This circuit was found in documentation from the 1980s. It is powered by the audio signal that must be cut. Transformers are the critical components and transistors are 2N2222.
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...