This two-tube circuit, one being a double, was found in Radio Electronics magazine from July 1958. The distance to the microphone depends on noise and losses. The cable must be shielded.
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...