Electromechanical Counter (CB21682E)

This photoelectric circuit from a 1960 publication uses a 6.3 V electromechanical counter, common at the time.

Astable with the LM3900 (CB4037E)
The frequency of this astable depends on the capacitor that can be changed over a wide range of...
Micro spy (CB24456E)
This circuit is from a documentation from the 90s. The transistor can be any general-purpose FET...
Intercom with common phones (CB9292E)
In the figure we have one of the many options found in this section for an intercom using ordinary...