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Fitting coach lighting to a Harry Potter coach.

The following describes fitting coach lighting to a Hornby Harry Potter coach. The lighting is controlled by a TCS FL-4 four function decoder, but this can be omitted (see details below). The coach has individual compartments, each requiring its own lighting. The aim was to make the lighting as self-supporting as possible. This installation is not flicker-free as I didn't have super caps to smooth the power, but this would have defeated the purpose to some extent, as it would have made switching the lights on and off impossible. By mounting a 100uF 25V cap across the function outputs, it improves performance somewhat, but it is not perfect.

What you will need:

1. Soldering iron (15 watt) and solder.
2. Small long nose pliers to bend brass rods with.
3. Side cutters.
4. Tiny piece of strip board or similar.
5. Epoxy quick set glue.
6. 0.5mm brass rods (from model shops).
7. LED flexible strip (from model shops or Maplin). I preferred the "warm white" LEDs although any colour could be used. You can also use normal
3mm LEDS instead of the flexible strip LEDs - they are typically cheaper.
8. Resistors (values discussed below).
9. Stanley knife or craft knife.
10. Shrink tubing.

Making the support frame.

The support frame is made from the 0.5mm brass rods as it will also serve as the power rails to the LEDs.

1. Dismantle the coach and lay the interior insert on the workspace.
2. Lay a 0.5mm barass rod accross the the top of the insert as far to the one side of the compatments as possible (see picture).

3. Cut away small V-shaped grooves for the rod to lay in all along the length of the insert.
4. Cut the rod to length and lay down in place again.
5. Using the soldering iron, mark each position where an LED will be installed. Be careful not to obstruct the screws that hold the body together when selecting where to install the LEDs.
6. Lay the rod on the work surface.
7. Cut the LEDs from the LED strip as close to both sides of the LED without damaging the LED.
8. Lay one of the LEDs on the work space, with the LED facing up and the cut-off corner (negative side of LED) closest to you and on the left.
9. Properly tin the position on the brass rod where the LED will be fitted.
10. Lay the brass rod accross the other side from the cut-off corner of the LED touching the solder pad of the LED. This should be done on a flat surface to ensure the assembly remains flat.
11. Momentarily heat the brass rod untill the solder on the LED to flexible strip has melted and fused with the tinned brass rod. Be careful not to apply too much heat as it will damage the LED. Test the joint by trying to lightly seperate it.
12. Repeat this process for each position where an LED is to be fitted. Make sure to always solder the brass rod onto the side of the LED that does NOT have the cut-off corner as this brass rod will be our positive power rail.

13. Prepare a second brass rod, similar to the first, but on the opposite side of the LEDs from the first rod. Make cutouts in the insert in which the rod can lay.
14. Decide which LEDs will be switched on/off together, e.g. passages, toilets and compartment groupings. Normally the passages/tiolets would require the most LEDs on a single switch.
15. Select the group with the most LEDs that will be switched together and on the most centre LED solder a piece of brass rod perpendicular to the first rod on the negative side of the LED (side with cut-away corner) to form a T-shape.
16. Lay the assembly in place on the coach insert. Position the second rod and using the T-rod from the first rod, swivel it into position so the LEDs are facing down and T-piece is touching the second rod. Solder it in place.
17. Repeat this process for all the LEDs that will be switched on/off together. If you are not using a decoder, then all LEDs will be connected in this way.
18. In a similar way connect all the rest of the groups of LEDs that will be switched together.
19 You should now have a 'ladder' of LEDs, with brass rods connected to both sides of each LED, that can be installed across the top of the coach insert.

Testing the prepared assembly.

To test the assembly you will need a 12 - 16Volt DC power supply. You could also use a train transformer but you will need to connect a 1N1004 diode in series to rectify the power (sort of). In the latter case connect the one output of the transformer to the diode and the diode to the the

resistor as described below. The side of the diode marked with the band is the positive side.

1. Connect the positive terminal of the power supply or diode to a 3k3 (3300 ohms) resistor.
2. Solder the other side of the resistor to the positive rail of the assembly (the side opposite the one that has the cut-out on the LEDs. It should be the rod connected to all the LEDs.
3. Touch the negative to the other rail(s) in turn and check that all LEDs come on. This is prudent to do as the soldering might have damaged some LEDs if they got too hot.

Selecting resistor values.

The brightness of the LEDs can be controlled by the resistor selected. The more leds connected in parallel, the lower the resistor value for the same amount of light.
The following is a rough guide:

# LEDs Resistor
1 3K3
2 3K3
3 2K2
4 2K2
5 1K

The above values are not absolute - it reflects my preference for brightness. I have not connected 12 LEDs together, so unfortunately I don't have an appropriate resistor value if you don't use a decoder. I would guess a value of about 330 ohm would do the job.

Install the resistors.

Connect the selected resistor to the negative rail of each group of LEDs, one resistor per rail. Make sure that the resistors do no rise above the assembly when installed and also that it does not interfere with the the walls between compartments or get in the way of the screws holding the coach together. They should also be high enough not to be visible through the windows.

Install the decoder.

1. Lay the assembly in place. If required you can apply some glue to hold it in place.
2. Install the decoder in a place that will not be visible through the coach windows, e.g. the tiolets. Thread the rail wires through to the bottom of the coach making sure they do not get pinched or are subject to too much bending. Solder the function output leads of the decoder to the resistors installed above. Solder the common output of the decoder (normally blue wire) to the positive rail of the assembly (no resistor). The positive rail will be the one that all LEDs are connected to or the rail connected to the side opposite the cut-off corner of the LEDs.

Preparing the coach pickups.

1. Cut a piece of strip board to a size that will fit nicely between the wheels of the coach with the copper strips running in the length of the coach. The piece should be about 4 copper strips wide and about 1cm long.
2. Tin the strip board's entire surface fusing together 2 pairs of 2 copper strips each. You should now have 2 connector pads of solder.
3. Glue the strip board in place on the bogey and let the glue set. I found that positioning it as far to the front or the back of the bogey works the best. There should be a solder pad positioned for each side of the bogey.
4. Bend a piece of the 0.5mm brass rod as in the picture, leaving a spare piece of rod to handle the contact when installing it.

5. Position the contact in the bogey so it touches the wheel as close to the axle as possible and the other end over the solder pads on the strip board. Solder in place.
6. Repeat the process for the other side.
7. Cut off the excess rod.
8. To ensure constant connection, it is advisable to install a set of contacts on both bogeys.
9. Solder the decoder wires to the contacts.
10. Bend the contacts slightly to ensure good contact with the wheels. Check the wheels still run freely (not too much friction). Adjust until happy with contact versus drag.
11. Using a multi-meter check that the contatcs make good contact with the wheels. This can also be done with a light (LED) and power supply. Much like the test of the assembly above.

Mounting caps if you want to:

These are installed between the outputs of the decoder BEFORE the resistor. Make sure to get the polarity right. If you are using the FL-4 decoder with four functions, you will need four caps(100uF 25V). These are small and will fit in the tiolets with the decoder. Make sure to insulate all parts using shrink tubing. For my part (I come from South Africa) lights in real trains flickered and without being totally flicker-free is more realistic. If you are using a decoder, do NOT use super caps as they will defeat the control of the lights. If you are wiring the lights permanently on then you can install a flicker-free unit.

Assemble the coach and enjoy.


1. If you use the TCS FL-4 decoder the functions can be remapped to different functions. By default they are mapped F1-F4.
2. For DC operation all the lights can be wired through a single 330 ohm resistor with all LEDs in parallel.
3. I opted for giving the lighting decoders a seperate address so I am not restricted as to which coach has to be with which locomotive. Depending
on your controller, several lighting decoders can have the same address and be mapped in groups of four functions per coach.

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75 Posts
After some more exprimentation I have come up with the following:
If you increase the resistor value to 10K Ohm with a 100uF capacitor the time that it will keep 6 LEDs alive without power is about 300 milli seconds before it starts dimming. This might actually be enough in most cases to stop flicker and still allow the lights to be controlled from the decoder. I will do some more investigation and post the results here. The big question is whether the lights will be bright enough with a 10K resistor.

Note. This implementation can be done cheaply if you don't want to switch the lights on/off as you don't need the decoder ( around £10). The rest of the materials is cheap:
0.5mm brass rods £3.50 for packet of 10 from dccsupplies.
LEDs on flexible strip from Maplin about 50p each.
resistors about 17p from Maplin.
Total cost about £10 for 12 lights fitted to a coach with many spare brass rods (enough for another 6 coaches).


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I have a coach with 4 caps fitted (100uF each) one per function output and the FL-4 seems to cope fine. It also does not cause any resets. To put a resistor in series with the caps would require a high wattage and they will get warm if the objective is to be achieved.

As far as cost is concerned:
The main cost is the LEDs. If you source these cheaper then the cost would seriously drop. The flexible strip ones I used was because I had them as I thought they could just be glued to the coach roof. The spacing was wrong and I had to come up with a different idea. Any LED could be used. The brass bars makes for very neat fitting on the insert and easy subsequent maintenance. The brass rods are not that expensive as you get 10 x 300mm for £3.50 (enough for about 7 coaches). The rest is just soldering. The main idea here is to use the brass rods. The caps are an afterthought and, depending on cost when you have many coaches, the flicker-free unit might be better. Driving the flicker-free unit with a decoder might pose a problem depending on how much current it draws when charging the super cap. Charging a 100uF cap without current restriction causes a very short current draw and the FL-4 doesn't seem to mind. If you use bigger caps, you might run into problems.
The way to control how long the lights remain on without power can mostly be controlled with the resistor and I found that 10k ohm gives ample light with the LEDs I am using.

I would guess you could use one flicker-free unit per coach and use 4 releays pulled by the decoder to switch the power from the flicker-free unit to the LEDs that are switched on. I have no idea what micro relays would cost.

Where can I get flicker-free units and how much are they?

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75 Posts
DC users dont bother to read on as this solution is primarily for DCC using a decoder. For all other cases the flicker free unit with the super cap is a much better solution.

OK, I have gone away and done my homework.
This is a circuit diagram for a simple flicker-free unit at very low cost that works nicely with DCC and decoders.

a - Rectifying diode e.g. 1N4001 (Maplin QL73Q 14p)
b - resistor 1 KOhm 1/2watt (Maplin M1K 15p)
c - resistor 10 KOhm 1/4watt (Maplin M10K 15p)
d - Electrolytic capacitor 100uF 25V (Maplin KQ70M 17p) (ebay 10 for 90p)
e - LED to light the coach or whatever. (ebay 2.99 for 20 3mm white from dovebabysilk)

2 of 0.5mm brass rods (dccsupplies 46p each)
Total cost: £4.52 (includes all leds(20 leds), resistors, diodes, caps and 0.5mm brass rods).

Supply voltage assumed 16v - 18v.

Circuit analysis:

Supply voltage assumed 16v - 18v.

At (d) maximum current (assume cap has zero resistance) = 18 / 1000 = 0.018amps = 18ma.
At (d) Watts = .018 * 18 = 0.324watt which is less than 0.5 watt so 1KOhm, at B, resistor at 1/2watt should be OK.

Using a 10 KOhm resistor at ©
At © current is limited by 10 KOhm resistor (ignore LED resistance as is negligible compared to 10K).
At © maximum current = 18 / (10000 +1000) = 0.0016 amps = 1.6ma.
At © watts = 18 * 0.0016 = 0.0288. This is less than 1/4watt so the 10KOhm resistor can be 1/4watt.

Using a 4k7 resistor at © (for brighter lighting).
current = 18 / 4700 + 1000 = 0.0032 Amp = 3.2ma. This should still leave enough to charge up the cap but will halve the time to discharge.

Total current (max) at (a) = 18ma.

This solution will work best on DCC and not on DC as it needs a reasonably constant power supply. If you want to fit coach lights on DC you are much better off with the flicker free unit mentioned above. For DCC this will allow switching lights on and off with a decoder without hassle or possible overload.

Charge up time for the capacitor is less then 500ms so hardly noticable when switching on the lights.

I found the best way to assemble this unit is to support the resistors on the cap and then put the whole lot in some shrink tubing to insulate it.

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75 Posts
Lowering the resistor at b will allow more current to flow and could lead to an overheating problem unless you use a 1W resistor, although with the short time the cap takes to charge it will most likely not cause a problem with a 1/2W.

I had a play with different resistor values after the original post and trying to make it somewhat flicker free. The 10k or 4k7 resistors give a nice amount of light on up to 5 LEDs and also discharges slow enough so the lighting becomes almost flicker free. If you lower the 10k resistor too much then the cap discharges too quickly and you might need a larger cap, but as you know in most cases space is a problem. These 100uF caps fits four into a single toilet, with the decoder, keeping wiring to a minimum.

Note that these were used with the LEDs off a flexible LED strip.
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