Title
----- 
Crossing Guard v1


Goal
----
If you have a track plan containing a crossing and you want to run two trains
on the track, then the crossing must be secured (guarded) that the trains
will not crash into each other.


Solution
--------
When using a crossing you always have 2 loops: loop1 and loop2. Create a 
buffer at the end of loop1 just before the crossing. Do the same for loop2.
The buffer is a piece of track which is isolated from then main loops. 
The buffers are individually powered by the RCX and the loops are powered 
by a speed regulator. Put light sensors just before the buffers to detect 
the presence of a train.
Starting situation is that no train is in front of the sensor and the buffers
have no power. When a train is detected by a sensor it's allowed to pass 
through. The RCX will power the appropriate buffer. As long as the train is 
passing the sensor the other must wait. When the second train passes the
second light sensor, then it automatically stops on the buffer because no
power is lead to the buffer. After train1 has passed the sensor, power is 
set to the second buffer and that train travels across the crossing.


Bill Of Materials
-----------------
1x RCX
2x Light Brick
2x Light Sensor
4x Train Track Contacts
6x Electric Wire
1x Track Crossing
.. Track Straight/Curved


Limitations
-----------
This concept only works with trains with only one motor. Also the motor must 
be the in front of the locomotive (the first set of wheels). If a train has 
a puling loc and a pushing loc both powered, then the train will not stop 
on the isolated buffer becasue the pushing loc will still receive power from 
the main loop.


Tip
---
I have elevated the crossing and the two straight tracks to and from the 
crossing by 1 brick. I did this because a lot of wires are required and 
with the elevation it's easier to lay down the wires.



Stefan Sanders
StefanSanders@Wanadoo.nl