Mobile Model Railroad Speedometer Car

Updated On 28 February, 2021 at 04:32



  In this speedometer implementation the display remains in a fixed location or can be carried by the operator.

  The Speedometer uses an Aluratek Bluetooth transmitter and receiver pair to send the speed pulses to a CATEYE VELO 5 bicycle computer to provide a remote speed indication and an odometer, other models of computer should work but the ones that are wireless are not suitable as they have a very short range. All of these items are available from a number of commercial and on-line sources.

  The stated range of the transmitter and receiver is 33 feet which should be more than adequate for most layouts and clubs.

  This implementation uses a bicycle trip computer and an adapter for a digital display, however, with a suitable app and a smartphone could also be used for the display but this is beyond the scope of this page and has already been done commercially.

  A LM2917N-ND - Frequency to Voltage Converter IC, can be used after the receiver to provide an analog DC output voltage that is directly proportional to the speed. This voltage could be used to control a throttle to make a form of cruise control and an analog milliammeter to be used as a speed indication. A dual voltage comparator and perhaps two optoisolators in a window configuration would be needed.


Circuit Schematic

  The next diagram is a schematic diagram for the 2020 Speedometer circuit.

2020 Speedometer Circuit Schematic


Adapter Terminal Block

  Note that the block in the picture is not an ED1601-ND, it's an extra one that was available so I put it to use on this project. ( Any 5mm or 3.5mm block should work as long as there are 4 positions available. 2 Blocks of 2 Positions = 4 as shown.

  Something Relevant May Go Here In The Future.

  The adapter terminal block holds transistor that interfaces between the Bluetooth receiver and the Speedometer's display unit.

  The adapter terminal block takes the place of a reed switch that would normally be mounted on a fork for a bicycle installation and provides an input to the computer.

  The adapter circuit consisting of an NPN transistor and a resistor was constructed with a 4 position terminal block made with 2 position 5mm blocks. Other sized blocks or a small pcb can be used as long as there are 4 positions to work with.


Model Railroad Speedometer Car Axle Magnet(s)

K&J Magnetics B821

1/2" Long x 1/8" Wide x 1/16" Thick

K&J Magnetics B821 Link


  NOTE: Two axle magnets are shown in the photo. Therefore, there will be two pulses per wheel revolution and the circumference is effectively halved. The bicycle computer can be set to a diameter of 16.5 inches or the circumference to 51.8" (4.32 feet) to compensate for this and the displayed distance and speed readout is automatically halved.   This is true for any scale as long as the wheel diameter is correct for that scale.

  A 33 inch freight car wheel has a circumference of 8.64 feet. Which equals 611.1 revolutions per mile (5280 feet). (Or, 1222.2 pulses per mile for 2 magnets.)

  A 40 inch locomotive wheel has a circumference of 10.47 feet. Which equals 504.3 revolutions per mile (5280 feet). (Or, 1008.6 pulses per mile for 2 magnets.)

  NOTE: 2 axle magnets are used for low speed performance and accuracy. With one magnet the display might drop out at very low speed as these have a minimum input rate to keep them active. (Even though the speed indication may drop out, the odometer will continue to count the distance. An icon on the display will blink every time a pulse is received.)


Magnets On Axle Diagram

Magnets On Axle Diagram

  No Units are shown as it's all relative at this point, assuming the wheel is correctly scaled to 33 inches in diameter. The magnets are K&J Magnetics B821 - Nickel Plated - 1/2" Long x 1/8" Wide x 1/16" Thick (N42 Type).


Model Railroad Speedometer Car Speed Table

For 28 To 81 Inch Drive Wheels




Wheel Data Calculator Set up Image

Set up the calculator as shown in the image above and the guide below.

Choose a Calculation

Find A, C and r | Given d

Use this calculator to find the data for other size wheels.

Wheel Data Calculator Link


  1. Divide the circumference by 12 to get feet. (103.672 Inches / 12 = 8.639 Feet) (8.64 Feet).
  2. Divide 5280 by the circumference (8.64 Feet) to find the number of revolutions and pulses per mile ( 611.1 ).
  3. For 1 Axle magnet - Set up the bicycle computer for a wheel circumference of 8.64 feet or 220 cm to find the number of pulses per mile. ( 611 ).
  4. For 2 Axle magnets - Set up the bicycle computer for a wheel circumference of 4.32 feet or 110 cm to find the number of pulses per mile. ( 1222 )
  5. Power up the transmitter and receiver and allow them to sync.
  6. Place the car in a train and head out on the road.

Model Railroad Speedometer Car Computer Example (CATEYE Velo 5)

Bicycle Computer Notes

  The speedometer uses a bicycle computer to perform the speed and distance calculations and provide a real time display.

  Just about any bike computer will work, however, it should not too bulky so that it is easy to carry around. If the computer remains in its bike cradle, which provides the connections to the computer module, it could be put back on a bicycle quite easily for normal riding use.

  The adapter circuit consisting of an NPN transistor and a resistor was constructed with a 4 position terminal block made with 2 position 5mm blocks. Other sized blocks or as small pcb can be used as long as there are 4 positions to work with.

  A wrist band or a small box could carry the receiver, adapter block, and a computer. Or, a small box that fits in a pocket with the display attached to it so that the display could be placed in an easy to see location such as a control panel or held in the hand for walk-around use.

Aluratek ABT01FTx Transmitter Aluratek AIS01FRx Receiver

Basic Speedometer Parts List

  Go To the bottom of this page for poorly formated a parts table.

  Some of them are manufactures part numbers but they will work fine for an in house search.


Speedometer Car Notes and Updates

  1.   The example shown is a CATEYE VELO 5 but the VELO 7 and many other models should work. It's best if they have an actual reed switch in the pickup head.


  2.   This car uses a 9 Volt transistor radio battery but the power consumption is very low. A good source of batteries is to save the old ones from the annual smoke detector battery change and reuse them in this project. There's still plenty of life left in them.


  3.   Initially, the plan was to use the battery in the transmitter to power the oscillator but there must be a diode at the power input to the transmitter.


  4.  



PCB No. 1 Model Railroad Speedometer Circuit Board - Detector And Oscillator Circuit Board - Digikey number SBB1002-1-ND

PCBs 1 and 2 Model Railroad Speedometer Circuit Boards

PCBs 2 and 1 Model Railroad Speedometer Circuit Boards

  This board has an Digikey LM358P dual OPAMP that is used as an oscillator and comparator as well as the Digikey J211-D74Z FET used as a gate that sends a signal to the transmitter.

  This board is powered by a 9 Volt battery. The ( Digikey 36-232-ND ) connector for the battery can also be seen in the photo.

  Also visible is a 1/8ths inch stereo audio jack that connects the oscillator to the transmitter. Only one channel is connected although some other information could be sent with the other channel although I don't know what that would be though.

Model Railroad Speedometer Car - Digikey number SBB1002-1-ND

  The SBB1002 board was used to build the detector and oscillator circuitboard that rides in the gondola.

Search for SBB1002-1-ND ( 1.40" X 1.10" ) on any Digikey Site.



Making Contact

email to: rpaisley4@cogeco.ca

The Message Tittle Will Be: 2020 Speedometer Car

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Updated On 28 February, 2021 at 04:34


21 February, 2021 @ 07:40


More Other Information

Model Railroad Speedometer Frequency To Voltage Converter Schematic

  The circuit above uses an LM2917N-ND Frequency to Voltage IC to convert the pulses from the Speedometer car into a DC voltage. This allows a low current, analog milliammeter to be used as a speed display.

  Also, this voltage could be used to control the output of a throttle to adjust the output voltage to control the speed.

  No signal conditioning would be needed to drive the circuit the Interface unit would suffice to change the oscillating signal from the speedometer into pulses for the LM2917N-ND.

  The LM2917N-ND is not particularly easy to use and requires careful selection of the component values for good results. Follow the information in the data sheet carefully.

Speedometer Circuit From 1994

  The next diagram is for a speedometer that was developed in 1994 for the LMRG.

  This circuit worked in O Scale but could not be reduced to use for HO Scale because of the actual components used. The Transmitter was very large, it would only fit in an O Scale gondola. The odometer display is the one used for the 1994 Odometer car article in Model Railroader Magazine. It reads up to 140 MPH but I don't recommend doing this on a layout.

  This TMXxxx units shown on the diagram will fit in an HO scale car but are not exactly easy to use although they seemed to work fine.


First Speedometer and Odometer Displays

1994 Model Railroad Speedometer Car Schematic

  The next photo is for the original speedometer display that was developed for the LMRG. It is from an old laboratory pH meter and has a built in odometer. The same display as use in the 1994 Odometer car.

1994 Model Railroader Odometer Car Article At This Site Used By Permission.

Model Railroad Speedometer Display Built For An O Scale Example


1994 Model Railroad Odometer Car And Display

CIRCUIT PART QTY Supplier PART NUMBER
B821 Magnets 2 K&J Magnetics B821
Hall-Effect Switch 1 Digikey APS12205LUAABU
2 Pos Terminal Block 2 Digikey ED1601-ND
0.47uF Capacitor 1 Digikey B32529C0474K000
"CATEYE VELO 5 or 7" Bicycle Speedometer* 1 Other See text
LM358 Dual OPAMP - 8-DIP 1 Digikey 296-1395-5-ND
2N3906 1 Digikey 1514-2N3906PBFREE-ND
J-FET 1 Digikey J211-D74Z
9 Volt Battery Connector 1 Digikey 36-232-ND
PNP Transistor 1 Digikey 2N3906FS-ND
100K Resistor 5 Digikey 100KQBK-ND
22K Resistor 1 Digikey 22KQBK-ND
15K Resistor 1 Digikey 15KQBK-ND
Oscilator PCB 1 Digikey SBB1002-1-ND

  LEFT Click on the part number, then RIGHT wait for the prompt and do a Google search for the part number.

  Scan the results for the country that you normally use.

  Chances are that your browser will default to the correct country even if you choose a different contry.

  MAKE SURE THAT THE SITE IS IN THE CORRECT CURRENCY FOR YOUR ACCOUNT.


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Updated On 28 February, 2021 at 04:32