This is a simple IC 7805 based Turn-On VR Circuit Diagram. IC 78XX Series includes three terminal positive voltage regulators commonly available in TO 220 package. These are useful in wide range of applications to give precise output voltage. These regulators have internal current limiting, and thermal shutdown circuitry etc.A logic level can control a 7805 regulator with this circuit. Q2 is a series switching transistor controlled by Ql. Ql is turned on by a logic voltage to its base.
IC 7805 Turn-On VR Circuit Diagram