Coach Lighting Version 2

Coach lighting is commercially available for around €10 per coach. It contains wheel pickups, LED’s and electronics, sometimes all conveniently mounted on a strip. Still it is fun to see if we can do it ourselves. And … yes we can! 🙂

While the first coach lighting experiment worked, the Christmas lights pointed in all kinds of directions except the one you’d want them to :). I still liked to do it again and experiment with the use of a DCDC converter and the much nicer LED strips. Three coaches have been fitted with the new lighting.

Components used:
– LED strip, 20 cm per coach (link: €4,60 per 5m). This strip has a warm white led every 0.83 cm and it can be cut every 2.5 cm.
– Diode bridge rectifier. Almost any type will do, a small size is nice of course.
– Resistor, 82 ohm, ¼ W.
– DCDC buck converter. (link: €0,76 a piece).
– Some 20 cm of thin metal wire.
– Some 30 cm of thin and flexible electrical wire (isolated).

The wiring:

The purpose of the 82 ohm resistor is to limit the peak current to the input capacitor of the DCDC converter. That helps to minimize wheel and DCC pickup sparking.

For the DCC wheel pickups, commercial solutions are available. I liked to try something ‘home made’. All 4 axles got a metal wire wound around it. This has a low mechanical friction and it turned out to be electrically reliable. A wire through a hole in the bottom brings the DCC inside.

To make room for the DCDC converter, some material from the cabins and chairs inlay had to be cut away. In this coach too mch is cut away, that’s because there had already been made room for a LiPo battery with the first lighting attempt.

The diode bridge rectifier is soldered on top of the DCDC converter, together with the resistor. The package is too large to be hidden, it can be seen through the windows. A few pieces of paper could be used as window blinds.

Why use a DCDC converter and not just a (super) capacitor and resistors? Well … the price is  just a few cents and I like the ease and the accuracy (10 turn pot) with which the light output can be tuned. Also the converter is a regulated device, it eliminates flicker due to rail dust (but there still is flicker, see further down). But the main reason is for the next part of the project: to switch the light via an AUX output of the DCC decoder in the loco. The chip has an ‘enable’ input. Will need to run a wire between the loco and the coach.

The LED strip is self adhesive. It is stuck to the ceiling. IMPORTANT: before connecting the LED’s, check with a Volt meter that the DC output is tuned to a value around 8V, otherwise smoke may come out of the converter, or the LED’s, or both. Then connect the LED strip and tune the light level to your liking. I measured the LED strip current, it was some 1omA (one coach, 8×3 LED’s -> current through the LED’s is 1.25 mA).

Remark: the picture shows an overdoese of wires. That’s because 2 LED tail lights with their resistors are connected too.

To light more than one coach, the options are:
1: Build the wheel pick ups and electronics in every coach.
2: Build the wheel pick ups and electronics in one couch and connect the others via wires and mini connectors. The latter is what I’ve done (it was already in place with the previous lighting  attempt).

The result?
+ Overall light impression looks OK to me
+ Very cheap at around €2,50 for three coaches.
– The converter covers two windows. A ‘window blind’ (piece of white or grey paper) may make that look a bit better.
– Even though the converter is regulated and contains two capacitors, there still is light flicker when running over turnouts and also in some curves.

Might try an extra capacitor at the input of the converter to see if that helps. But actually I already know it will not be enough. I have spots on the track where the light is off when the train stands still. Apparently the wheels lose contact with the track, a capacitor won’t help out here. With the previous battery operated lighting this was no issue at all. Well … life is full of choices. 🙂

Brainwave. Since the ‘hole’ in my carriage’s interior is already big enough, I can put the battery back in and then use a ‘step-up’ converter to upgrade the 3.7V to some 9V. They can be had for €0,80. They’re small and flat, since there are no capacotors on. The converter fits below the battery. Can get rid of the DCC wheel pickups again. This converter chip (mt3806) also has an enable input, so the DCC decoder can still switch the lights. This will combine the best of both worlds: the nice LED’s from a reel and the battery that has absolutely no flicker. It is 600mAhr, it will last some 10 hours on a charge, that’s enough to me. Yes, that is what I’m going to try next.