Speed-tester

This tool (a Maxxus F09) is sufficient to show us current, maximum and average speed, and measures travelled distances, too. Such a device sells with the actual computer and a wheel sensor. As a prototype, I mounted it on an old passenger car bougie. Of course, this construction wasn't very useful: due to the high gravity and small supporting surface, the car tilted at sharp curves, bounced off at switchpoints, ...
As a sensor, I used a reedrelais. On one of the cars axis, I mounted a magnet. The measured result wasn't reliable, however. An even bigger problem was the magnet's influence: it attracted metal parts, got the wheelset to wobble,... so a magnet was obviously not the way to go...

The only affordable method was a LED/sensor switch. These things are nowadays easy to buy and affordable, and can be recycled from broken computerprinters, CD-players, ... too. They are often mounted in one single housing: betwee nthe two components is a small gap where the lightbeam is interrupted and therefore, motion detected. The downside is that is has a light emitter, and thus it needs power supply. As we don't want friction to influence measuring results, taking the power from the rails is no option. An extra battery is.
The design shows no dimensions, as these depend on the supporting car's size. However, choose a short, 4-axle flatcar. Why bogies? Well, they get the car to roll through curves with as little friction as possible, they support the rather heavy load best, and the light-sensing unit can be built easily in a bogie.This unit is mounted near the second axis, where a disk with a small gap rotates and interrupts the lightbeam.
Some simple electronics (just a LED, resistor and switch) get power from a battery (at the right). The construction method makes it easy to remove the battery and the bicycle computer.

The schemati shows little components: on the upper lef is a 9V-battery, powering a control-LED and the one in the light-sensing unit through a resistor. This circuit can be switched of to save battery power. The right circuit part shows the light-sensitive transistor and the bicycle computer. As polarity matters, you should check the current needed with an electronics meter.
These simple elektronics can be mounted on a small piece of PCB, either etched or wired on-site.
The freight car can be any spare short 4-axle car. When using a flatcar, you can start building right away, when using another type of car, you will have to dismantle is upper part.

The first thing to make is the rotating disk with its little gap. I used an old plastic wheel and milled it to shape: 1mm thin, and with a 7mm diameter. The gap is sawed in with a 1mm thick blade, as deep as possible, but without damaging the wheel's central hole.
The light-sensing unit gets it's wires: one common wire, connecting the led's negative pole and the phototransistor's emitter. Use flexible, thin, coloured wires. Lign up the unit with the disk, so it can rotate freely, but as deep as possible in the unit's gap.

The support's base is made from styrene sheet. On one end, we make a wider part, followed by a thinner part. These make room for the computer's supports.
5 marks are prepared for little screw holes: one right in the middle, the other near the corners, leaving room for the buffers and couplings. Holding the sheet in place, drill thin holes through the sheet and the car's top plate. Now that we have correspondinh holes, you can widen them to fit the future screws.

To enable us to dismount the bicycle computer unit, we make flexible supports from 4mm H-beam styrene profiles. They are filed and cut to fit the device's bottom. Glue them to the styrene base sheet, using the computer to hold them in place while the glue sets.
For the battery, we make a fitting open styrene box at the other end of this bottom sheet, leaving room for two brass strips to conduct electricity from the battery to the central electronics.

The picture shows the desired result after gluing. The styrene needs to be protected from sunlight, so I painted it using an airbrush and “Hull Red” from PolyScale.
Drill some more holes to let the wires from the light-sensing unit through. Take care the car's bougie can swivel easily.

Now for the electric parts: this shouldn't be much of a problem to surface-mount them. A piece of thin flexible bronze sheet is used to cut two 3mm wide and two 5mm wide strips, which are soldered at the edges.
The widest strips will conduct battery power, the thinner will make contact with the bicycle computer unit.

Now, the bottom plate can be screwed to the flatcar at the corners. I used very small screws to do this.

The bronze strips are folded to fit the battery contacts, the other are bent a little to make an easy contact with the computer.

What can be measured with this device? Well, current, maximal and mean speed, distance and total amount of kilometers or miles.

Not to difficult to make, we have a measuring device for model railroading. The most expensive part can still be used for its original purpose. Just take it of an adjust the wheel diameter to that of your bicycle.