Transistors to switch leds on/off

Cool! thanks for your help!

Been using relays but I want a much smaller, quieter set-up.

This should get me started well!

Cheers!

I have a question about using transistors instead of relays.

My example is- I have a switching (positive or negative) source for a circuit that is aprox. 3 volts and I would like to control a positive source of 12+ volts.

I think I understand that the Collector and Emitter would bear the 12volt end of the circuit and the Base would be the 3 volt switcher. (correct?)

Now, Should the base be opposite polarity of the Collec. and emitter?

And if the base receives a variance of voltage does it affect the volatge being passed from collector and emitter? (such as a dimmer circuit)

Thanks!