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Wednesday, 11 March 2009

Laser Pointer - Across and Along The Track Detection.

Laser Pointer - Across and Along The Track Detection.


This train detector makes use of hand held laser pointer devices that are widely available to detect trains over long distances.

WARNING Laser Pointers must be used with great caution as they may cause eye damage. Follow the directions that are supplied with these devices carefully or permanent injury could result.


The following diagram shows the basic laser pointer circuit. It is identical to the infrared circuit except that the infrared LED's have been replaced by the laser pointer unit.

Due to the long range of these devices this detector method can easily span great distances and could be used to detect trains in a long section of straight track such as in a lader yard or across the throat of a very wide yard.

Very Long Range Laser Pointer Detector


Determining the value of R1 for laser pointer diodes.

Laser pointers are not designed for this type of application and careful selection of R1 is required. testing should be carried out to determine the best resistance value for a particular pointer.

It is best to start with a high resistance for R1, 500 ohms for example, and decrease it in steps until the pointer will produce enough light energy to saturate the detector.

If you can see the light hitting the phototransistor this should be more than enough for the circuit. The pointer used for testing this detector needed about 38 milliamps for it to give off enough light to be easily seen by the naked eye.

Not enough current and the pointer will not 'lase'. - Too much current and the diode can be damaged.


Wiring The Pointer For Testing

Laser pointers are not designed to be taken apart, so for testing purposes the following method might be used to connect an external supply to the laser diode without damaging the pointer itself.

In this way if the detector is not to your liking you still have a functional laser pointer and not an expensive junk box oddity.

  • Use a mini test lead clip such as Radio Shack part number 270-372B to reach inside the body of the pointer and connect to the spring terminal.

  • If the laser pointers body is made of metal a small alligator clip can be used to connect to the body where the end cap screws into the body.

  • To hold the switch closed, clamp the pointer in its mount so that the button is depressed or hold the switch closed with a small wire tie.

  • Be careful to determine the correct polarity of the terminals or the laser diode could be ruined.

  • The leads from the clips can then be connected, through R1, to the power source.


The next diagram shows a possible mounting that could be used for testing a laser pointer detector.

Laser Pointer Mounting Method


Emitter/Detector Alignment

Aligning the emitter and detector should be straight forward as the the light can be seen hitting the detector. For best results the height of the beam should be at coupler height.


For information on Voltage Comparators please see the Voltage Comparator Information page at this site.


LM339 Data sheet - National Semiconductor (.pdf)

LM393 Data sheet - National Semiconductor (.pdf)

Pinout Diagram For Various Devices.



Please Read Before Using These Circuit Ideas

The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.


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