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.

Overcurrent oscillator (CB9627E)
The frequency of this circuit depends on the crystal. The transistor supports a modern equivalent. We...
High-gain composite amplifier (CB8069E)
Indicated in Maxim's documentation from the 90's, this circuit has a gain of 40 dB and an output...
Timer with 741 (CB12281E)
The time obtained in this circuit, from a 1978 publication, depends on R3 and C. R3 can have...