Found in Popular Electronics from the 90s, this circuit uses a reed switch on the rails as a sensor. Instead of lamps, in a modern version, LEDs can be used.
Found in a 1960 Sylvania manual, this circuit can be implemented with a modern equivalent...
This configuration, already presented in other points of this section and website, is of a...
We found this precise crystal oscillator for the 144 MHz band, depending on the crystal used. The...