Found in American documentation from the 1980s, this circuit prolongs the ringing of a doorbell or chime powered by a 6 V continuous voltage. The transistor can be a BD136.
The figure shows how to add a protection circuit to a 7812 regulator with increased current. The...
This is an amplifier for low intensity audio and low frequency signals. The transistor can be a...
This circuit is from a 1992 publication. The transistors can be the BC549 or BC548 and the power...