This circuit uses an LED (called SSL) to regulate current in a configuration with complementary transistors. The circuit is from 1970 General Electric documentation.
This circuit was found in a 1991’s PE Hobbyst Handbook. The circuit is based on altering the sound...
The transistor for this flashlight brightness control may be the BD135. The circuit is from 1978...
This circuit is from the component's own datasheet, which is a quadruple equivalent of the 741....