The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
This circuit was obtained in the publication 101 Electronics Projects of 1979. The circuit can be...
We found this circuit from a commercial megaphone in a Japanese publication in 1967. The circuit is...
This circuit was found in a Raytheon manual from the 60s. The transistor is the CK722, one of the...