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.
This circuit was found in old documentation and can be implemented with other operational...
Found ina 1970 American documentation this fast-reading circuit uses a photodiode. Transistors...
This circuit is from a publication from the 70s, but it can be assembled with any diac and a Triac...