Using a NPN transistor as a switch

Adding on to my post below, an NPN transistor (e.g TIP31C) can be configured to operate as an ON-OFF switch. To turn the switch ON, both V _BE and V _BC have to be forward biased to operate in the saturation region where transistor acts like a short circuit. To turn the switch OFF, both V _BE and V _BC have to be reversed biased to operate in the cut-off region where transistor acts like an open circuit.

Happy hacking!