N Scale Kato FEF into Reading T1

[propmeup1]

As a machinist i can do pretty much any cosmetic work on an engine including turning the wheel diameter down.  I'm using a 3d printed RDG T1 shell that i was involved in it's making along with the tender.
    My weak point is lighting. I can install my own sound decoders as well and in this case i'm using another Tsunami 2 TSU-1100.   Point is I must remove the LED that is installed in the engine on the board up top.  Reading had their headlights mounted away from the smokebox face on it's own bracket and were not centered.  What I want to do is put a small LED in the 3D printed light fixture and run the wires through the smoke box face and hook them up to where the existing LED is.  I'm coming up with 2.93 V when i test it while running.  I imagine i need a 3V LED or is there a different size to use with a resistor ?   Also I'm not very sure of the LED size i will need once the decoder is installed. Again, Soundtraxx TSU-1100
    I don't want to toast anything.     Some of my other installs I could use fiberoptic wire but in this case it would stand out a little more then I'd like with the gap between back of light fixture and smokebox face.

  Any suggestion are appreciated.
Thank you,
Keith

[peteski]

As a machinist i can do pretty much any cosmetic work on an engine including turning the wheel diameter down.  I'm using a 3d printed RDG T1 shell that i was involved in it's making along with the tender.
    My weak point is lighting. I can install my own sound decoders as well and in this case i'm using another Tsunami 2 TSU-1100.   Point is I must remove the LED that is installed in the engine on the board up top.  Reading had their headlights mounted away from the smokebox face on it's own bracket and were not centered.  What I want to do is put a small LED in the 3D printed light fixture and run the wires through the smoke box face and hook them up to where the existing LED is.  I'm coming up with 2.93 V when i test it while running.  I imagine i need a 3V LED or is there a different size to use with a resistor ?   Also I'm not very sure of the LED size i will need once the decoder is installed. Again, Soundtraxx TSU-1100
    I don't want to toast anything.     Some of my other installs I could use fiberoptic wire but in this case it would stand out a little more then I'd like with the gap between back of light fixture and smokebox face.

  Any suggestion are appreciated.
Thank you,
Keith

LEDs are not rocket science.    But unlike light bulbs, they need their current limited. The voltage just for reference.

If you are simply unsoldering the existing white LED from the decoder then mounting another LED remotely (soldering its wire leads to the pads which used to be connected to the on-board LED, then there is nothing else you need to do. Just solder the new LED leads to the pads which used to hold the on-board LED.  The decoder already has a current-limiting resistor in that circuit.

If you are adding a new LED to the decoder, but using the decoder's function output pad and a common positive pad (blue wire) which has +12V potential, then you need to add a series connected resistor to the LED.  1000 ohm (1k ohm) is a good value for white LEDs.  If the common positive lead of the decoder is less than +12V then the resistor value will be different.  Also, some decoders have resistors already included in the additional function outputs - that would be mentioned in the decoder's manual.

Small SMD (Surface Mount Device) LEDs come in several small sizes.  Like 1206, 0805, 0603, 0402, 0201.   Those numbers are actually the LED's footprint or size.  Split that 4-digit number into two 2-digit numbers, then add "0." to each number.  For example size of 0603 LED is 0.06" X 0.03". That is its footprint.  That shoudl help you out in determining the size of LED you need.

[jdcolombo]

Like Peteski said:

If all you are doing is wiring up a new LED to the existing light board, all you should have to do is cut the old LED off and solder the new LED's leads to the pads on the board where the old LED was. You do have to worry about polarity: Anode is +, Cathode is negative.  It's sometimes hard to tell on SMD LEDs which is which, particularly if you buy them pre-wired.  If you have any question about which wire is which, solder a 1000K resistor to the negative "spring-y" terminal of a 9V battery.  Then solder what you think is the negative side of the LED to the resistor, and touch what you think is the positive to the positive terminal of the 9V.  If the LED lights, you know which wire is which.  If it doesn't light, reverse the wires and test again.  The board you are soldering the wires to might have + and - marked; if it doesn't, make sure you know which is which (you apparently do, since you measured the voltage).

I echo: it's not rocket science.  Fear not.

John C.

[propmeup1]

Thank you guys, that helps. i will keep you posted how it works out.

[peteski]

Yes, as John added, LED polarity matters, but hooking it up in reverse (using the voltages we encounter on decoders) nothing will be damaged.

As far as that simple 9V battery tester, here is one I made couple of decades ago, and which I use often: https://www.therailwire.net/forum/index.php?topic=46039.0

[propmeup1]

That's perfect, thank you.  Yes I always worry about frying a decoder, been there done that twice over the years.  $110 bucks gone in an instant, POOF !  But they say, if you not breaking anything you not learning.

Keith