Found in old American documentation, this circuit is intended for the control of heating elements. The SCR must have current according to the controlled load. Transistors are from the time.
This circuit was found in a Electronics World of 1968. The integrated circuit is the MC1320P from...
The circuit presented is from the Linear Applications Handbook of the National Semiconductor of...
Using an LDR as a sensor and an operational amplifier this circuit was found in the Encyclopedia of...