Christmas Light Project

Hello, bhankiii here. frankd20 suggested I repost this on his Workbench Creations blog.

About this time last year, I was watching Star Trek - Generations, - the part where Picard is caught in the Nexus and he's looking at the Christmas tree. There are these glass balls on the tree that light up with little slow motion light explosions inside.And that made me want to make some. So, over the past year, with some input from CR4 members, I made a set.

I bought a bunch of blown glass balls - I think they sell them as garden gazing balls - at Hobby Lobby. They are in two sizes, soccer ball or basket ball size, and come in a variety of colors. Each ball has a neck about two inches long and 2 inches in diameter.

The idea was to place a circuit in each ball that would control a 12V xenon lamp - the kind they use in outdoor landscape lights - and have the lamp come on and off slowly, like you were controlling it with a dimmer.

My original plan was to make an analog circuit, using a 555 timer, and a ramp generator and pulse width modulator to control the light. I discussed the idea here on CR4,Someone recommended using a PIC microcontroller. I wasn't keen on the idea because "microcontroller" to me means a chip with 100 pins on it, and I didn't want to deal with that, but then I stumbled across Microchip's family of 8 pin microcontrollers, and I was sold.

I use the PIC chip to drive a MOSFET switch to pulse width modulate the lamp.

I programmed the microcontroller with variables for the ramp up/down time, the on time and the off time, and a value for the initial delay after power on. At first I thought I would make each ball with different timing variables - some would blink quickly, others would blink more slowly. Eventually I realized how boring that would be, and I modified the program. By playing around with a spreadsheet I was able to put together a set of values for each of the variables that gave me acceptable duty cycles. By cycling through the variables in a software loop, I was able to create 336 different timing combinations in a sequence that lasts about 1 1/2 hours.

The balls come on in sequence, over about three minutes, then execute the same series of 336 combinations, and then repeat. The effect looks somewhat random, but it's actually not, each timing instance moves from ball to ball. But the movement is so slow that the pattern is only subconsciously felt, if at all. The desired effect was to make people stop and watch for a few moments as they passed by.

This was my first prototype:

After verifying the electrical design I had to design the circuit board. I downloaded a free CAD program from the board manufacturer (PCBExpress) and laid it out:

I ordered 20 boards and parts, and built 18 units - enough for my 17 balls, plus a spare:

The next trick was to figure out how to mount the circuit board in the glass balls. It needed to be weatherproof and watertight, but also accessible in case I needed to repair something. I discovered that 2 inch PVC fittings just fit over the necks of the balls, so I glued threaded couplings over the necks, and mounted the circuit boards into threaded caps, sealing all the holes with silicone. I used standard brass lamp hardware and chain to put it all together:

Now, I just needed to program the microcontrollers. I had designed in a connector on each board that connected to the PIC programmer, so that was easy.

Next was the initial testing / burn-in. I set them up in my den and ran them 24 hours a day for a week. Here's the link to a youtube video of that:

http://www.youtube.com/watch?v=coVGu0wy3X4

All that remained was to string the 12V cable and hang them in my tree:

Night fell, and this is how they look:

http://www.youtube.com/watch?v=O2V5ekXMxjU

They've been up for a about a week now. The response from the neighbors and passing traffic has been great.

If you're interested in the details of the schematic, or the PIC assembly code, shoot me an email and I'll send it to you.