The signal box has optical sensors built-in and has a sensitivity adjustment. Just apply power and put them next to the track. They have separate inputs and outputs so they can be daisy chained. They have two solid state relay (0.9A) outputs, which are complimentary, so they can directly operate other signals or accessories using lamps or solenoids. Specially designed for American Flyer semaphores. Can be used for non-de-railing on 2-rail track with our switch machines.