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 uses only passive components and must be tuned to the operating range. The circuit is...
This circuit is found in a 1975s documentation for radio amateurs. It was developed to recognize a...
This configuration triggers an external circuit or simply an LED for time given by R1 and C1 (T =...