This circuit was found in a Hands On Electronics from the 90s. It turns on an LED to indicate that the phone is in use.
Capacitor C and resistor R in this circuit determine the time the output remains at the high level...
The purpose of this circuit is to extend the range of applications from a Darlington pair to DC....
Found in an old English publication this circuit uses a dual gate MOSFET. Coils not specified....