Basic - Infrared Across Track Detection

  This page presents information on infrared - 'Across Track' train detection circuits. The circuits are built around the LM339 comparator chip and can use a wide assortment of emitter detector pairs.

  The "Basic" circuit shown below has been tested with sensor gaps as wide as 12 inches but a distance of 8 inches or less is more reasonable. There are also "Long Range" across track detection methods that could be used across yard throats.

  This type of circuit is in use at the London Model Railroad Group's club layout and spans seven "O" Scale tracks using one infrared LED and a lensed phototransistor.

  In the circuit bellow the LED will turn on when the infrared beam is broken. Other output options and general information regarding comparators can be found on the Comparator information page link shown below.

  This "Basic" circuit could be used to experiment with for your own detectors as it has a 1Megohm variable resistor R2A to control the sensitivity of the phototransistor.

  In actual circuits, R2 and R2A would be replaced by a fixed value resistor to save money. In most cases the value can be 1 Meg or 500K ohm with good results but every situation is different and some experimentation might be needed.

  The "Basic" circuit can be used to test emitter/detector pairs when they are installed and before they are connected to the actual detector circuit.

Basic Infrared Detector Schematic

Circuit Operation - When a train breaks the infrared beam the phototransistor will go dark and turn OFF. The voltage at the MINUS input of the comparator will rise above the reference voltage, approximately 9 volts, that is between R3 and R4. This will cause the output of the comparator to conduct and the LED will be ON.

  The LED can be made to turn OFF when a train is detected if the PLUS and MINUS inputs of the comparator are reversed.

  The following schematic shows how the LM339 might be used for a multiple detector unit.

4 Infrared Detectors Schematic

•     There is a wide variety of Infrared LED's and phototransistors available that will work with this circuit. When choosing devices try to select ones with the same wavelength sensitivity. Next decide what case style will be needed, small devices are easier to hide but may be more difficult to work with.

  Some emitters and transistors have the leads placed so that the lens points to the side when the device is mounted vertically. Consult component catalogues for the various case styles.

•     Infrared phototransistors are also sensitive to visible light and therefore will need to be shielded in someway to prevent the device from being swamped by the rooms lighting. This can be done by hiding the phototransistor inside of a lineside structure or placing it inside a short piece of opaque tubing. If the phototransistor is mounted as shown on the schematic the back of the transistor can be painted black to prevent room light from entering.

•     Alignment of the emitter and detector is also important, especially if the gap is large. This can be done with a piece of string stretched between and in line with LED and phototransistor. A length of dowel or thin rod could be used if the mounts are rigid enough. The rod can also be used to accurately set the height of the emitter and detector.

  Another method that can be used for longer distances is a laser pointer shone through one of the mounting holes. Refer to the Alignment Methods diagram further down this page.

•     The LM339 integrated circuit contains four separate comparators. As shown on the schematic above they all share the same reference voltage, also any number of LM339 packages could share the same reference voltage. This will reduce the parts required if a large number of detectors are needed.

•     In order to save on the total current flow when a group of these circuits are to be used the infrared LED's can be connected in series and the value of R1 adjusted to supply the needed current. In this way the LED's share the same 20 milliamps instead of using 20 milliamps each. This is shown at the left side of the schematic.

•     The parts values shown on the schematics are guidelines only and may have to be adjusted for your particular needs. For example the sensitivity of the phototransistors is controlled by the value of resistors R2, 6, 8, 10. These values can be determined by using the basic circuit shown above.

•     The value of R1 can be decreased in order to produce a stronger infrared beam. Be careful to not burn out the emitter though.

•     If you want to use any of these across track detectors, take time to do some experimenting with mounting and alignment methods at the work bench first. This can save a huge amount of frustration later when installing the sensors on your layout.

  The next diagram shows two methods of aligning the emitter and detector mountings. For best results the height of the "beam" should be at coupler height.

Emitter/Detector Alignment Methods

  The next diagram shows a method of shielding the detector phototransistor from room light.

Detector Shielding Method

  The next diagram shows an inexpensive method of increasing the range of infrared across track detectors by using additional LEDs to supply a higher infrared light level.

  Operation is the same as the Basic Detector but the increased IR energy allows for greater distances.

  Making use of phototransistors with lenses built into the case would also be helpful for this particular application and may reduce the number of extra LED's needed to span a given distance. This is a situation where some bench testing can be very helpful.

Long Range "Infrared" Detector

  The "Long Range "Infrared" Detector" is a variation on the "Day and Night Phototransistor Detection" system shown on this site. A link for this page follows.

  The next diagram is for a small printed circuit board that could also be used to mount the infrared emitting diodes.

Quad Emitter Printed Circuit Board

  Below is a link to the "Voltage Comparator information" page. This page has more information on the use of comparators.

  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.