This page is about automatic reversing loop switch machine control circuits for twin coil type switch motors. The first switch machine controller is very basic. More complicated circuits follow.
The original version of this circuit was designed for use on the trolley section of the London Model Railroad Group's "O" Scale layout. The circuit is designed to operate twin coil type switch machines through a capacitor discharge system
As the power for the trolley cars is supplied through the overhead wire, just as in the real thing, there was no provision for changing the polarity when the train exits the loop.
The next diagram shows the switch machine control circuit mentioned above. Also shown is how the phototransistor sensors would be placed along the track.
This sets the turnout the 'normal' or strait through position.
The turnout will be set to the diverging route and the trolley can exit the loop.
The only other factor would be: When the train is entering the loop from the lead track; to allow enough time after Q1 is uncovered for the delay associated with IC 1B to run out and for the 2200uF capacitor to charge before the train reached Q2. The time delay is approximately 1 second and the capacitor charging time about 5 seconds. Unless the train is very long this should not be a problem though.
The following is a rough diagram of the loop and yard area where the original of the circuit shown above is installed. The reverse loop itself is about two feet in diameter which as you can imagine is extremely tight for "O" Scale.
When the reverse loop is under automatic operation the Georgetown trolley yard would not be manned and the trolley car would be operated from the Vicsburg station which is out of sight of the loop.
The "Interlock Protected" crossing shown on the diagram is controlled by the original version of the Rail Crossing Diamond Protection circuit that is shown at this site.
The #2 reverse loop switch machine control is for use with twin coil type switch machines. The circuit is designed to throw the turnout to the exit side just before the departing train leaves the loop. The next train to approach the loop will then enter the on the same side that the last train departed from. This means that trains enter and leave the loop in the alternate directions.
The issue of reversing the track power on the lead track of the loop will not be addressed as this depends largely on the particular layout conditions.
The next diagram shows #2 the switch machine control circuit. Also shown is how the phototransistor sensors would be placed along the loop track.
For the purposes of the following explanation the loop turnout is set so that the approaching train will enter towards the top of the loop and cover phototransistor Q1 first. This means that the switch machine's - COIL "A" was the last to be activated.
In this circuit both capacitors, C1 and C2, will be charged when no trains are covering the phototransistor sensors.
No changes will occur however as the switch machine is already thrown to the 'A' COIL side.
The turnout will now be aligned to allow the train to exit the loop from the top.
Power could be supplied from an existing switch machine supply transformer through a separate rectifier bridge. If this is done the circuit must not share a common with any other circuit.
If more or less energy is needed to throw the loop turnout the values of C3 and C4 can be increased or decreased respectively. Changing these values also changes the charging time accordingly.
Infrared LED's have been shown on the schematic to indicate how they might be connected.
It is not known how a particular switch machine will behave and the next train entering the loop might find the turnout not properly thrown to either side. If a train is already in the loop there will be no problem as the turnout will be reset when the train exits.
The next diagram shows an alternate output for the #2 Automatic Loop Control circuit. This circuit could be used for twin coil switch machines that have a common connection between the coils.
The wiring is slightly more complex but the function of the circuit is otherwise same.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.