We found this circuit in a documentation for radio amateurs from the 70s. The transistors can be the BC548. With it, small speakers of 4 or 8 ohms can be used as microphones.
This blocking oscillator circuit was found in a book by Rufus Turner from 1954. The circuit uses...
This is a configuration much explored by assemblers, being found on this site in dozens of similar...
This circuit is from a June 1992 Radio Electronics. The circuit is for 4.5 MHz signals from old...