Train RR Crossing LEDs Scheme 2

In order to implement a railroad crossing scheme, it is necessary to detect the position of the model train relative to the railroad crossing.  A pair of Reed Switches will do the trick in a relatively cost effective manner.

What is a Reed Switch? A Reed Switch is comprised of a set of swich contacts hermetically sealed in glass. For this application which is appriximately one inch and about one eighth inch in diameter. It has a sausage-like shape.   The switch contacts are normally open and will be energized when it is placed in close proximity with a magnet.

The following figure depicts a pair of Reed Switches, associated magnets, and a Twin Coil Latching Relay used to turn power on and off to the LED Flashing circuit previously described.

Let’s walk through the figure in detail.

1) Assume that the train can be going in either direction from Reed Switch A toward Reed Switch B, or from Reed Switch B toward Reed Switch A passing accross the railroad crossing.

2)  As it passes over Reed Switch A, which is located between the train rails, Magnet A attached below one of the train cars causes Reed Switch A to momemtarily causing Coil A of the latching Relay to to enegize through the Panel Mounted Switch. This in turn causes the +12 vdc return (Power Supply negative) to turn on the Flashing LED circuit, which are wired to the Cross Bucks.

3) P.S.  The latching Relay is needed because the Reed Switch (not a latch) will only close when it is close to the magnet. There is also a Panel Mounted Switch (Double pole, Double Throw, none momentary) which will permit bi-directional train sensing.

4) As the train passes over Reed Switch B, it will momentarily close causing Coil B to reset the Latching Relay back to its normally open position, thereby disconnecting the +12 Vdc Power Source from the Flashing LED circuit.

5) Reed Switch A can be located a suitable distance from the crossing (appoiximately 5 feet).

6) For bi-directional operation operation, Reed Switch B must also be located pass the crossing by about 5 feet.

7) Now when the train travels from Reed Switch B to Reed Switch A, the Panel Mounted Switch must be thrown to the down position in the figure causing the B Coil of the Latching Relay to become the “set coil” which will thereby connect the +12 Vdc Power Source to the Flashing LED circuit.

8) As the train travels over Reed Switch A, Coil A will act as the “reset coil”, thereby causing the +12 Vdc Power Source to be disconnected from the Flashing LED circuit.

9) There is a reason why the distance to and from the railroad crossing are both about 5 feet.  This is because it is wise to use one magnet placed as close as possible to the engine, allowing the Cross Buck lights to first turn on 5 feet before the crossroad and turn off 5 feet after the train has passed the crossroad in either direction.

If you wish to acquire a better understanding of Electronics Theory, I suggest you go to  the following  link: Electronics