A TRAIN DETECTOR

THAT WORKS

 

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An inexpensive detector that works

 

There are many types of train detectors on market today

    All with varying degrees of dependability

    Almost all expensive if you need a lot

 

Here is a circuit

      Reliable up to one foot away in all lighting conditions

      Relatively low cost

      Flexible

 

How it works

    The circuit is essentially an LED flashed about 10000 times a second

    Next to this is a light detector connected to a filter

          In simple terms, a few simple components that produce a constant signal

             In the presence of a intermittent signal

     The weak signal is amplified    

          

      

 

     By far, the most reliable seems to be the IRDOT-1 circuit

     Can be used to detect trains from as far away as three feet and more

Only problem is expense

 

 

 

Simple detection

    Ambient or visible lighting

         Simple light-sensitive detector (cadmium cell)

        Cheap, simple effective

        May not work under certain lights or “night” lighting

         Can be triggered by shadows etc.

 

Infrared line-of-sight

    Fairly dependable

           Simple, inexpensive

     May be hard to conceal

 

Infrared – reflective

    Most dependable when set up right

     But can be a bit tricky

 

      Tone decoder

 

       Lm324 circuits

 

Considerations

     Make sure emitter/detector wavelengths match

          Do not have to be exact but should be close

              The two most popular emitter frequencies are 880nm and 940nm

          Many detectors will handle both theses ranges but check to make sure

 

     Do not under power emitter

           Devices not liner

           Little output to minimum threshold value

           Much better output between threshold and max

           To much current will damage LED

      Beams must overlap

           While detector circuit may work at two to three inches

            Quite possibly not work at ½ inch

          Included angle of emitter must illuminate

                Area in view of detector

                     Parallel beams

                     10 degree slant

                      20 degree slant

                      Greater angles may be difficult to drill with accuracy

      I made a jig to standardize positioning of these holes

          For standardized installation

 

 

Circuitry to detect trains not that difficult to assemble

   The trickiest part is the mechanics of assembly

       To match the sensitivity and dependability required

 

Working with infra-red can be tricky

     A reflective surface may  detect a car at six inches

          A rough, black surface may be invisible at a quarter inch!

    Not to worry

    Once you know what to look for

        Building a reliable detector is a fairly simple matter

First step is to boost up the power to the emitter

      To shine a brighter light against the target

Second, most LEDs have a fairly narrow beam

     Typically ten to twenty degrees

     Angling the emitter and detector to about 20 degrees from centerline

          To converge about a half inch over railhead can do wonders

     On my detectors I used a brass sleeve to separate the beams.

         I found that blackening the inside of the sleeve

         And positioning the tip of the LEDS to just outside the sleeve

         Yielded maximum sensitivity but this can vary depending on

              LEDs and configuration used

I built a small test fixture to test the positioning of the light guns

    And a Shiny side/rough black side target

        To test both extremes of reflective surfaces

 

The best circuit I’ve seen to date is the IRDOT circuit that works

     Surprisingly well with no adjustments. The only problem

     Is that each circuit costs $40 so if you need 50 detectors

             Well, you do the math

The other extreme is the circuit built around a LM567 tone decoder

     At around three or four dollars per circuit

The circuit I built costs about six or seven dollars per channel

     Using (1) LM734, (1) LM339 and (1) 7404 chip for each pair of channels

             (The 7404 chip can be eliminated if you use a more expensive LED)

              And the LTV4 Opto-isolators are a bit of overkill

               But absolutely guarantees safe, fool-proof interfacing with computer

 

All circuits work more or less the same way.

    A pulse circuit drives the emitter somewhere between 1khz to 10khz

    The detector circuit contains filters to reject higher or lower frequencies

    The result is passed through a diode to charge a capacitor

    The last stage amplifies the charge on the capacitor to drive some output device

          (LED, relay, computer input)

 

The more sensitive you make the control circuits

      The more leeway you have in the adjustment of the LEDs

The better you align the LEDs, the less critical your circuit

      Adjustments need to be

It s very easy to get some flickering output of most circuits

      But to get a rock-solid signal it may take a bit of engineering

 

 

    

 

 

 

 

 

 

   

 

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Bob Van Cleef, MMR

Last update  07/24/2012