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.

Controlled Active Cycle Multivibrator (CB15535E)
Two inverters of a 4069B type CMOS integrated circuit form this oscillator which is characterized by having...
Difference Amplifier OP37 (CB19724E)
Raytheon's used operational amplifier is rare today, but the configuration can be adapted to use...
Battery charger (CB9127E)
Found in a radio amateur magazine of the 1980s, this circuit uses components that can still be...