TTSS3L Sensor Signal (LH Feather) 3 Aspect Home Distantith the use of the Sensor Signal developed by Train Tech you can easily add automatic signalling into your layout in seconds. The signals work by detecting a locomotive, or full train passing by it using a very small infrared sensor built in to the signal base. As soon as the locomotive has passed the sensor the signal changes to red and will stay red for about seven seconds to let the train clear the block. It then reverts back to yellow for a