We found this circuit in a 1965 magazine. The circuit can be assembled with current equivalent transistors and the precision in generating the frequencies depends on the tolerance of the capacitors.
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...