Okay I'm making a very simple transistor circuit. The problem I'm having is that the 5v control is separate from the 12v supply for the load (there two entirely separate sources with separate grounds). I need to be able to control and supply the transistor with two separate circuits.thanks in advance for help.

 

Welcome,

Can you connect the grounds of the two supplies together?

If yes, do it.
If no, use an opto-isolator on the +5V source control to the base. That will maintain the circuit isolation. Google: 'opto-isolator' if required.

Good Luck

 

The best way is to connect both grounds together.

 

The transistor is connected as an emitter follower. When the base is at +5.0V then the emitter and load are only +4.3V, not +12V.