	REWIRING CHEAP CHRISTMAS LIGHT LED STRINGS FOR LOW VOLTAGE USE Cheap strings of molded LED Christmas tree lights are finally hitting the market in quantity. Cheap as in $14-$20 a string, depending on color. Under $30 for a string of 70 - 100 white ones. Many of you remember paying over 75¢ for a single white LED, so the implications of hundreds of 10 cent LEDs in sockets with insulated wire tails is not likely to escape you. Unlike traditional green plastic incandescent Christmas tree lights, which have a multi-part plug socket arrangement at every lamp to allow replacement of the incandescent bulb when it breaks or the filament pops, these sets are injection molded, with an LED sealed up in the middle of two shots of resin. Farther down the page I will show how to retrofit the more traditional incandescent strings with LEDs, because those old plug and socket strings are both ubiquitous and versatile but for now, replacing the incandescent bulbs with LEDs just increases the complexity of the project. Here is how we rewired the cheap 120vac series-wired LED strings for Festival of the Forest. We used the mini-ice strings that we got from inirgie and other sources and rewired the LEDs into parallel and series-parallel strings for light toys, balls, parasols, etc and where we put the batteries and resistors to provide the correct power supply voltages various colors require.
	Yes, it was quite a bit of work - that's why they make Christmas lights and most of the other crap you haul around in your pack in sweatshops in Asia - but in fact all you need to do this rework is a table, some yogurt tubs, some nail-clippers and some tape. Wire-nuts are a cool way to fasten stuff together. So is solder, but soldering irons are HOT. Soldering tools and soldering skills are therefore totally optional.
	OVERVIEW The goal from my POV is to get the current consumed in the diode relatively large - or at least exactly as bright as you want it to be - and the current that is wasted in the resistor realtively small, or at least as small as is practical. This goal is obviously far more important when you are running off coin batteries or non-rechargable Lithium cells than when you are using rechargable AA NiMh or when you are running off 12 lead-acid car batteries in the RV (which should read something like 13.6 volts when fully charged)

	There are 3 wires in a series LED String Clip out the LEDs leaving 2 wires attached.
	POLARITY Before you can use the LEDs that you have cut out of the 120 vac series strings, you must polarize them. Actually they are already polarized - but the polarization is hidden inside - which is the reason that you have to mark them on the outside so that you can recognize their polarity and connect them correctly.
	Marking the polarity requires a test fixture. I use a pair of C batteries, connected together in series by a few resistors, with an LED wired across the hot end of the batteries to let me know that the power is on. it does not matter which side of the LED you mark - all that matters is that you ALWAYS mark the same side the same way. I do this by soldering wires to the battery with a clip on one wire and a bare end on the oither. When it correctly connected, the LED lights up and I mark the side where the clip is attached.

	ECONOMIC MOTIVATION Each string gives close to 100 of these LEDs, so the unit cost of an LED with a diffuser and a cool easy to handle package is very very low. Like 15 cents each. Far less than people would charge you for just the diffuser alone, and the socket is free.
	OPERATING VOLTAGE CONSIDERATIONS If you run a single 20 ma 2 volt yellow or red LED off a 12 volt supply (10 AA NiMh batteries) you need a 570 ohm resistor, but that gives you 20 mw to make light in the diode (it will run very bright at 20 MA) and 179 mw of wasted heat in the resistor. Better to connect six 2 volt LEDs in series for 12 volts - then all the current goes into making light and almost none is lost in heat.
	EXAMPLES Here is a series string with 6 yellow 2.0 volt junctions, each junction dissipates 20 ma at 12 volts at full brightness. Which means the whole string of 6 LEDs draws far less current than a single light in an incandescent string


	2 volt junction + 2 volt junction + 2 volt junction + 2 volt junction + 2 volt junction + 2 volt junction = 12 volts
	20 ma junction + 20 ma junction + 20 ma junction + 20 ma junction + 20 ma junction + 20 ma junction = 120 ma current
	However, if you do it this way, when the battery voltage drops below 12 volts, the LED lights will go out. So it is better to put 5 LEDs in series for 10 volts and use a 100 ohm current limiting resistor this give 200 mw of light and 20 mw of heat.

	2 volt junction + 2 volt junction + 2 volt junction + 2 volt junction + 2 volt junction = 10 volts
	20 ma junction + 20 ma junction + 20 ma junction + 20 ma junction + 20 ma junction = 100 ma current

	CURRENT LIMITING RESISTOR CONSIDERATIONS chosing the right resistor(s) for your light toy is really easy if you understand how to use the online current-limiting resistor calculator that Rob Arnold put up - I have a link to it near the top of my slice through the internet page so if you lose the bookmark or lose this page, you can find it on my site. The basic rule of thumb is that a single 47 ohm resistor will drive a 2 volt LED from a 3 volt source at 20 ma. A pair of 47 ohm resistors in parallel is worth 24 ohms (47 divided by 2) three is worth 18 ohms (47 divided by 3).
	MIXING LEDs WITH DIFFERENT VOLTAGES since you cannot necessarily safely mix diodes with different junction voltages on a series string you can mix colors that use different voltage diodes 2 ways: either as separate diodes, each with its own resistor(s) to set the brightness of that color, or as parallel groups of series strings of different colors. That is how we did it at FOTF.
	EXAMPLES Two groups of colored LEDs (imagine purple and gold or purple and red or white and blue) each color a series set wired off its own resistor pair, with the series strings connected to the power source with a common wire. On a few really cool toys where the balance of brightness between colors really mattered, we used a tiny variable resistor in series with the current limiting resistor so we could reduce the brightness MORE INFO REAL SOON

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