POVTree; geek out your holiday!
I was talking to my wife the other day about projects I am working on and, as often happens when I have to break down what I am working on in easily digested chunks for those less technically inclined than myself, my mind began to wander down another fairly unrelated path. Typically my last sentence starts to trail off and I look glazed and go "oooh... ooooooooooh..." as I rapidly ponder the concept, then I regain my focus and finish my conversation. The result of this "oooh" moment was the "POVTree", a Christmas-style tree-shaped persistence-of-vision state-of-the-art thoroughly-hypenated take on the modern aluminum pre-wired never-die (oooh, more hyphens!) Christmas tree. I explained this concept to my wife and she thought it sounds interesting. She says that about almost everything I am working on because I think she is afraid I would take all the time necessary to explain all the painstaking details to her, and I am pretty sure she would rather chew off her own foot than stay stuck in that trap. So, I filed my concept away for "develop before next year's Xmas".About 2 weeks later MAKE Magazine's RSS feed had a headline about some guys who made a POV Christmas tree! Oh well, this happens to me all the time; come up with a trivial geeky concept and somewhere between two days and two months someone posts video to YouTube of it working away. Hey, million geeks with a millon microcontrollers and all that. Quite often the person who actually developed the idea does a nicer job than I would have and I feel like a hole in the space-time continuum has been sealed, but this time... well, not only do I have some free time on my hands but the fine gentlemen who beat me to the punch on this missed only ONE absolutely critical detail, and the difference between what they did versus what I did virtually defines the whole look of what you can now call a tree. At the end of this article I will sum up the only significant change to the concept those gentlemen used in their project, and you can be the judge of how important it really is to your own efforts to build a POVTree.
Here is the thing in all its hackety glory. I'm proud I did not use ANY duct tape in this project! ;)
On the left you have the "tree boughs" (3 levels of boughs) wired with 12 T1-3/4 (5mm) LEDs, using point-to-point wiring (for another hyphen-fest... and another for using hyphen-fest hyphenated!) to connect much of them together. On the right is the counterbalancing "dark limb" and the 9v battery that drives the LEDs. At the top of the column is the (far too) small DC motor I used to spin the limbs, just below that you can see the motor power leads coming out of the column and running down the face to the base, and at the bottom you can see the elaborate "cardboard and twist-ties and double-sided tape" assembly (good for two more hyphens!) that tie both limbs together at the base and allow the whole thing to rotate about the column pretty freely. At the motor shaft I gobbed hot glue all over a little plastic nib that fit the shaft and poked through the holes in the metal tape, which seems to have locked the whole thing together quite well. You will note there is a base mounted on a base; this thing has SIGNIFICANT force behind it when spinning at modest speeds, and the original base would try to throw itself off my workbench when rev'ed past a very very low point. The extra wood almost complete solves the problem, and I think a little effort in trying to evenly weight the two limbs would all but eliminate it.
Hot glue. What can I say? Use it by the gallon in all your projects! I didn't really need to glue in the LEDs in the reamed-out (-!) small holes, but it was required for the large holes. It was the only practical way to attach the motor to the limbs, and I even used a bit to hold the limbs together at the motor axel just to add strength.
I honestly didn't pay much attention to how the Zedo guys built theirs, so there was about a whole day of time spent just figuring out how to get it all to work. That is one of the reasons I ended up with the onboard 9v to drive the LEDs and an external 9v to drive the motor; I simply couldn't QUICKLY come up with a clean, integrated solution. I really wanted to use ALL external power but connecting the LEDs would have meant some kind of contact brush system similar to a brushed DC motor, and I was not in the mood to fab contact strips out of beryllium copper. Call me a damn cheater. I hope someone takes up the challenge and wires their own tree totally externally.
One of the more time-consuming things was to figure out how to drive 12 LEDs off a 9v battery. I tried calculating topologies, I tried using some really nice 'net calculators (I just freaking love this one), but in the end what worked best was to use my benchtop variable power supply in conjunction with a basic solderless proto board. Using the proto board I could easily set up power buses and contact points to allow me to experiment with series-parallel relationships. For example, I found if I connected 4 of my LEDs in series it took about 7.5v to drive them to a decent brightness. Since I needed to drive 12 LEDs with 9v the best answer seemed to be using 3 parallel banks of 4 series LEDs, as crudely shown here:
However, 7.5v is quite a ways away from 9v if you are an LED, and I could fully expect that if I applied 9v all 12 of my LEDs would either explode or burn out. I played with various additional limits, with the best success appearing to be a 1K resistor in series with the power. After I got the tree built up enough to view the lower branch spinning I realized that only a fully-charged (hyphen alert!) battery would drive the LEDs at my desired brightness. I then found a pretty good approach would be to replace the 1K in series with a 120 ohm in parallel (across the power lines), but this burns up such a tremendous amount of power that my resistor got extremely warm and would have eventually failed. So, something across the power would dissipate high power and heat, while something in series worked well but needed a smaller value than 1K. Irony of ironies, I have plenty of resistors between 1 and 1000 ohms to choose from in my stock... but they are almost all SMT! Doh! I was tempted to make an "Radial Thru-hole SMT resistor" (yet another hyphen too) which is the gentle art of using thru-hole leads (like trimmed from my LEDs) and soldering them to the sides of a SMT component, forming a very fragile leaded part. Nah, too stupid. Then inspiration struck.
Another LED, in place of the series resistor! It gave a perfect voltage drop to drive the other 12 at the right apparent brightness using 9v on my benchtop power supply, and it works well with a partially-discharged (-!) battery.
The support disc at the bottom is made of cardboard, making it very easy to cut to the right size and with the unexpected bonus of having a contact surface that is akin to felt due to the "fraying" of the cardboard fibers as it gets drug around the column. I used a little double-stick tape to hold the limbs ot the cardboard while I was positioning and assembling but I am not really sure how necessary it was. This was Scotch double-sided (-!) tape, very sticky stuff, and it does feel like it it holding on extremely well so I think it was worth it since it adds almost no weight.
The result? POVity goodness!
Now, the single biggest difference between my concept and the Zedo guys is this:
--Their paper design called for a cone
--Their hardware was a gourd-like blob ;)
--My design calls for "boughs", distinct levels of nested triangles
They did a VERY nice job, but it ain't a tree... it's an "amorphous POV Christmas gourd"! ;)
I know people will be screaming "why didn't you post a video clip?!?" and the answer is simple; I do not have the proper gear. The problem with filming a POV-based device is that you need to keep the shutter open at least 1/2 second for the streaks of light to properly register, but video camera shutters run a minimum of 1/30 second and as a result only capture a very small portion of the streaking. I include this video clip for those among you who have no idea what I just said... and I pity your children.
Final thoughts:
I might consider making the bottom support (is it a slip ring?) out of something far more rigid like a CD jewel case or a very large washer, but I am not sure it would work better, or even at all! I am pretty sure that the flex in the cardboard, as well as the felt-like (-!) surface in contact with the column, actually allows this to spin far easier than a rigid disc would. There is a trade-off (-!); if the whole thing is really balanced and tight you would likely lose efficiency as the cardboard felting acted as a friction brake versus a perfectly smooth and balanced disc of plastic or metal, but if your design is not dead-tight (-!) then a rigid disc may not be able to spin freely 360 degrees, and the cardboard flex and felting would allow for considerable imperfection in the design.
I'm not sure you could make this much smaller because I think each "bough" should have at least 4 LEDs (note my top bough violates this design goal by 1 LED... my shame is extreme...)
I actually envisioned this as quite a bit bigger, 4x or so, to allow for enough LEDs per "bough" so that the "surface" could be used for POV dot-matrix display. Imagine a tree with multiple boughs... one says "Merry Christmas", one says "Felix Navidad", one says "Happy Chanukah" and one says "Joyux Kwanza", all slowly rotating around the tree at different speeds and in different directions! DAAAAAMN, G! However, that is a truly monumental task and far more effort than I wanted to spend. I STRONGLY ENCOURAGE someone to try this idea out, particularly if you happen to have a half-dozen POV kits just sitting around you could quickly hack together. I also encourage someone to go psycho-nuts (-!) and use RGB LEDs, thereby allowing color animations and virtual ornaments, as well as driving the design complexity beyond the stars. ;)
Lastly, please note that this is NOT a "project"... I am not even remotely telling you how to do this yourself! This is just a presentation of something I did, and it turns out this thing is semi-dangerous thanks to spinning around a chunk of metal tape at decent speeds. If a cat walked into this it could die in a split-second, if I child touched this it could lose a fingertip. DON'T UNDERESTIMATE THESE FORCES OR DARWIN WILL PWN J00!
Merry Christmas!
(C) 2006 Don Stratton
two comments:
Hi Don,
Nice tree!
Posting to zedomax.com
:) Max
max () (link) - 12 18 06 - 11:27
Thanks, Max! I’m really glad you guys did your tree or I might never have gotten off my rear and done mine. ;)
—Don
Don Stratton () (link) - 12 18 06 - 11:47