Diode Matrix

Layout wiring can become very complicated in the extreme and if you can "keep it simple" then you will find it easier to solve future problems should they occur.
One of the things that add lots of wires to your layout is cables for point operation and your control panel can soon become a mass of buttons or switches for operating your points. Most people will be familiar with the magic pencil idea of operating points where you follow the route and touch the tip of the wired pencil or probe to a contact stud that sends current to the point and throws it in the selected direction. This is a good system which works well and for a small layout there is nothing at all wrong with using it.
But as layouts grow and the number of points increase it can get tiresome and indeed cause a problem if you miss just one of the points on your intended route, causing your loco and train to charge down an unwanted path.
Route selection can be accomplished by the push of one button if you work out and employ a "Diode Matrix" to send the current to the correct combination of points.
The example here will consider a 4 road fiddle yard but there is no reason why you could not add to this and make it 6,7, 8 or more if you need to.
tracks
The first thing we need to do is come up with some rules that we can apply to create our "Logic" pattern. These are quite simple and I have used the following :-
1. All points will be given a unique number e.g P2.
2. The direction a point is switched to will always be refered to by the direction a train would take if it entered our drawing above from left to right. So for example if point P1 was switched left then trains would carry on towards point P2. If P1 was switched to the right then trains would continue on towards P3.
3. All routes will be given a number e.g. 4

Having agreed our rules then we can apply them as follows to find out what point combinations we need to use to route trains in the drawing above.

Route 1 = P1 Left + P2 left.
Route 2 = P1 Left + P2 Right.
Route 3 = P1 Right + P3 left.
Route 4 = P1 Right + P3 Right
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The above text and drawing shows the combination of diodes needed and how you could arrange them on a piece of copper strip board.
All that is needed now is to wire the outputs of the matrix to the point motors and the route input wires to either push buttons on your panel or to contact studs if you want to use a probe to do the switching.
For the wiring you now have 2 options. The matrix board could be mounted under the section of your layout where the points are and 4 wires plus the common brought back to the control panel. Or you could mount it in the control panel and run 6 wires plus the common from the point motors back to the panel.
If you have enough wires available then I always prefer the second option with the diode matrix in the panel. The reasons for this are that each of the wires is carrying less current as they each only feed one motor. If however you have hefty wires and you are not operating too many points at the same time then you could locate the matrix close to the points and save some wires.

All of the above is based on the convention that the "Live" output from your point operating power supply is Positive and the common return is Negative. Suitable diodes to use are 1N4004 available from places like Maplins at less than 10 pence each.
If you are going to use the system above to operate more than 2 points at once then you will need to "Beef up" the output from your C.D.U. (Capacitor Discharge Unit). I prefer to build my own using a 18 volt transformer with a bridge rectifier feeding a large capacitor of around 10,000 Microfarads or more.

See this page for the circuit of a home made CDU

Summary.
The article above has only considered a simple route into a fiddle yard but there is no reason why you cannot use it as we did on the layout called "Kirkfield" where we had a number of combinations of routes for trains travelling along the front of the layout which could either be straight ahead, divert into the station or switch from fast to slow lanes or both.
In this case we used a colour combination on the track plan for every possible combination and then used a rotary switch and push button to select say the Blue route or the Red route etc. This made operating a doddle with a push of just one button needed to select the entire route for your train.

John Essex
Heywood Model Railway Group

This page updated 3rd July 2010