Hi,
Could someone have an idea on what to do if you want to switch on a 12vdc relay coil with a signal of 2vdc that is lighting up a certain LED? My setup is that I connect the 2vdc to the base of the transistor via a resistor and the connect 12vdc voltage source to the terminal A1 of the relay coil and then connect the the terminal A2 of the relay coil to the collector. Finally I connect the emitter to the ground and the I am done. Is this the right thing to do? Ideally what I want to achieve here is to have the relay coming on when the LED comes on and vice versa. Then I use the relay contacts either as digital inut signals for some control or to run some load e.g a lamp or a small motor. Please help.

 

I think so, but verbal descriptions are sometimes hard to follow. For best results I suggest you come up with a schematic diagram. This is the universal language of electronics.

 

Don't forget the BEMF diode across the coil.
Also look at using a 2n7000. Mosfet.
Max.

 

Your circuit sounds right, except for one vital missing component. You need a reverse-biased diode connected across the relay coil to kill the nasty voltage spike which occurs when the coil switches off. Diode cathode goes to +12V, diode anode goes to collector. Whatever supplies current to the LED must also be able to supply the required transistor base current.