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Track Circuits And Train Detection

Track Circuiting.

This is going to require an awful lot of work post installation and is something that you should do prior to laying it. The track circuit idea is very simple; the wheels and axles of the train short the two rails. An electric current passes between the rails and the sum resistance of the rails gives you the position of the train. This means that there has to be electrical continuity along the complete track lengths.

Prior to laying the track you solder small wire "fly ends" to the rail. Once the track has been laid the fly ends are then twisted together and soldered in situ. The only method of doing this once the track has been laid requires duplicating the electrical connections either by soldering or mechanical screw connection.

Train Detection.

This is easier to do post installation. You only need to have some form of electrical switch to detect the train. This can either be by just shorting the rails, a magnet closing a reed on the sleeper, or an IR LED and sensor on the sleeper.

By far the easiest method is using a magnet and reed. You will need a DPDT relay and two types of reed. These are the Normally Open N/O and the Normally Closed N/C types. Into the circuit of the "coil" connection of the relay you insert the N/O relay. Thus when the magnet passes over the reed the reed closes and power flows to the relay coil. One side of the DPDT relay is wired as a "latch" circuit. Thus a 12 Volt connection between the N/O and COM terminals of the relay is used to supply power to the coil. Thus even if the N/O reed opens the relay still remains active. At the other end of the COM to N/O circuit is the other reed which is N/C. When the magnet passes over it the reed opens and power is cut to the DPDT relay.

The other side of the DPDT relay is available for you to use as required.


Now that you have some idea of where your train is -what do you intend to do with this information? The obvious one is to feed this to some sort of indicator panel or to use it to automatically control signal or crossing gates. This raises the design problems normally called local and remote control.

Local Control.

This is the simplest. The detection system has its own power (a battery), and its action is based on a simple trigger. An American friend of mine has a small sideline in selling a line-side toilet whose door closes when the train approaches and opens when it has passed. It can be used to change the signals from Red to Green etc. The problem with local control is that fact that each unit is an individual and has to be serviced as such. But an advantage is that if one goes down it does not affect the whole as the remainder will continue to function. Personally I would only use local control for "trivial" things rather than relying on it for main operations. In a power points system it would be easier to use local control to "close" points that are not used by the loco in its travel and then to open them if a possible route is selected that would use them.

Remote Control.

This is far more complex than local control as the actuator has to connect to a display panel and then the operator has to initiate the function. This means that there has to be "wiring" or "plumbing" to the device to be controlled and then ideally some form of feedback to the control panel. The alternative is "blind send" in which the operator flips a control and "hopes" that the action has been completed. This may be all that you need and it is not a second class solution.

Typically a true remote control system has a main power feeder wire with a control wire and return wire all sharing a common earth. The main power feeder straddles all the devices and has a large reserve behind it. The earth return should be very heavily rated as in theory all the devices will feed to it regardless of the amount of power fed by the main feeder wire. The control wire and the return wire can be quite flimsy as their function is simply to carry the switching voltages. It should be noted that there should be a "dropper" resistor of about 1Meg Ohm that is switched between the power feeder and the earth return cables to prevent the so called "black wire corrosion" that is an ionic attack of the negative side of the wiring by positively charged ions. Also this helps prevent electric shock as the two cables act as a capacitor and hold their charge for some time and here I speak from personal experience!


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Page last modified on August 15, 2017, at 11:20 AM