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.
The battery charger suggested in the figure has its current limited by resistor R1. This component can...
The operation of this circuit is not common today, but equivalents can be experienced. The circuit is...
This circuit generates various types of sounds and can be used as a toy. The circuit is from Radio...