Marble Maze- part 3- Ball Bearing Detector

My marble maze has two parts that detect when marbles go by. One detector works when any type of marble hits it. The other detector only works with metal marbles (ball bearings). It works because the ball bearings are made of steel which is conductive. It consists of 2 pieces of foil tape in series with an LED string. The balls themselves complete the circuit. It was the first system on my marble maze to be powered off 2 D batteries. The ball bearing detector only works on one of the four paths through the marble maze.