A nifty little LED Christmas tree made from nothing but LEDs, resistors, and a found power supply.

Step 1: Parts

This tree has made of 17 Red, Green and Yellow LEDs - the cheapest the electronics store had in stock. (no idea what manufacturer)
Their specs: (same for all colours)
forward voltage drop = 2.0V
max continuous current = 15mA
If you can, try to get LEDs that have the same specs - it's easier to design your tree.

The power supply used is from an old printer found out on the street - any sort of DC power supply will do, but the higher the voltage, the better. The one I am using supplies 30V, at up to 400mA. Enough power for 300 LEDs, but that's overkill. :-D

Step 2: Design - Electrical

There are three possibilities when designing the Christmas tree circuit, depending on the number of LEDs, their forward voltage drop, and voltage of the power supply.

1. The LEDs drop LESS voltage than the power supply supplies.
(That is for example, if you have a 12V supply, and you have 5 LEDs - each with a forward voltage of 1.8V - then the LEDs drop only 9V)
If you connect the LEDs in series directly to the power supply, too much current will flow, and at least one of the LEDs will destroy itself (hopefully breaking the circuit, and protecting the rest)

You need a resistor to restrict the amount of current to a safe level. To calculate the total resistance needed:
R = (Vs - Vf*N)/Is
Vs: Supply voltage
Vf: Foward voltage drop across 1 LED.
N: Number of LEDs
Is: Safe current for a LED.

My initial design looked like schematic A: R1 and R2 are each HALF of R_total (for symmetry) because resistors in series are added to calculate the total resistance.

2. The LEDs drop exactly the same voltage that the power supply ... supplies. Great! No resistors are needed, just connect every LED up in series, and wire to the power supply terminals.
Be careful you have calculated correctly though, or a LED may fry.

3. The LEDs drop MORE voltage than the power supply can supply. Bad news - you can't connect every LED in series. You CAN however split the LEDs into parallel chains. If you look at schematic B, you can see that there are two paths from Vcc (+) to Gnd (-). The path on the left only has 2 LEDs, so it needs a resistor to keep the current at a safe level. (Scenario 1) The path on the right has 15 LEDs, which because each LED drops 2.0V, and the power supply is 30V, gives me exactly the right amount of voltage drop - no resistor needed. (Scenario 2)

Once you have your power supply and desired number of LEDs, you can see which scenario you have, and design your tree!

Step 3: Design - Aesthetic

Artistic skills time!

When designing your tree just remember:

1. It has to be an electrical circuit (see previous step), and so try and visualise the path the electricity takes.

2. Try not to make the distance between connected LEDs any more than twice the distance of the LED's leads, or you'll need to use extra wire (complicated)

If you look at the design B, you can see that there are two paths between the two terminals. The wires on the underside of the lowest green LEDs only connect to one terminal, and bypass them on the way around the outline of the tree. The other terminals of the lowest green LEDs are connected through the resistor, making two parallel circuits.

Step 4: The Jig Is Up!

This project does not use a circuit board, and as anyone who has tried to solder components together without one will know, it's HARD! The tree is even more difficult, as the wires and placement of components is aesthetic - you want the wires to be straight, and the tree to be symmetrical.

To overcome this, I used a jig - print out your design (or hand-draw it), and glue it to a scrap piece of wood, at least 5mm (1/4inch) thick. If you have a smooth wood like pine or MDF, you could just draw directly on it.

Find a drill bit the same size as your LEDs (3mm or 5mm, usually), and drill a shallow hole where each LED should go. Ideally each LED will fit snugly into the holes, without wiggling around.

Step 5: Soldering the LEDs

Figure out whether your tree should have (conventional) current flowing clockwise, or anti-clockwise. This will depend on the location of your power supply contacts, and which way you want it to be oriented to have the tree face forward.
Now, reverse it, because you'll be working upside down (I missed this, so my tree has the brick facing the wrong way)

Put each LED into the jig holes, making sure that they are oriented so that the positive lead of the first LED will go to the power supply (possibly through a resistor first), and that the negative lead of each LED goes to the positive lead of the next LED.

Carefully bend the leads of adjoining LEDs toward each other, and trim any excess so that ~1cm of overlap exists. Align them closely, then solder together.

WARNING:
LEDs are temperature sensitive - get them too hot, and they will burn out.
SOLDER AS FAR AWAY FROM THE LED AS YOU CAN
Try and melt the solder and put it on the connection, rather than heating up the wires until the solder melts onto them.
If you are not successful with soldering a connection in the first ~10 seconds, wait for the LEDs to cool down before retrying. If you're soldering two long leads together, the risk is only small, but if the LEDs are very close together (like the yellow LEDs in my design) then you have to be much more careful.

LED diagram is from wikipedia

Step 6: Almost There...

(If you rush the extraction, you'll warp your tree)
With a pair of pliers, go around the jig carefully teasing out the LEDs. The more care used here, the less your tree will be bent. For example, pull each LED out a fraction, then move to the next one, then come back and pull each out a little further, until the tree is free.

Once removed, the tree needs to be connected to the power supply. If you have a nice compact brick like mine, then you can use it as a solid base as well... otherwise you may need a small wooden block.

(If the terminals have any gaps around them, this is much easier)
Insert the tree legs into the gaps, or bend the legs at 90 degrees, and solder to the terminals - CHECK you have the polarity correct... :-D

Now that the tree is firmly anchored, you can correct any warping that has occurred by gently bending the tree. Make sure that no wires are shorted together before you plug it in.

This instructable has not shown the checking at each stage of construction that was done to ensure that each LED is around the right way, that the electrical design will work, that the power supply actually supplies the amount of voltage that it says it does, that the LED's forward voltage drop is what the spec sheet says it is, and that no LEDs have overheated during construction.
Exercise due care, (equivalent to 'measure twice, cut once') and you won't have things go wrong. ;-)

Step 7: Done!

Hooray! A new Christmas tree that doesn't take up heaps of space when not in use.

If you want to make it more durable, you could look at:
- Making the Jig out of clear Perspex, and leaving the LEDs embedded in it.
- Casting the entire thing in Lucite! Muahahahaa....

The only bad thing about my tree is the lack of present space. Oh well! I guess I'm buying everyone MicroSD cards!