This circuit with zener diodes was found in documentation from the 90s. The zener diodes determine the lighting point of the LEDs. The circuit is for 12 V automotive batteries.
This circuit that can amplify signals from 0 to 30 MHz was found in a 90s Popular Electronics....
This circuit makes use of a low impedance microphone. The relay is according to the supply voltage...
This circuit for incandescent lamps was found in a 70s documentation. The circuit uses an SCR...