Spring switches are used by some prototype railroads to save the crew from having to re-align the switch points after passing through the switch.  The switch is sprung in one direction (typically the mainline), allowing any train to pass through the switch from the frog end without manually aligning the points.  When entering the mainline from a siding, this allows the train to continue on its way without having to realign the points to the main – the spring action takes care of this automatically.  Modeling this unique feature can be easily accomplished with an IR sensor and switch machine.

One approach to modeling a spring switch follows the prototype almost exactly – applying a spring force to the points.  However, since mass does not scale well, this can cause operational problems and derailments if the wheelsets cannot overcome the spring force on the points.  This approach may also require periodic adjustment of the spring tension.

Another approach is to use a motorized switch machine and simulate the behavior of the spring switch.  By actively driving the points the appropriate direction, the operational problems can be avoided while still maintaining the illusion of a spring switch.

This application uses a CKT-IRSENSE reflective infrared proximity detector to detect the approach of a train on the diverging route.   Normally, the points are aligned to the main track.  When a train is detected on the diverging route, the IR sensor shorts its output to GND, toggling the control input to an MRServo switch machine that controls the turnout, causing the points to align with the diverging route.  As long as the IR sensor remains covered, the points remain set for the diverging route.  Once uncovered, there is a programmable delay (up to 20 seconds) before the output of the IR sensor is released.  This allows time for the train to clear the points before they are automatically set back to the main route.

Manual control, to allow a train to take the diverging route when approaching from the point end of the switch, can be accomplished with a toggle switch wired in parallel to the IR sensor output.

A demonstration of this spring switch simulator can be see in the video below.  Upon the initial approach to the turnout, the IR sensor detects the train, causing the points to be set to the diverging route.  The points are then automatically set back to the mainline after a 4 second delay.  After traveling on the mainline, showing that the points do not change position, the switch is manually lined for the diverging route so the train can back in from the point end.