This circuit for old telephone lines was found in the March 1997 issue of Electronics Now magazine. The circuit drives a relay based on the voltage monitored on the line.
In the figure we have a bistable multivibrator using an operational one. The circuit is from old...
This is another of the Star Trek red alert circuit that you can find on this site. We took this...
This circuit for testing zener diodes by lifting their curve on an oscilloscope was presented in...