I made this simple "On Air" sign out of a rear deck brake light. This is all 12 volt DC. Although I already have a cool "on air" light in the shack already, I had this rear deck 3rd brake light here that I have never used and decided to convert it to an "on air" light for the shack or some other use. I had all the parts I needed from around the house so didn't have to spend any money on it at all. That was the goal.
