This is the schematic I used as an outline for the efies I made. The resistors values can be varied based on what you have laying around and what voltage source you use. Just make sure the pot center tap voltage varies from 0v to at least 400mv. As you can see, the fixed resistor is double the pot value. If you keep with this ratio, you will be in the ballpark. I prefer 9v batteries because they are square and the battery mounting clips are shiny. In my experience, anything above @ 300mv with cause trouble for the vehicle.