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 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.
General wiring advice:
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.
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:
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:
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.
Interrupt sensing (sensors look across the track)
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.
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:
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.
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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