Telephone in use warning button (CB9465E)

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.

Current sensor with 100 M ohm input (CB9663E)
This sensor circuit for currents of the order of nanoamperes was found in an American...
MW Experimental Radio (CB4517E)
This circuit was obtained from a 1964 magazine and has some interesting features such as the input...
Automatic call recorder (CB8656E)
Suggested by a 90’s Radio Electronics, this circuit automatically activates the recording switch of an...