This 5.1 MHz circuit was found in a 1980s documentation. The frequency is given by the crystal and general purpose transistors can be used.
The component values depend on the current, which in this case has a maximum value of 5 A. The...
We found this cable tester in a Popular Electronics from the 90s. The circuit uses a connector...
This circuit can be used in the car to convert the 12 V from the battery in 6 or 9 V up to 2 A....