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 is an improvement on the previous circuit with a preamplification stage. The circuit is from...
This bench circuit was found in the February 1998 issue of Electronics Now magazine. It can be...
This circuit was found in a 1967 Electronic Design. FETs are from the time, so modern equivalents can be...