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Guide to Using Prewired Suface Mount LEDs

Posted by on Apr 30, 2015 in Miscellaneous, Tips Decoders and Installations, Tony's Tips |

Some LEDs come prewired with magnet wire.  In this case we’re using 0603 smd type LEDs. The magnet wire is very thin and has an insulating coating which has been removed from the ends for 12mm/0.5″. The leads are of different lengths, with the longest being the [+] connectionto the blue [common] wire from the decoder. A 1.5 KΩ resistor is to be soldered to the short lead, which is then connected to the function wire from the decoder.


Do not replace the magnet wire. The additional heat cycles due to soldering will reduce the lifetime of the LED, possibly to zero.

It is best to make a tester and check that the LED functions after eac step of the installation. The tester is a 9v battery with two leads and a 1kΩ resistor soldered into the circuit [snap on clip leads are from Radio Shack].


For soldering, wrap the magnet wire a few times around the component lead or wire to which you want to make a connection. Slide a piece of heat shrink tubing onto the magnet wire before soldering. Apply flux and a little solder to a clean iron and let the solder flow from the iron into the joint. If you want to shorten the magnet wire leads, cut to the desired length and follow the same procedure. The heat from the iron will melt the coating so you can make the connection without trying to remove the coating. TEST THAT THE LED STILL LIGHTS!


Preparing the LED for installation in your loco will depend on the specific configuration but there are two basic situations:

  1. The loco has either a light housing twith a lens in place[steam loco headlight or diesel dith lights] or there is a light bar which carries the light to the proper location and there is a a lens at that point [in many diesels, especially N scale]. Position the LED in the light housing or behind the light bar and hold in place with tape or even white glue across the wires. For a little added security to prevent the LED from being accidentally broken off the leads, the LED and solder points can be encased in clear epoxy, essentially giving it the the form of a bulb. Since the light is emitted directionally throught the yellow cap, this has the advantage of providing a more uniform light.4
  2. The loco was supplied with incandescent bulbs and the bulb itself is the lens. The opening in the shell for the bulb is generally too small for the epoxy bulb [above] to work so a lens is fabricated from acrylic rod.


  • Cut a length of clear acrylic rod of the proper diameter and square up both ends; 0.5″-0.75″ long will work. Glue the LED with the yellow cap down onto one end with CA adhesive. Once this has set, a small amount of epoxy around the joint will strengthen the assembly.


  • Apply shrink wrap then a puddle of clear epoxy on the end to form a a rounded lens [support the assembly vertically to a form a uniform dome shaped lens].


Your replacement light is ready to be installed on your loco.