In use telephone indicator (CB8657E)

Found in a 90s Popular Electronics, this circuit keeps an LED on when a phone is in use. Can be used with extensions.

2 transistor radio (CB22272E)
We found this circuit in an American documentation from the 60's. The transistors are made of...
Digital Capacimeter (CB4312E)
The accuracy of this circuit depends on adjustments made with capacitors of small tolerances of known...
OR function with Relay (CB19773E)
The figure shows how to implement an OR function with two inputs, using a relay. The relay...