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.

Digital Voltmeter (CB18145E)
To use the instrument for AC measurement, the tips are A and B. For CC use tips A or B for...
CMOS Amplifier +1 and -1 Gain (CB15751E)
The interesting circuit shown in the figure, can have its gain digitally controlled and present two...
Three Transistors MW Radio (CB12261E)
This simple MW radio was found in a 1975 publication, but it can be mounted with BC548...