This circuit is from old documentation, but it can be implemented with modern transistors such as 2N2222 for bipolar and BF124 for FET. The crystal determines the operating frequency.
The figure shows three circuits for using headphones in amplifiers with speakers that do not have...
In this charger, the charging current is basically limited by the LEDs. The transformer is a low...
This configuration is from an old integrated circuit manual. The circuit can be applied to modern...