Found in Motorola documentation from the 1970s, the circuit presented uses a component from the era that is not common today. The characteristics of the coils are given next to the diagram.
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...