Speedometer Sensor Options

Revised 06-30-06

Sensor Options

  1. The sensors for the MSTS are made up of a pair of infrared LEDs and a pair of infrared detectors.  As mentioned earlier, the IR LEDs are pulsed at a frequency of 38 KHz and the detectors only respond to IR at that frequency.

  2. The MSTS comes with one set of sensors that are mounted in a clear, plastic "U" shaped bracket.  This mounting method allows users to place the sensors over the track so that they can easily be moved from place to place on the layout or to another layout entirely.  This is shown in the photo at the top of this page.

  3.  The sensors can be mounted in any number of trackside scenery items.  The photos below show two "G" scale installations, one inside of two wooden barrels and the other inside of  piles of railroad ties. 


This photo shows the indicator LEDs that light red when the IR emitters and detectors are properly aligned.

Mount  Your Own Sensors

A package consisting of two IR LEDs, two red indicator LEDs, 4 current limiting resistors, two 38 KHz IR detectors, one filter capacitor and a connecting cable that is terminated in an RJ-12 plug that fits into the Speedometer board is available for those of you who would like to mount your own sensors in a building, or other track-side piece of scenery


To mount your own follow these steps:

  • Select a pair of objects or pieces of scenery that can be placed on either side of the track and have enough space inside for the LEDs or sensors.  In this example I used 4 model milk cans.  Pairs were glued together.




  • The sensors and LEDs must be precisely mounted exactly 1" (center to center) apart from each other to preserve accuracy.  If you have access to a drill press it makes this much easier




  • To mount the LEDs drill two 1/16" holes in your object, 1" apart.  These holes will allow the LED "light" to exit.  If you have difficulty drilling small holes you can drill larger holes and place small pieces of brass tubing with a 1/16" inside diameter in them.  The important thing is that the amount of IR that is permitted to escape is limited to what can pass through a hole no larger than 1/16".


  • Drill out the back of the 1/16" holes to the diameter of the LEDs which is about 3/16"




  • The two longer leads of the LEDs are positive.  Shorten them to about 1/4" and solder one 470 ohm resistor to each positive lead as shown below.



  • Wiring is very simple.  The remaining ends of the two resistors are connected together and the two negative leads of the LEDs are also connected together.  If you have difficulty identifying the leads, the longer one is positive and the negative one is next to the flat spot on the side of the LED's base.  You must take care that the two leads of the LEDs don't touch and short out.  Bending them as in the photo helps to keep this from happening. 




  • Solder the two resistors and the two negative leads together.  Solder wires to each connection as below.

  • Solder a two conductor length of insulated wire to the points shown below.  The wire needs to be long enough to extend under the track, or under your layout, to the sensor unit across from it.


  • The completed IR LED unit is below.



  • The sensors are mounted in much the same way as the IR LEDs shown above.

  • Start by drilling another pair of 1/16" holes, spaced exactly 1" apart, in the other set of cans.

  • Increase the size of the back of these holes, shown below, to 3/16" to accommodate the lens that protrudes from the front of the IR sensors


  • Note the other two holes.  They are 3/16" and are for the indicator LEDs.

  • Glue the sensors to the backs of the drilled cans as below. 

  • Solder one of the 470 ohm resistors to each of the indicator LEDs as you did with the IR LEDs in the first section.

  • Insert one of the LEDs into each hole taking care that the leads do not touch.  Solder the two resistors together as below.  The other LED leads are soldered to the output pins on the sensors. 

  • Carefully solder the other leads from the sensors as below.  Note that a small piece of insulating tubing is used to keep the outer lead from the left sensor from shorting against the inner lead.

  • Carefully trim the leads from the capacitor to 1/4".  Place the capacitor under the two power leads as shown below.  Note that one of the capacitor's leads is marked with a "+".  This lead MUST connect to the lower of the center leads of the sensors.

  • Here is a close-up.

  • Connect the wires from the RJ-12 cable as shown below.  The ORANGE wire goes to the negative, center, lead on the sensors.  The ORANGE/WHITE striped wire goes to the joint where the two LED resistors join.  The GREEN wire goes to the right lead on the left sensor and the GREEN/WHITE lead goes to the right lead on the right sensor.  The BLUE and BLUE/WHITE wires will go to the IR LEDs.

  • Connect the BLUE wire from the RJ-12 to the wire that goes to the negative leads on the IR LEDs.  The BLUE/WHITE wire goes to the connection to the two resistors as show below.  Use tape or heat shrink tubing to insulate the joints.


Custom Sensors

Custom sensor mounting is available.  Due to the time involved in making and testing custom sensors the price is based on time and materials.  The price for the sensors, wiring and other parts is in the price list.  Most custom sensor installation takes between 1 and 2 hours which is billed at $30.00 per hour.