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.
Found in an EEE this circuit uses FETs of the time and a bipolar one. Current equivalent...
This capacitive switch or approximation relay was found in a 1980s US publication. Power should...
Although it is a common configuration today, this circuit was taken from an old publication. The...