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Installation tips and examples for Azatrax optical proximity sensors for model trains

infrared proximity sensor kit

The Azatrax infrared train detectors are designed to be easy and simple to use, yet versatile and flexible. The detector consists of a small circuit board and two sensing elements: an infrared LED light source (IrLED) and a phototransistor that senses the infrared light that is produced by the IrLED. The infrared light is not visible to the human eye.
The IrLED and phototransistor are pre-assembled with wire leads and are supplied with plastic tubes 3/16 inch outside diameter to facilitate installation.

The train detector circuits may be set up with the sensing elements mounted above the layout surface so that they look across the track, or for reflective sensing, with the sensing elements embedded in the track roadbed.

 • Reflective sensing can see a train at only one spot. This is fine for trains in motion and for signaling to other devices that a train has arrived. But if a train is parked and a gap between cars happens to be over the sensor, the detector may not sense the train and thereby give a false 'clear' indication.
 • Interrupt (across-the-track) sensing can cover a longer stretch of track when the IrLED and phototransistor are placed at an angle across the track. The shallower the angle, the longer the length of covered track.

General wiring advice:

  • Where two or more wires are connected to one screw terminal, tightly twist the bare ends of the wires together before inserting them in the terminal block. Loose connections can be frustrating to track down.
  • Route detector wires (including power supply wires) away from DCC power and control bus wiring. If detector wiring must cross DCC wiring, have the cables cross at right angles as much as practical.
  • Wires to the sensing elements should be twisted pairs. Twisting the wires together reduces electrical interference with other electronic systems.
  • Use a power supply of 8 to 14 volts AC or DC. Twisting the pair of wires to the power supply will also reduce electrical interference.

vertical sensor mounting

Reflective sensing (sensors in the roadbed)

When wired as 'reflective mode,' the relay contact will close when the infrared light beam is sensed by the detector (normally open contact).
When wired as 'across-the-track mode,' the relay contact will open when the infrared light is sensed (normally closed contact).

Drill two #12 (4.8mm) holes through the centerline of the track roadbed. A 3/16 inch hole often works, but can be a tight fit. Drilling the holes vertically is easiest, and sensing works well for HO and larger scales. Press the sensors and tubes into the holes. Be sure the sensors are below the tops of the ties, and that no light can pass directly from the IrLED to the phototransistor.
If infrared light 'leaks' through the roadbed it will cause a false detection of a train. This can be prevented by putting a metal or black plastic sleeve over one of the elements. The sleeve should be 3/16 outside dia. and 1/4 inch long. An easier solution that often works just as well is to push the pointed end of a hobby knife blade into the roadbed between the LED and phototransistor. Be careful when breaking off the end of the blade!
angled sensor mounting

To improve sensing at close range, such as with N scale, and to reduce false sensing of structures or other objects above the track, install the sensing elements angled toward each other. Imagine a line extending up through each sensing element. The lines should cross above the center of the track at a height approximately equal to the bottom of typical rolling stock.

If your sensor indicates a train is present when in reality there is no train, then infrared light is either leaking through the roadbed or the light is being reflected off another object above the track. To reduce reflections off other objects, you can:

  • Paint the object flat black
  • Move the object farther away
  • Change the angle of the object (tunnel ceilings, for example, can be angled at 45 degrees)
  • Mount the sensors at an angle in the roadbed.
optical track sensors Also make sure the detector is wired properly. This is rather obvious of course, but we've all overlooked the obvious at some point or other...

If your sensor fails to detect a train, then either insufficient infrared light is being reflected back to the phototransistor, or the phototransistor is being blinded by bright ambient light. Normally the train itself provides shade from ambient room lighting, but some rolling stock such as tank cars allow light from side sources to reach the detector. To solve this, you can:

  • Make sure the light path from the IrLED and to the phototransistor is not obscured by ties or ballast
  • Push the phototransistor a bit deeper into the roadbed
  • Place a structure or scenery to the side of the track to provide shade
  • Change the light bulb from incandescent to fluorescent (much less infrared light)
  • Use across-the-track sensing (see below).

Installation photos:
Installation with N scale track.
Track was temporarily placed on the roadbed. Red marks were drawn to show where the gaps between sleepers are located. Track was then removed and the mounting holes were drilled.
top view
Track has now been replaced. Infrared LED (left) and phototransistor (right) are angled toward each other at 45 degrees.


Jim Reising uses MRD1 detectors to detect trains on hidden staging tracks on his magnificent N scale model railroad empire, the New Oakville Sub. He took these photos as he installed the detectors in reflective mode using angled installation.

Drilling angled holes on the track centerline for the infrared sensors.
top view
Top view of sensing elements. These tracks will be hidden, so Jim is not concerned here with cosmetics.
sensors from below
Below decks view of the infrared sensing elements.
circuit boards
Circuit boards mounted at the edge of the benchwork.

Interrupt sensing (sensors look across the track)

horizontal sensor mounting
When wired as 'across-the-track mode,' the relay contact will close when the infrared light beam is blocked (normally open contact).
When wired as 'reflective mode,' the relay contact will open when the infrared light beam is blocked (normally closed contact).

Interrupt mode works best when the infrared LED and phototransistor look across the track (or multiple tracks) at an angle. This avoids flickering caused by the gaps between cars and reduces the chance of not detecting a parked train. The shallower the angle, the longer the length of track that is covered. Sensing elements may be as far apart as 18 inches or more, but alignment becomes more critical as the distance is increased.

Drill two 0.185 (or 3/16) inch holes through the layout base. Pull the IrLED 1/4 inch out of its tube, then bend the leads at a right angle. Do the same for the phototransistor.
Insert the wires into the hole for each element, then push in the tubes. It will be a snug fit, no glue should be required. This allows you to adjust the height for optimal operation.

It is possible for the phototransistor to be saturated by bright ambient light. If this happens, the detector will indicate that the infrared light beam is blocked even when the path is clear. To remedy this situation:

  • Shield the phototransistor with scenery or a structure
  • Swap positions of the IrLED and phototransistor so the phototransistor is not pointed at the offending light source
  • If the light source is an incandescent bulb, change it to fluorescent (fluorescents put out much less infrared).

two sensors per detector Double coverage with two sensors: When used in across-track sensing, one Azatrax train detector circuit can be wired to two sensor pairs. The detector will turn 'on' when a train blocks either of the two infrared light beams. See the double sensing page for details.

Installation photos: Ed Reutling took the following two photos of interrupt-mode train detector installation on his O scale model railroad. Ed prefers to keep all his wiring on the top side of the layout, running wires through the expanded foam base material.

trackside installation
Wires stay above the benchwork. Photo transistor is at the extreme left of the picture.
top view
Circuit board cleverly hidden in the basement of a trackside shack. Infrared LED is just beyond the circuit board, photo transistor is in the bushes at top right.

Automatically switch power to storage tracks, staging yards and track crossings.

The MRD1 detector can switch power to electrically isolated track sections when a train arrives at a given location. Mount the IrLED and phototransistor at the location where you want the train to trigger the power cutoff. Wire the detector and an auxiliary relay according to the how to automatically stop trains page.

To automatically protect trains at crossing tracks, see How to Avoid Collisions at Track Crossings.

Simple trackside signals.

You can have basic functioning red/green trackside signals using the MRD1 model train detector. Get started here...

Three-color red/yellow/green trackside signals can be operated with the MRD2 dual detector.


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