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 can be used to recognize a tone by its frequency. When this tone is present in a...
Found in a 1980s documentation, this amplifier uses an operational aplifier of the time with two...
This circuit, from a publication of the 1970s, can compare the intensity of two light sources,...