AC voltage control (CB8904E)

This circuit was found in a 90s Hands On Electronics. The semiconductor component is a Triac of the time.

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...