Found in an old Popular Electronics this circuit appears in other variations on this site and in this section. It uses a 7-segment common anode LED display as an indicator.
Many reed switches can be wired in series to perform na efficient alarm as shoen in the figure....
This simple operational amplifier and dual source circuit was found in the March 1996 issue of...
Found in a Maxim documentation from the 90s, this circuit produces a voltage of 5 V from an input...