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 found in a publication of1994, originally obtained from the National...
This circuit was found in a 1977 Radio Shack documentation. The circuit is TTL, which must be...
This circuit was found in an American 1980s documentation. It analyzes the exhaust gas of a...