I am trying to use a transistor to switch a 12v relay ?
I am using a micro controller to switch a transistor which will then switch on a relay. I have heard that you need a resistor in between the output pin and the base of the transistor. Why is this and how do you calculate what size. Also the micro controller is running off a 12 battery which is regulated down to 5 volts for the controller. Can a transistor use a 5 volt source to active the base and then have 12 volts running through it activating the coil on the relay. I apologise for my poor electronics knowledge.