One of the mysteries that some railway modelers face is the "reverse loop." This is where you have a a section of the track that forms a complete loop so that the train will go round it and come back in the opposite direction on the same rails.

The major problem when trying to achieve this is the fact that the loop puts a short circuit on the track. This could be prevented by putting a break in each of the rails.

Train on the reverse loops are controlled by manual switches and/or manipulated throttle lever, the train operator must flip them in proper sequence. A moment of inattention results in a short circuit or derailment. This is especially likely (and embarrassing) when operating the layout for visitors.

For automate DCC railway control systems have the auto-reverse module. But there's nothing comfortable for DC control systems. This instruction is another attempt to create such device.

One direction entrance

There are several options for passing the reverse loop. My instructions provide for the following option – the train entering to loop in one direction only.

You can change the speed and direction of your train inside the loop and even stop it. The only limitation of this option is that your train must pass from point C to point A without changing the direction and the last car of the train must go this distance before the countdown time expires.

IR sensor

A way to modify the standard infrared module for Arduino is shown in the video. After this modification, the sensor trigger signal is inverted up to HIGH level.

Switch Machine

The ULN2003 chip is used to control the turnout, but you can also use a ready-made stepper motor control module. Connection method is exactly the same.

A classic switch machine for change direction the turnout is connected to the outputs of this chip. Some, especially old switch machines, will require an increase in voltage to 14 - 16 volts. To increase the voltage, you can use the DC-DC Voltage Step-Up Converter Module for Arduino.

Control system

This manual uses train control through the Arduino Train DEMO 2 mobile app.

If you are not mistaken when connecting the wires, then your result will be similar to that presented in the video.

Update 4/18/19

The free application has been updated to version 2. The application now supports the data transfer Protocol version 2.1, which means 9-step traction control.

EXPANDED TECHNICAL DETAILS

Model Railroad Power Logic Matrix

A technical solution for a classic model railroading problem: managing the electrical polarity of a "Reverse Loop" without causing a short circuit.

• Optical Train Sensing: Features IR break-beam sensors at the entrance and exit of the loop. The Arduino tracks the train's position as it enters the circular track section.
• Automatic Polarity Flip Control: Manages a DPDT (Double-Pole Double-Throw) relay. Just before the train exits the loop, the Arduino flips the relay to match the main-line polarity, allowing the train to exit smoothly.

Performance

• Verified with Arduino IDE: Coded with a "Safe-Buffer" delay to ensure the relay flip happens precisely in the gap between the train's wheel-sets, preventing any arcing or motor stutters.