Found in an old publication, this is a transmitter with a spark coil. The high frequency oscillations of the high voltage spark generate the signal thrown at the antenna.
Found in an American publication from the 1980s, this circuit can be found in commercial sound...
With the use of an additional diode and a resistor, the configuration of the figure can be...
This sound meter or sonometer was found in a Popular Electronics from the 80's. The circuit can be...