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.
We found this circuit in an American documentation from 1977. The circuit must not be powered by a...
This circuit uses two Raytheon RC4558 operational amplifiers that are similar to the 741. The...
This light-sensing probe was found in a 90s Popular Electronics. The circuit detects light sources by...