3-way control (CB7047E)

This is a simple circuit to control a set of lamps from 3 different points. We found it in an old Popular Electronics.

Digital inverter with transistor (CB25625E)
In the figure we have the simple way to obtain a digital inverter using a general-purpose...
Two way intercom (CB20505E)
This circuit works with only 1.5 V and can be powered by a solar panel. The circuit is from 1991....
Period to voltage converter (CB8869E)
This circuit was found in a 1969 EEE. The circuit uses transistors from the era that can be...