SPDX 2018 Model Railroad Speedometer

  This page has information on a model railroad speedometer that can be used with HO and larger scales. The circuit could also be used for other limited range telemetry purposes.

  The speedometer is a working prototype and is not designed for comercial purposes.

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Design Objectives For The Speedometer

  1. The average hobbyist can build the circuit with little difficulty and no special equipment or tools.

  2. A digital display showing speed and distance.

  3. The range of the speedometer should be about twenty feet.

  4. Electronic components that are easily available.

  5. Assembled hardware that is widely availble and easy to find.

  6. Assembled hardware that can be used for their original design purpose when not in use on the layout.

  The only input adjustment needed to calibrate the speedometer is setting the wheel diameter in the bicycle computer to match the car's wheel. For most freight cars this will be 33 inches but any diameter wheel can be used if the the computer can accept that diameter for a bike wheel.

  The bicycle computer must be able of accepting a manual input for wheel diameters that are not standard sizes for bicylces. Check the instruction sheet for the computer do determine this capability.

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Design Limitations

  1. The bluetooth transmitter has a stated transmittion range of 30 feet.

  2. The mobile portion of the speedomenter circuit is powered by a 9 volt battery.

  It was desired that the battery of the transmitter power the sensing circuit but its charging jack is isolated by a diode and cannot supply any power.

  A battery that is physically smaller than a 9 volt battery could be used but at greatly increased cost. The current requirement for the sensing circuit is very low so batteries changed out of smoke detectors yearly would be ideal for this circuit.


Major Component Assemblies

  To make the Speedometer as practical as possible, the two major components are items that can be purchased at sports shops and consumer electronics dealers or online.

  1. A bicycle computer* to display the speed of the train and the distance traveled.

  2. A Bluetooth, audio transmitter and reciever to send data to the display**.

Aluratek Transmitter And Reciever

  * - Any bicycle computer that uses a magnet and reed switch as the wheel sensor can be used with this circuit. Other types of pick ups could be adapted to the circuit.

  ** - For this circuit a digital display is used but analog displays could also be used with an adapter circuit. Data could also be sent to another system using pulses.

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Speedometer Schematics

Though Not The Actual Circuit Schematic

  The schematic shows the transmittier and receiver portions of the circuit

Alternate Component Uses

  When not on the layout, the computer can be used on a bicycle. The magnet and reed switch can be left on the bike if a plug and jack are used to disconnect them.

  The computer's mounting bracket will probably be needed connect to the display and therefore will have to be removed from the bicycle.

  When not on the layout, the bluetooth transmitter and reciever can be for wirelessly transmitting audio.

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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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18 July, 2018

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