Found in a 1993 Electronics Hobbyst Handbook, this circuit uses components that are still common today. It alerts you when an extension is being used.
This optically coupled circuit was found in the February 1968 Radio Electronics magazine. The...
We found this circuit in the April 1978 issue of Radio Electronics magazine. The power supply must...
This circuit supports several sets of coils and can oscillate from 1 to 44 MHz. The transistor is an old...