This automatic brightness control that uses an LDR as a sensor, was found in American documentation from the 1980s. Resistors in series with LEDs depend on the desired current.
Found in a Popular Electronics from the 90's, this circuit detects the approach of objects by the...
This old 4-channel circuit was found in the December 1994 Electronics Now magazine. The circuit uses...
The figure shows how to implement an alarm buzzer in an LM3914 to warn about crossing the scale...