This 4-tube counter was found in Radio Electronics magazine from December 1959. The circuit uses a coil as a sensor to detect the passage of metallic parts or magnets.
This simple CMOS touch switch with bistable action must not be powered by a source without a...
This one-transistor alarm can be used with much more sensors than the two shown by the figure....
This circuit was suggested by Sprague Electric in an 80's documentation. The circuit uses a...