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White LEDs from Christmas Lights

by Pat Suwalski, January 2009

At the end of the Christmas season, just about every hardware store sells off its remaining Christmas lights at heavy discounts. They do not want to store large supplies of stock for over ten months. The LEDs in Christmas lights are completely regular LEDs and can be removed for use with lighting models.

While the information in this article can be applied to any colour of Christmas LED lights, quality white LEDs are the most difficult to obtain at a decent price. Therefore, they are the most worthwhile to take the effort of actually removing them from a solidly made chain of lights.

Background

Light Emitting Diode technology has progressed significantly in the last decade. The picture below compares a typical 12V train model wheat bulb to a typical "cool white" LED, the same painted with Tamiya Clear Orange to soften the colour, an old "soft white" LED, and a modern "soft white" LED. What is important is the colour at the center of each bulb and the colour of the fading "halo" in the diffusing light.

Photo 1

Clearly, the cool white LED produces very blue light. The painted LED loses much of its brightness. The old soft white LED is far too yellow. However, modern soft white LEDs have a very similar colour temperature to the wheat bulb. The diffusing light does not have the dramatic orange hues, but that is to be expected with the LED's limited light spectrum.

For all intents and purposes, the modern soft white LED is a practical scale model representation of an incandescent bulb.

Small Light Strings

In 2007, a chain of 35 of these could be purchased for about $8 after the holidays. They are somewhat tricky to crack open to extract the LED, but it is definitely worth it, because 3mm soft white LEDs are actually difficult to find otherwise. The Noma brand is shown here.

Simply cut around the already-scored line with a hobby knife. This will remove the clear plastic diffuser and provide direct access to the LED itself.

Photo 2

Next, with a hacksaw or rotary cutting tool, cut 5mm further down from this scored line, where the flat taper begins. This cut should be all the way through, to sever the LED completely from the string. Don't do this with the string plugged in.

Photo 3

At this point the LED should be relatively easy to pluck from the plastic collar with pliers.

Photo 4

Bare LED Strings

The 2008 season seems to have brought around a new product from Noma, which is a string of 5mm soft white LED lights, but without a plastic cover. Additionally, these LEDs have the conical indent in the lens, which diffuses light in many directions rather than at a tight angle.

Photo 5

Logic would dictate that these lights would be even simpler to deal with than the 3mm design above. Unfortunately, that is not the case. The LEDs are soldered right up to the base of the lens, which makes it extremely difficult to remove them. A combination of heat and rigorous cutting seems the only way to get them out, and it takes a lot of work.

Photo 6

However, simply cutting off the entire encased unit is simple. In many cases this solution is completely sufficient. At 15 dollars for a string of 100, these can definitely be useful.

Wiring

White LEDs are typically powered by 3.3V to 3.6V, at around 30mA. Running at 3.5V seems to be a good target. Therefore, they should be wired in-line with a resistor. Here is a table that can help:

Battery VoltageIdeal ResistorCommon Resistor
5V57Ω47Ω
6V90Ω100Ω
12V283Ω220Ω+47Ω

High voltages such as 12V may be more practical to wire in series. For example, with a chain of three white LEDs in series, only a 47Ω resistor is required. Two LEDs in series can be safely used with a 220Ω resistor.

Conclusion

The 3mm LEDs are definitely the best deal. They are difficult to purchase individually, they are simple to remove from the string, and they probably can be used in more scales than the larger 5mm components.

Pat Suwalski

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