Infrared Track Occupancy Detector Circuit
This is a Track Detector circuit that uses an Infrared LED Emitter and Photo Transistor Detector to sense movement of traffic on a track. The output has a relay that can handle up to 5 amps. It uses parts available at Radio Shack . I have included options for two types of sensors. A below the track reflective  sensor and an above track interrupted beam sensor. I used the below the track type on my layout and have included pictures. This sensor uses the one half of a 556 timer for a detector and the other half is a pulse stretcher that smoothes out the pulse from the cars going over the sensor. It is set to be about 10 seconds long. Reducing the value of the 1 meg ohm resistor will shorten it if you think it is too long for your application.  This circuit can be used in many ways for throwing turnouts, grade crossings, signals etc.  

Adjusting sensitivity

The indicator LED will light when sensor is tripped and is a basically tally light that you can use when setting the sensitivity with the 47k variable resistor. Depending on the amount of ambient infrared light on your layout, you may need to adjust it to just the below threshold of the background lighting. Incandescent lighting has more  background Infrared light  and fluorescent has the least and therefore is the best to use . As far as using incandescent with IR sensors, it works if you use strategic placement of both lighting and sensors. The problem with the under the track type with they look up into the lighting. Beam interrupter type are not as susceptible to the problem. The under the track type allows a prolific placement of them if you are using them for signaling as I do. Sometimes moving a an Incandescent track flood light a bit will improve the performance of a sensor on the layout. General purpose incandescent fixtures would not cast as intense beam as a track flood and may not be as big of a problem.


                                (Click on drawing to enlarge)

 Parts list:
    NE556/ML556 Timer IC
    Radio Shack #276-1728
    or Mouser Part 512-LM556CN

    Radio Shack #276-1101
    Matched Infrared
    Emitter/Phototransistor Set
    Radio Shack  #276-142
    or Mouser
    Photo Detector Part 512-QSD723
    Photo Emitter Part 512-QED523

    Relay 12 volt  DPDT #275-249

    10uf 35v ElectrolyticCap.
    LED (used to adjust sensitivity)
    Variable Resistor 47k 
    (sensitivity trimmer) #271-283

    1 meg 1/4 watt resistor #271-1356

    270 ohm 1/4 watt resistor

    15k ohm 1/4 watt resistor

    470 ohm 1/4 watt resistor


Here are the sensor and emitter assembled together with heat shrink tubing. Note that the sensor also has a piece of black heat shrink on it to act as a light shield. The Infrared light is projected up and reflected off of the rail car's trucks, back into the adjacent sensor.   
Here is the assembly mounted in the roadbed of my layout.
This is what it looks like after ballasting.

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