The red lights are simply painted. Not acceptable for a recent model (but this one is about 20 years, I guess), so let’s drill at diameter 2 mm, fill with Kristal Klear (in several times to speed up drying). There is probably a better solution: plastic optic fibre for example, but I had none.
At first disassembly, I had destroyed the white lights LED, then the light guide. So, I remade a tiny printed circuit of which here are the diagrams.
The specified colours are those of the decoder wires. The 1.5 kΩ resistor, common to all LEDs, will be mounted on the lighting circuit.
I know that some will think that I make my life unnecessarily complicated… I make the PCBs myself, which allows me to respect the pitch of the compartments. For an RIB car, there are four compartments and three platforms, which makes seven LEDs. Since we can not reasonably put more than four white LEDs in series (otherwise the voltage drop would be too high), I need two circuits, one with four LEDs, the other with three. In this case, depending on the voltage (DCC is different), the voltage dropped by the series resistor will not be the same in each circuit, and thus the current and the brightness may be different (Yes, I know, I quibble). To compensate for this difference, I place a (non-electroluminescent!) diode, which will have about the same voltage drop as an LED, in series with the three-LED circuit. It is a Zener diode, in this case having a 3.3 V Zener voltage.
Here are actual values measured with a 1.5 kΩ resistor. You will check that VR / I gives (approximately) 1500 Ω:
|UPWR (V)||VLED (V)||VR (V)||I (mA)|
You may find the current too low. It is indeed very far from the rated current, but, for me, the luminosity is satisfactory; it depends of course on the type of LED. Choose LEDs as bright as possible, they will consume less current for the low illumination we need.
How do you say? The capacitor? Ah yes! It is used to store a little energy and to restore it in case of bad electrical contact. See this page for an explanation of how it works.