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.
The next diagram is a schematic diagram for the 2020 Speedometer circuit.
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.
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
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).
Set up the calculator as shown in the image above and the guide below.
Choose a Calculation
Find A, C and r | Given d
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.
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.
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.
Updated On 28 February, 2021 at 04:34
21 February, 2021 @ 07:40
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.
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.
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.
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|
|2 Pos Terminal Block||2||Digikey||ED1601-ND|
|"CATEYE VELO 5 or 7" Bicycle Speedometer*||1||Other||See text|
|LM358 Dual OPAMP - 8-DIP||1||Digikey||296-1395-5-ND|
|9 Volt Battery Connector||1||Digikey||36-232-ND|
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