Automatic Reversing Circuit - Mk VII

DC Throttle Output       or       FWDC Throttle Output

  This page shows information for the 7th version of the Automatic Reversing Circuits. This circuit is designed for use with HO and N scale trains but could be used with other scales if the current draw was low enough.

  The time that the SHUTTLE waits at each end of the track can be individually adjusted from approximately 30 seconds to 6 minutes. The reversing circuit has adjustments to control the acceleration, braking rates and maximum speed of the SHUTTLE.

  The BRAKING distance can be as long as 1.5 meter (5 feet) with an HO scale locomotive traveling at approximately 20 MPH.

  The SHUTTLE can be a single car or a train of any length.

  This Automatic Reversing circuit is capable of being controlled by one or more Automatic Station Stop circuits. The Station Stop circuit is shown further down this page.


Automatic Reversing Circuit - Basic Operation

  1.   To start: The SHUTTLE is sitting over the EAST end STOP sensor (Q1). LED 1 is ON - Timer IC 3B is active.

  2.   When B timer completes its cycle its OUTPUT will go LOW and LED 6 will turn ON. The direction relay, RELAY 1, will be set for EAST to WEST travel and the SHUTTLE will begin to accelerate.

  3.   The SHUTTLE will continue to accelerate to the maximum voltage set by R26.

  4.   When the SHUTTLE covers the BRAKE sensor Q3, LED 3 will turn ON. The IC 3B timer is reset and the IC 3A timer is activated.

      At the same time as SHUTTLE covers sensor Q3 the the SLOW brake will be activated and the SHUTTLE will decelerate.

  5.   When the SHUTTLE covers the STOP sensor Q4, LED 4 will turn ON and the QUICK brake will be activated. The SHUTTLE should stop in a distance of approximately 1/2 car length or less.

  6.   The SHUTTLE will wait at the WEST end of the track until the IC 3A timer completes its cycle.

  7.   When the IC 3A timer completes its cycle its output will go LOW and LED 5 will turn ON. The direction relay, RELAY 1, will be set for WEST to EAST travel and the SHUTTLE will begin to accelerate.

  8.   When the SHUTTLE covers the BRAKE sensor Q3, LED 3 will be ON but there will be no change in the circuit.

  9.   The SHUTTLE will continue to accelerate to its maximum.

  10.   When the SHUTTLE covers the BRAKE sensor Q2, LED 2 will turn ON. The IC 3A timer is reset and the IC 3B timer is activated and begins its timing cycle.

      At the same time as SHUTTLE covers sensor Q2 the the SLOW brake will be activated and the SHUTTLE will decelerate.

  11.   When the SHUTTLE covers the STOP sensor Q1, LED 1 will turn ON and the QUICK brake will be activated. The SHUTTLE should stop in a distance of approximately 1/2 of a car length or less.

  12.   The SHUTTLE will wait at the EAST end of the track until the IC 3B timer completes its cycle.

  13.   The cycle will now repeat itself with the SHUTTLE traveling between and waiting at each end of the track.


Phototransistor Placement

  The next diagram shows how the phototransistor sensors are placed along the SHUTTLE track.

  The BRAKE sensors (Q2, Q3) control the direction of the SHUTTLE and activate the braking that gradually slows the SHUTTLE. The BRAKING distance can be as long as 1.5 meter (5 feet) with an HO scale locomotive traveling at approximately 20 MPH.

  The STOP sensors (Q1, Q4) activate a QUICK braking action that will bring the slowly moving SHUTTLE to a stop over a distance of less than 1/2 of a car length. The STOP sensors should be positioned just short of where the front of the SHUTTLE should stop.

  No specific distances are given as these will depend on the length of the track, the distance desired for braking and the operating characteristics of the SHUTTLE's motor. The SHUTTLE track itself can be of any length desired.

  The brake sensors might have to be placed unequally to compensate for differences that the SHUTTLE motor may have when slowing in one direction versus the other. The only way to determine the exact placement of the sensors is by experimentation but with good quality motors this should not be a large factor in setting up the circuit.

  Diode stopping sections at each end of the track are optional and provide runaway protection should the circuit not operate properly.


Automatic Reversing Circuit Schematic - DC Output

  The following diagram is the schematic for the Automatic Reversing Circuit Mk VII circuitboard.

  Operation is controlled by four infrared/visible light sensitive phototransistors (Q1, 2, 3, 4) that are mounted through the roadbed and between the rails. As the SHUTTLE passes over each sensor the corresponding section of the circuit is activated.

  Light Emitting Diodes (LED 1, 2, 3, and 4) indicate when the phototransistor sensors are covered and braking will begin. Light Emitting Diodes (LED 5, and 6) indicate which direction the SHUTTLE will travel when the wait timer has run out.

  A DC throttle has been incorporated into the circuit and features automatic current limiting and a short circuit indicator - LED 7.

  The circuit uses a 7400 Quad NAND logic IC to provide a memory function for the circuit. One half of this device controls the timers that determine how long the SHUTTLE will wait at each end of the track while the second half remembers the SHUTTLE's direction and controls the reversing relay.

  A schematic for a FWDC version of this circuit is at the bottom of the page.


The Brain Of The Reversing Circuit

  The next diagram shows a more detailed view of how IC 2 is configured and its simple memory functions. The use of a 7400 Quad NAND gate as a dual - SET / RESET type of Flip-Flop is a text book example of a memory device.

  Depending on which BRAKE phototransistor is covered the bottom half of the 7400 chip activates the corresponding timer.

  When the output of one of the IC 3 timers goes to a LOW state, the top half of the 7400 sets the direction the SHUTTLE will travel via the direction control relay.

  This portion of the Reversing circuit can use a 7400 type IC from almost any family of logic devices. The 74LS00 is the preferred device but it can sometimes be difficult to find. A 74AC00PF IC from DigiKey was used as it was easiest to obtain.


Reversing Circuit Operation Notes

  1.   If the SHUTTLE stops before it reaches a STOP sensors, (Q1, Q4) the circuit will still function correctly and reverse the SHUTTLE when the wait timer completes its cycle. Stopping short may occur until the motor has warmed up.

  2.   If the room lights are turned OFF the reversing circuit will go into a QUICK braking mode and stop the SHUTTLE.

      When the lights are again turned ON the circuit will resume normal operation, however, depending on the direction of travel and how long the lights were OFF the SHUTTLE may have reversed itself when operation resume.

  3.   The QUICK brake is only active when one of the STOP sensors, Q1 and Q4, is covered by the SHUTTLE. Therefore, the throttle's braking rate setting (R29) should be adjusted so that the SHUTTLE is moving slowly enough for it to stop in less than the length of the first car.

  4.   The reversing relay is controlled by the top half of the 7400 IC. The circuit is designed so that the direction of travel is changed when the SHUTTLE is ready to leave its stopping point. This is so that the relay cannot change the direction direction before the SHUTTLE is fully stopped.


General Notes

  1.   For the best operating results this circuit requires a locomotive with very good running qualities such as one with can type motors. The running qualities of the locomotive will ultimately determine the BRAKING and ACCELERATION rates and the stopping distances for this circuit.

  2.   The SHUTTLE does not have to be covering any of the four phototransistor sensors when the circuit's power is turned ON. However, the SHUTTLE must be between the STOP sensors to ensure proper circuit direction control.

  3.   The waiting times for the reversing circuit are controlled by R12/R13/C5 for the IC 3A timer and R18/R19/C6 for the IC 3B timer. For the values given, the calculated waiting times are from 33 seconds to approximately 6 minutes. Actual times may be longer.

  4.   The specifications of phototransistors Q1, 2, 3 and 4, are not critical and any that respond in the infrared and visible light ranges should work for this circuit.

  5.   The phototransistors use normal room light to control the circuit but the circuit could be adapted for day and night operation by using infrared LEDs to supply artificial light. For one possible method of doing this refer to the Day and Night Infrared Detection page at this site.

  6.   There are 0.33 microfarad capacitors (C1, 2, 3, 4) at the sensor input terminals to reduce the chance of electrical noise causing the circuit to operate improperly. Careful routing of the phototransistor wiring can reduce many noise problems.

      If noise problems still persist, a 10K ohm resistor can be placed in series between the input terminals and the phototransistors.

  7.   The STOP sensor Q1 receives its power from the output of the IC 3B timer (PIN 9) while the STOP sensor Q4 receives its power from the output of the IC 3A timer (PIN 5).

      This is so that the STOP sensor that a SHUTTLE is covering, is disabled when the circuit changes direction at the end of its wait. This causes the QUICK brake to turn OFF when the SHUTTLE is ready to leave its stopped position.

  8.   The SHUTTLE can cover both the BRAKE and STOP sensors simultaneously without affecting the operation of the circuit. This allows the SHUTTLE to be longer than the braking distance.

  9.   The circuit is designed to operate on an AC or DC supply of between 12 and 16 volts at 2 Amps. The voltage of the supply determines the maximum output voltage of the throttle.

      For an AC supply voltage of 12 VAC the maximum output of the throttle will be approximately 8 volts. This would be more than enough to operate N and HO Scale SHUTTLES at a reasonable speed. (The ATLAS locomotive used to test this circuit traveled at approximately 20 MPH at 5 Volts.)

      For an AC supply voltage of 16 VAC the maximum output of the throttle will be approximately 11.5 volts.

      The advantage of using a lower supply voltage is a reduction in the amount of heat produced by the throttle's power transistor (Q7).

      The control portions of the circuit operate at 5 Volts DC allowing the AC power input to the circuit to be as low as 10 Volts without affecting the overall operation of the circuit.

  10.   The throttle has a designed current capacity of 0.8 Amps. Resistor R32 sets the current limiting level for the throttle.

      The next diagram shows the terminal connections for the Automatic Reversing circuit and the Automatic Station Stop circuit. The Automatic Station Stop circuit to provides timed stops between the ends of the shuttle's track.

      The Station Stop circuit uses the same brake and acceleration rate settings as the Automatic Reversing circuit.

      More than one Automatic Station Stop circuit can be used with a single Automatic Reversing circuit.


    Station Stop Circuit - Basic Operation

      The Automatic Station Stop circuitboard connects directly to the Automatic Reversing circuitboard and uses the main circuit boards 5 volt supply.

      The Automatic Station Stop circuit uses the Brake and Acceleration settings of the Reversing circuit to control the shuttle.

      The Automatic Station Stop circuit is controlled by 3 phototransistors that are mounted between the rails.

    1.   The Shuttle departs the East end of its track.

    2.   When the Shuttle covers phototransistor Q1, the upper 556 timer of IC 2 is triggered causing its output to go HIGH and the BRAKE output to go LOW.

    3.   When the BRAKE output is LOW the voltage is slowly drained from capacitor C9 on the Automatic Reversing circuitboard and the Shuttle will begin to slow.

    4.   When the Shuttle covers phototransistor Q2 the output of IC 1B goes HIGH causing the STOP output to go LOW and quickly drain the voltage from C9 on the Reversing circuitboard .

        The shuttle should come to a stop in less than 1/2 of a car length.

    5.   When the upper 556 timer of IC 2 runs out, its output goes LOW and the BRAKE and STOP outputs of the circuit go HIGH. The voltage across C9 on the Automatic Reversing circuitboard will increase and the Shuttle will start to accelerate and leave the station.

        Also, when the output of the upper timer goes LOW, the lower 556 timer of IC 2 is triggered and its output goes HIGH.

    6.   When the output of the lower 556 timer is HIGH, the upper 556 timer's input is disabled and it cannot be retriggered. (The lower timer has a fixed run time of approximately 22 seconds.)

    7.   The Shuttle must cross the Brake sensor, Q3, within 22 seconds to be able to continue on its way to the West end of the track.

        If the Shuttle crosses the Brake sensor, Q3, after 22 seconds it will slow to a stop and wait until the upper 556 timer again runs out before proceeding.

    8.   The Shuttle will not continue to the West end of its track wher it will be stopped by the Automatic Reversing circuit.

      When the shuttle leaves the West end of its track, the above steps will be repeated.


      The next diagram shows an Automatic Station Stop circuit's schematic. The Station Stop circuit takes its 5 volt power from the Reversing Circuit PCB.


    Station Stop Circuit Operation Notes

    1.   .

    2.   .

    3.   .







    Automatic Reversing Circuit Schematic - FWDC Output

      The next schematic shows a modification to the Automatic Reversing Circuit that allows the circuit to produce a full wave DC output from the throttle.

      A 12 volt transformer must be used with this circuit in order to produce the FWDC output.

      Circuit board modification information will appear here at a later date.


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    Please Read Before Using These Circuit Ideas

      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.

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    14 February, 2016