The title already said the question of this thread. As I searched the internet, transistors were configured in common-emitter configuration. They also show me that in order to activate the relay, the transistor should be operated in saturation and cut-off mode. How to make my BC547 Transistor operate to saturation mode (using common-emitter configuration) in order to activate my 24 VDC Relay using 24 VDC Source? How to decide proper Resistor Values?