Does this circiut look correct for interior lighting?

[daniel_leavitt2000]

Against better judgment, I'll be using a Digitrax FX 4 function decoder
This circuit uses self adhesive rope lighting for the interior lights (they are SMD LEDs wired in series and rated at 12v so do not need any added resistors.) This will be using function 1

Function 2 will operate the class lights, a single red SMD LED.

Functions 3 and 4 will operate the ground/ditch lights, also SMD LEDs.

How does this look? What ratings would I need for the resistor and capacitor?

[F1 would always be on when F2-4 are operating. F2 will always be on if moving. F3,4 will be on normally, but will alternate if the horn or bell is activated.]

[wvgca]

it's an odd way to do it, with LED's that have a built in resistor [rated for 12 volt], but kinda would work except for the capacitor which should be in parallel, not series ....  in series it's just a low pass filter, and turns all the lights off ..
put the capacitor in parallel, value can be pretty much anything you have, the added resistor will need a minimum value of 1/8 watt as the added ditch light leds, and others past the added resistor --may-- consume 20 miiliamps each ..

[peteski]

Not sure where you got the idea for adding the series capacitor.  It will not pass any DC current, so none of the LEDs will light up.

The string of 4 LEDs (assuming that they are white LEDs) will work without a resistor, but it is not the best practice. I would recommend against it.  If the DCC system puts out a slightly higher voltage then you might end up with either very brightly lit interior, or even with some magic smoke released from the LEDs and/or the decoder.  Plus you will not have much control over the brightness of those LEDs.  I would recommend hooking the interior lights as 2 strings of 2 LEDs and a resistor in series.  Choose the resistor value to suit your taste for how brigthly lit you want the interior to be.  You don't want it to be as bright as a surgical suite during surgery.

Come to think of it, if you are using one of those commercially available 12V white LED strips then those are usually connected in groups of 3 LEDs in series, with each group having its own resistor.

As for the ditch and class LEDs on a single shared resistor, if more than one is lit at the same time, then they will dim (since each illuminated LED will be only passing a portion of the current allowed by the 1k resistor).  Each LED should have its own 1k resistor.  SMD resistors are very small (I recommend the 1206 size).

If you are hoping for alternating ditch ligths, you will be disappointed in the way Digitrax handles dimming LEDs.  I know I was. The other decoders mentioned (ZIMO or ESU) control the LED dimming much better.

[daniel_leavitt2000]

So each function circuit should have its own resistor in series with the LED and a capacitor connecting directly to the ground and function wires, correct?

Maybe I should wire the ditch lights together. I have not seen them operate as normal ditch lights:

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[peteski]

So each function circuit should have its own resistor in series with the LED and a capacitor connecting directly to the ground and function wires, correct?

Maybe I should wire the ditch lights together. I have not seen them operate as normal ditch lights:

I'm really confused about the capacitor.  Is it supposed to be a part of the lighting circuit, or are you attempting to add a stay-alive cap to keep the decoder powered during brief track power interruptions?

If the ditch lights don't have to alternate then you can hook them up to a single function output.

As far as resistors go then yes, each function should have  a dedicated series-connected resistor.  Each function can have more than one LED connected in series (and with the series-connected resistor), but the total voltage drop of the series connected LEDs should be few volts lower than the internal "blue lead" voltage of the decoder. That is done to have some voltage to be dropped across the resistor so it can control the current going through the circuit (thus control the LED brightness).

Online LED resistor value calculators are handy for single and multiple series-connected LEDs.
https://www.hobby-hour.com/electronics/ledcalc.php
http://ledcalc.com/
Many others too - just Google for it.

The LED forward voltage (Vf) is around 3V for white, blue and true-green, and around 2V for other colors (red, standard green, amber, and yellow).

For LED current you will probably want around 2-5mA for the ditch lights (if they are sitting right behind the headlight bezel) and maybe 5-10mA if they mounted further back and use fiber optic or light pipe to pass the light to the headlight.  The red marker lights will likely need 2-10mA (depending on the LED and your personal taste.  Interior lights are tougher as it is  a matter of personal preference, If you want them to be realistic brightness (at night) then the LEDs will likely need less than 1mA, but if you want a nuclear glow then 5-10mA might be needed.

[daniel_leavitt2000]

I'm really confused about the capacitor.  Is it supposed to be a part of the lighting circuit, or are you attempting to add a stay-alive cap to keep the decoder powered during brief track power interruptions?

If the ditch lights don't have to alternate then you can hook them up to a single function output.

Online LED resistor value calculators are handy for single and multiple series-connected LEDs.
https://www.hobby-hour.com/electronics/ledcalc.php
http://ledcalc.com/
Many others too - just Google for it.

The LED forward voltage (Vf) is around 3V for white, blue and true-green, and around 2V for other colors (red, standard green, amber, and yellow).

For LED current you will probably want around 2-5mA for the ditch lights (if they are sitting right behind the headlight bezel) and maybe 5-10mA if they mounted further back and use fiber optic or light pipe to pass the light to the headlight.  The red marker lights will likely need 2-10mA (depending on the LED and your personal taste.  Interior lights are tougher as it is  a matter of personal preference, If you want them to be realistic brightness (at night) then the LEDs will likely need less than 1mA, but if you want a nuclear glow then 5-10mA might be needed.

Yes the cap was supposed to be a keep alive/anti-flicker. My guess is that, like the resistors, I would need one for each function?

Something like this:

[wvgca]

not really, just one ...
from the blue wire [positive feed] to a ground on your decoder, easier if the pads / wires are already there ..
with a diode and current limit resistor in series to the capacitor ..

[peteski]

No Daniel, stay-alive capacitor is for the entire decoder (so it keeps the microprocessor, or the decoder's "brain" running too).  Installing caps for each function would not work.   The decoder you are using does not provide the ground (common negative) pad for installing a stay-alive cap. If you wanted to add a stay-alive cap then you would have to remove the heat shringk wrap off the decoder and find a place where you could solder the common negative lead from the cap.

The rest of your circuit looks ok, but doesn't the car have 2 red marker lights?  But do see that you understand the hookup correctly.  As for the interior LEDs, you can have more than 3 series connected LEDs.  Just hook up any additional strings of series connected LEDs and resistor in parallel with the others.  So all the series-connected strings of LEDs and resistors will be tied to blue on one end, and to the function output on the other end.