ARCC – Automatic Railroad Crossing Controller
The Automatic Railroad Crossing Controller (ARCC) is designed to operate signals and/or barriers as a train approaches a crossing, and then switch the signals off (and raise any barriers) once the end of the train is clear of the crossing.
The system works by having four infra-red light beams across the track, as shown in the diagram below. The source of each infra-red beam is a suitable LED, positioned on one side of the track, and its light is detected on the opposite side of the track by a matching phototransistor. Each of the four beams will be broken in sequence by the train as it travels along the track and over the crossing.
If a train is approaching from the left, for example, the crossing signals, etc., are activated as the front of the train (locomotive) breaks beam ‘A’. The signals stay activated until the last car in the train has passed through beam ‘C’, and the beam is intact again. Similarly, for a train approaching from the right, the crossing signals are activated as beam ‘D’ is broken, and continue until the complete train has passed through beam ‘B’.
It is important to appreciate that all four beams are broken as the train moves through. When travelling from left to right, breaking beam ‘B’ before beam ‘D’ effectively tells the ARCC to ignore the breaking of beam ‘D’ until the train has passed completely through the set of sensors, ie. until the last car has travelled beyond beam ‘D’. The ARCC is then returned to its “Ready” state, where the breaking of either beam ‘A’ or ‘D’ will trigger the crossing signals. A similar sequence applies for trains travelling from right to left, where the breaking of beam ‘C’ stops the ARCC from re-triggering the crossing signals as soon as beam ‘A’ is broken.
The ARCC is designed to handle only a single track, but it is possible to handle a railroad crossing with dual tracks, by fitting a second ARCC module, with its own set of four infra-red beam sensors. The two ARCCs are coupled together so that a train running in either direction, on either track, will operate the crossing signals. If you have trains running across the crossing on both tracks simultaneously, then the first train to reach an ‘outer’ sensor (its own ‘A’ or ‘D’) will activate the crossing signals/barriers. These will stay active until BOTH trains are clear of their own inner sensors (the relevant ‘B’ or ‘C’).
This project has been designed so that anyone with a reasonable level of skill can assemble an ARCC (or two) for themselves. Full details of the ARCC circuitry, and suggestions for its construction on stripboard, are given in the User Guide which is available for download