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 conventional window comparator configuration was found in an old documentation. It is valid...
We found this configuration in a Radio Shack documentation from 1977. Here we have the way to...
This is a single transistor stage for a phototransistor. The circuit is suggested by Texas...