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.

Twin Coil - Reverse Loop Circuit #1 - schematic

1.   When the trolley crosses the sensor - Q1, the output of IC 1A will go LOW and IC 1B will turn OFF.

2.   When the output of IC1B is OFF the transistor Q3 will turn ON and switch machine coil 'A' will fire.

   This sets the turnout the 'normal' or strait through position.

3.   When the trolley covers sensor - Q2, the output of IC 1C will go LOW and IC 1D will turn OFF.

4.   When the output of IC1D is OFF the transistor Q4 will turn ON and switch machine coil 'B' will fire.

   The turnout will be set to the diverging route and the trolley can exit the loop.

5.   When sensor - Q1 is again covered by the departing trolley the turnout will again be set to the normal route.

•   When the outputs of IC1 B and IC 1D are in their ON states the base currents for Q3 and Q4 are shunted to the circuit common and the transistors remain OFF.

•   Comparators IC 1B and IC 1D provide a 'snap' action to the circuit so that the switch machine will fire properly when one of the sensors is covered.

•   Switches S1 and S2 have been included to allow manual operation of the turnout.

•   Switch S3 disconnects the base's of Q3 and Q4 from the detection circuit so that the turnout cannot throw automatically while the turnout is being operated manually as might be the case during switching operations.

•   This circuit uses 12 volts DC in addition to the switch machine power. There is no reason these could not be supplied from the same transformer, such as the accessory terminals of an old power pack, if a separate bridge rectifier, filter capacitor and regulator were used to supply the 12 volts to the circuit.

•   The distance between Q1 and the points of the turnout must be greater than the maximum length of the train to prevent the switch machine from being SET to the normal route while the train is still in the loop.

•   Alternately, sensor Q1 could be placed closer to the turnout if the train exiting the loop covered Q1 before Q2 was uncovered. This would prevent the 2200uF capacitor from charging and therefore the switch machine could not be thrown by the departing train.

  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.

•   Other types of detection methods such as across track infrared and block current detectors could be used to trigger this circuit.

•   Some experimentation with sensor placement may be needed to obtain the best results.

•   The time delay created in the recovery of IC 1B and IC 1D prevents the the 2200uF capacitor from recharging until the train has cleared the sensors at either end.

•   It may be possible to add track power reversing by using auxiliary switch machine contacts or separate relays. As mentioned earlier, this was not a consideration for the original installation.

•   If fast recovery of the charge on the 2200uf capacitor is not needed then the value of the 500 ohm resistor could be increased to 1000 ohms.

•   Relays could be used for the circuit's output if desired. In this case the relay coils would replace "COIL A" and COIL B" shown on the diagram and Q3 and Q4 could be replaced by 2N3904 transistors. The relays would be powered from the 12 volt supply and the 2200uF capacitor and switch machine power supply would be an isolated circuit.

•   For more information on comparators please refer to the Voltage Comparator information page at this site.

  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.

Georgetown Trolley Yard on the L.M.R.G.

  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.

Reverse Loop Circuit #2 - schematic

  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.

1.   When the train covers the sensor - Q1, the output of IC 1A will go LOW. This will cause Q3 and Q4 to turn ON. Capacitor C1 will discharge and COIL 'A' will activate.

     No changes will occur however as the switch machine is already thrown to the 'A' COIL side.

2.   The train continues its travel around the loop.

3.   When the train covers the sensor Q2, the output of IC 1B will go LOW. This will cause Q5 and Q6 to turn ON. Capacitor C2 will discharge and COIL 'B' will activate and throw the switch machine to the opposite side.

4.   The turnout will now be aligned to allow the train to exit the loop from the bottom.

5.   The next train to approach will enter towards the bottom of the loop and cover sensor Q2. The circuit will be activated but no action will take place as the turnout is already set to the 'B' COIL side.

6.   The train continues its travel around the loop.

7.   When the train covers the sensor Q1, the output of IC 1A will go LOW. This will cause Q3 and Q4 to turn ON. Capacitor C1 will discharge and COIL 'A' will activate and throw the switch machine to the opposite side.

     The turnout will now be aligned to allow the train to exit the loop from the top.

8.   From this point the cycle repeats alternating the train route through the loop with each train.

•   Switches S1 and S2 have been included to allow manual operation of the turnout.

•   Transistors Q4 and Q6 are Darlington transistors, they were drawn as single transistors to save space on the drawing.

•   This circuit requires filtered DC in a range from 20 to 24 volts. This could be obtained from rectified and filtered AC from the accessory terminals of an old power pack.

    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.

•   Practical recharging times for capacitors, C3 and C4, is approximately 30 seconds for the values shown. There should be no problem with this long charging time as only the coil on the exit side of the loop will be needed to throw the turnout to the exit side.

    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.

•   Optional LED's have been added to the base circuits of Q4 and Q6, these can be used to indicate when a train has been detected. When a LED is ON a train is blocking the sensor and the coil will have been activated.

•   Other types of detection methods such as across track infrared and block current detectors could be used to trigger this circuit.

    Infrared LED's have been shown on the schematic to indicate how they might be connected.

•   The coils of the switch machine must be isolated from each other as shown in the schematic.

•   The 470K ohm resistor and diode between the PLUS input and OUTPUT terminals of comparators IC 1A and IC 1B is used to provide as a snap action to the detector circuit.

•   The 1N4001 diode across the 4.7K ohm resistors in the storage capacitor circuits allows the charge in the capacitor to be bled off when power to the circuit is turned OFF. This will prevent the turnout from throwing when the power supply is shut down.

•   If the room light is used to provide light for the sensors, when the lights are turned off the circuit will think that both sensors are covered and activate both coils at the same time.

    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.

Reverse Loop Circuit #2 - PCB

  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.

Alternate Output for Reverse Loop Circuit #2

  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.