This circuit was found in a 1924 Gernsback publication. The variable capacitors are dual. Listening is through a high-impedance phone.
An interesting type of sensor for specific applications is the sensor formed by piezoelectric...
This circuit is from Radio Electronics from the 90s. It supplies a voltage between 800 V and 2 000...
This circuit was obtained in a 1989 publication and is quite simple to assemble, but critical in...