This circuit was found in a 1924 Gernsback publication. The variable capacitors are dual. Listening is through a high-impedance phone.
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...