Hint #2) Now, if you are like me, your desk tends to get crowded with "stuff" and things occasionally get bumped around a little bit. In my case, I wanted to make sure that the tree that I had plugged into the docking station's USB port would keep standing upright. In other words, I wanted to add some weight to the bottom of Tree1.1.

I could have just installed a set of batteries. But what would have happened is that as soon as I undocked my laptop, the USB power would have turned off and Tree1.1 would have switched over to battery power. (If you want to entertain the folks that work the other shifts, then by all means go ahead. :-)  But if you really only want have your tree powered up while you are there, then you need another approach.

In my case, I did installed some batteries. (Again, just to weight the tree down and to keep it from getting knocked around my desk.) You could simply use dead batteries. But if you only have good batteries and don't want to drain them, simply turn one battery around and install it backwards. Then the tree's power selection circuit will see 0V for the "battery power".

The advantage of using live batteries, with one installed "backwards", is that you always have a full set of batteries handy...  if you happen to want to disconnect the USB port power and switch back to battery power for a portable Tree1.1

Short answer:

The current technologies for blue (or white) LEDs have about 4.0V forward voltage. Too much, I'm afraid, to run directly off of a pair of D-cell (i.e. ~3VDC).

Long answer:

We, too, like blue LEDs. We toyed with the idea of adding blue LEDs to Tree1.1. We could have added a power supply circuit that converted the battery voltage rail to a higher voltage to run blue LEDs. However, that would have added cost
and complexity to the design. It also would have reduced the battery life. In the end, we decided that a simple, cheap design that ran for a long time off of a pair of D-cells was more important.

We keep watching the blue LEDs and when they get down to the same forward voltage that the traditional red, green, orange & yellow LEDs have (about 2V), rest assured, we'll put them in a PicMas tree. :-)

Better Answer:

Tree 2.2 does Multi-Color LEDs!

(Ok I admit, I couldn't resist the title). 

Several people have asked what the extra metal screw/nut is for in the kit.  In the final version of the magazine article we left off the instructions for using the metal screws to align the battery clips for soldering (see Step 15).  Here's the full description of what to do.

Installing the battery clips

The screws supplied in the kit are nylon with nylon lock washer nuts. Although not absolutely necessary it is preferable to run a bead of solder on the inside of each battery clip. To do this you will need a metal 6/32 screw and nut. Start by using the metal screw and nut and attach a battery clip to the front and back side of the pcb. Check to make sure it is aligned and run a bead of solder down the inside of each of the battery clips (front and back) Once the clip has cooled remove the metal screw and repeat the process for the clips on the other side of the tree. After this side has cooled remove the screw and replace with the nylon screws and nuts. The nuts supplied in the kit are lock nuts and will thread on the screw with some resistance. Also since they are plastic screws please be sure to avoid over tightening.