Found in a 1924 book by Gernsback, this circuit uses a double variable capacitor, a component not very common at the time. Listening was done on a loudspeaker at the time.
Found in Precision Monolithics documentation from the 90's, this circuit equalizes octave signals...
This circuit can be using in demonstration to teach how light sensor, such as the LDR, can be...
Found in a 1980s publication for amateur radio, this circuit employs an MC1596G which is a...