Have you ever wondered exactly how long your mainline is?  Or have you given a thought as to how far a car travels during an operating session?  There is one way to find out.  Build an odometer car that measures distances.

This project makes a nice conversation piece and with modern components the circuitry is not that hard to build.

Here are two variations.  The first variation is embedded into a flat car.  This configuration has a lower profile and center of gravity, which in turn gives it a more plausible appearance.

The second variation is a “drop-in” portable unit that simply rests on a flat or gondola show here.  The only modifications required for the car is a small hole in the bottom over one axle and a special truck.

Both cars have the same electronics, operate the same way and are equally as accurate.

Distance is determined by counting the revolutions of an axle using an infrared light emitting diode and a photo detector.  Both point to a specially modified axle of a truck.

Almost any truck can be modified for this purpose.  Simply glue and wind a ½” strip of paper around an axle to form a drum about 9/32 in diameter.  The exact diameter is not critical.  paint one half of the drum black and the other half white or silver as shown

There are a number of counters suitable for this project. Some have up to eight digits and others have digits of different heights not to mention many other features.  There are also several configurations available.

I used a Red Lion “SUB-CUB 1” which is designed to “snap in” to a PC board.  It is rugged, easy to understand and one coin battery can keep the display active for a long, long time.

This evaluation board is available to test and learn about both the SUB-CUB1 and the SUB-CUB 2.  It can be cut and used as a daughter board for your project, or you use it as a template to lay out your soldering traces.  This module is a bit unusual as you will not see any individual pins.  There is a conductive ‘zebra’ strip at each end to make the required connections against bare copper conductive pads on a printed circuit board.  There is no direct soldering.

The specifications sheet for the counter includes a dimensional layout of the mounting holes and the pads required to make electrical contact.  Here a 1/16” single copper clad pc board is used and the separations between the pads are hand filed.  It worked just fine.

The “leads” from the pads are soldered using regular solder and the counter is snapped into place later.  Wiring connections on the other side are made with a Low-temp silver-based solder.

This represents the bottom of the car.  Two holes are drilled for the ¼” tubing used to hold the emitter/detectors in place.  Be sure to remove any sharp edges and smooth the inside of the tubing.  Note the pads witch will separate The perf board from the base with just enough clearance that the wiring connections are not crushed

This is the top view of the electronics.  The leads of the LEDs extend past the board and allow for a small amount of adjustment of how close they are to the axle drum.

This view shows how everything is arranged inside the box loaded flat car.   The counter is in the center with the reset button to the right and the main power on/off- switch to the left. 

Below is the 3v coin battery to the left, a 9v rechargeable battery in the center, and the circuit board to the right.  The three views below show the underside of the car.

The slightly larger portable or stand-alone version has the same parts with a slightly different arrangement. 

The three views below show a gondola modified for use with the portable odometer configured as a load.  Note the relationship of the view port and the axle-mounted drum.

Wiring schematic for the North River Railway Odometer Cars      

(B1) is a 9v rechargeable battery and powers the circuits that deliver a counting pulse to the counter.

(L1) is an infra-red light emitting diode that will illuminate the axle drum as the car travels along.

(R1) Will limit the current to the very small amount of 0.003 amps to keep the drain on the battery as small as possible.

(L2)  is the infra-red detector that changes resistance with the amount of light it receives.

(R2) is a fixed resistor that develops a voltage swing from about .3 volts to 7.5 volts under ideal conditions as the drum turns This is much more than is required to trigger the counter.  Unfortunately, you will find that when everything is assembled this voltage change may be much less due to less than optimum conditions.

C1 is a capacitor to prevent sudden and spurious changes of this voltage that may cause false counts.

R3 is a resister that prevents overloading of IC1 input while allowing slight changes in feedback at this input.

R6, R7 and IC1 serve only to improve the reliability of this circuit especially as the main battery begins to wear down.  R6 makes it harder to turn IC1 “ON” and once “ON” it is harder to turn it "Off.”  R7 sets the point at which the on/off occurs and IC1 amplifies the voltage changes it receives at it input so that it is either at 0v or close to 7.5v or the full battery voltage.  This circuit would work without these three components under ideal conditions but the mechanical positioning becomes much more critical for reliable operation.

R4 and R5 divide the voltage of the pulse signals to the counter in half so that it is lower than that which might damage the counter.

IC2 is the counter itself and requires only a tiny amount of power to display its current value so the coin battery (B2) is sufficient to keep it active for a long time.

R8 is required by the counter as part of the circuits to drive the LCD display

R9, R10 and C2 work in tandem to “gently” drop the voltage to the reset pin when clearing the count and then to allow the voltage to slowly return to high and allow counting

PARTS LIST^*

A few rough calculations to determine distance

Diameter of typical freight car = 33”

Circumference of wheel = (33”  x  3.1416)  = 103.6”

Distance / 10 revolutions =   103.6 x 10 = 1036”

Revolutions / mile =  (5280’ /  1mi)   x  (12” / 1’) x  1 revolution / 103.6”  =  611 revolutions / mile

If a scale mile (smile) is 1/6 of a mile

Then  6 smiles equals to 1 mile

And  100 revolutions of wheel = 1.00 on counter = 1 smile    (or 1/6 scale mile)

In HO scale a Smile would be equal to 5280’ / 87 HO scale  x  6 smiles / 1 mile = 10’

Each count on the counter would then be about 1 actual foot

999999 revolutions = 103672454 inches =  6639371.157 feet = 1636.2 HO scale miles 

In the real world this would be 18 actual miles

A train traveling at 10 smiles / minute (10 feet)  would be traveling at 60 miles per hour

Train crews should be encouraged to run their trains very slowly.

LM358 Operational Amplifier