine. The used...
We found this circuit in an old American documentation, but the circuit can still be assembled with...
Other operational amplifiers of the same family can be used in this circuit suggested by Motorola in...