Motorcycle headlight monitor (CB9117E)

This simple circuit was found in Popular Electronics from the 1980s. The circuit uses an LED as an indicator.

Automatic Night Light (CB10690E)
This circuit was published in an English magazine from 1972, but it can be easily implemented...
DVM to PC adapter (CB7924E)
This circuit from a Popular Electronics from the 90s allows you to use an old PC as a digital...
Tone Activated Relay (CB13696E)
With the values of the components of this circuit, tone recognition can be done in the range of...