This regulator can be controlled by a logic signal. The circuit is from an Electronics Now from the 90s. Note that the switching transistor is before the integrated circuit.
The circuit shown is from the Texas Instruments optoelectronics manual, applying to low power...
This circuit was found in a 1968 General Electric documentation. It has an input impedance of 2 M ohms...
This filter that rejects a frequency range determined by the double-T was found in the August 1993...