This alarm goes off if someone leaves a door open. We found it in the Encyclopedia of Electronic Circuits Volume 5 from 1995. The circuit uses components that are still common today.
An interesting and versatile filter circuit, suggested by National Semiconductor is the one shown...
Found in na 1970 EDN this circuit has a parallel regulation providing a symmetrical output of 15 V....
This circuit was found in an American magazine 73 for radio amateurs of the 80s. It is an...