Holiday Lights 2008

December 19, 2008 at 1:43 am

Today, I took a break in the snow and finished the installation of the new light display. It’s functional, except for one light that isn’t working.  I’ve been extra busy this year, so while the main displays are up, there aren’t as many additional lights as I would like to have.

Our recent snowstorm has changed the look quite a bit – normally you only get a little light from the streetlight on the left, but now there’s a ton.

On the left, there are 8 strings of multipcolored LEDs in a circle around the light pole. To the right in front of the truck are some other lights. Hiding behind the truck is the first animated display, the “tree of lights”. The big tree (about 40′ tall) has red leds around the trunk, and features to animated displays. At the top is the second animated display, the “ring of fire”, arrayed on the tree is the new display. To the right you can see the original animated display, santa in the sleigh and on the roof. Finally, outlining the house is a red/green/blue/white string, the last animated display.

Tree of Lights

16 channel sequenced controller, about 1500 lights total. From base of tree to top is about 14′.

The controller is 68HC11 based.














Ring of Fire

Ring of Fire is 16 high-output red LEDs driven by a custom 16 channel controller, supporting 16 dim levels per LED.

The controller is Atmel AVR based.

I wrote a fair bit about it last year.








The display that started it all. It animates as follows:

  1. Blue lights in foreground strobe towards santa.
  2. Reindeer, sleigh, and santa appear.
  3. Santa leaves sleight and hops up on the roof edge.
  4. Santa goes up to the peak near the chimney.
  5. Santa disappears, and then his hat disappears soon after.

Then the whole things reverses itself.

The display itself is painted plywood, with about 800 lights in total. After 12 years the lights had gotten a bit dim, so this year we replaced all of them. The santa at the top of the roof is usually a it more distinct, but he has a big snow beard this year.

The controller is based on the Motorola 68HC11, running 8 channels.

House Lights

The house lights are 4 individual strands in red, green, blue, and white, with a 4-channel controller that dims between the colors. So, the house changes between colors.

The controller is based on the Motorola 68HC11, with 4 channels, this time dimmable.

Tree Lights

The tree lights are the new display for this year.

These are jumbo lights lit up with C7 (7 watt) bulbs inside of of a colored plastic housing. They really don’t show up that well in the picture because of all the light coming off the snow, but even so, I think I will likely need to upgrade the bulbs in them to something brighter (say, in the 25 watt range). And I think I will go with clear bulbs – having both colored bulbs and colored lenses works well for yellow and orange but the blues and greens are really dark.

The controller can support up to about 100 watts per channel, though I’m not sure my power budget can support it.

The controller is Atmel AVR based (my new platform of choice), and the code is written in C. There are 15 channels, and each of them has 32 dimming levels.

You can find a lot more boring info here.

Holiday light project 2008 in pictures

December 19, 2008 at 1:42 am

A trip through the new project in pictures:

I was late in getting started on the project due to putting finished touches on the office, but I ended up with this wonderful new workbench. Quite the step up from the old door on top of shelf units that I’ve used for the last 35 years or so (really).

Left to right, we have the Tektronix dual-channel 20MHz oscilloscope (thank you eBay), a bench power supply, a perfboard with sockets on it in front of my venerable blue toolbox (also 35+ years old), a outlet strip with a power supply plugged into it, a perfboard, a STK500 microcontroller programmer, a weller soldering staioin, and a fluke voltmeter.

This the project in its first working version. On the far left partially clipped is the power supply. The upper-left part of the prototype board (the white part) has the zero crossing circuit, and the upper-right has a solid-state relay. A brown wire takes the zero-crossing signal to the microcontroller on the development board, and a brown wire takes the signal back to the relay. The Atmel AVR microcontroller that I use comes in a lot of different sizes, so the development board has any different sockets to support the. On the far-right is a white serial line which leads to my laptop – the AVR is programmed over a serial link.

Back to the zero-crossing circuit. To switch AC power, you use a semiconductor device known as a triac. The triac has a weird characteristic – once you turn it on, it stays on until the voltage goes back to zero. That happens 120 times per second, so to implement diming you need to control when you turn on the power for for each half-cycle.

Here’s a picture that should make it easier to understand.

The wavy part is the AC sine wave, and the nice square pulse is the zero-crossing signal, which goes high whenever the AC voltage is low enough. The microcontroller gets interrupted when the zero-crossing signal goes high, and then waits a little time until just after the real zero crossing happens.

If it turned the output right at that point, it would stay on for the whole half-cycle, which would means the light was on full bright. If it never turned it on, it would mean the light was (does anybody know the answer? class?) off. If it turned it off halfway in between, the light would be at half brightness. To implement the 32 levels of brightness means dividing the half-cycle into 32 divisions of each area, corresponding to areas of equal power.

(To be better, I should take into account the power/luminance curve of the incandescent bulb that I’m using and use that to figure out what the delays are. Perhaps the next version).

To do this for multiple channels, you end up with code that does the following:

  1. Check all channels to see which ones should be turned on at the current time.
  2. Figure out when the next time is to check.
  3. Set an interrupt to that time.

That gives us a set of lights stuck at a given dim level. To animate, you need to change that dim level over time. That is handled at two levels.

The interrupt-level code handles what I call a dim transition. At least, that’s what I call it now, since I didn’t have a name for it before. We have a vector of current dim levels, one for each channel, and a vector of increments that are added to the current dim vector during each cycle.

So, if we want to slowly dim up channel 1 while keeping all the others constant, we would set dimIncrement[0] to 1 and set the count to 31. 31 cycles later, channel 1 would be at full brightness.

If we want to do a cross-fade, we set two values in the increment vector.

That all happens underneath the covers – the main program loop doesn’t know about it. The main program loop figures out what will happen next after the current dim transition, and then blocks.

My early controllers were all table-based, with the tables pre-computed. This was because I was writing in assembler. The current system could also use that approach, but with only 2K of program memory, the procedural approach is more compact, though it is considerably harder to debug. I have a C# program I use to create and test the animations, but I need to rewrite it to use DirectX because I need a 120Hz frame rate to match what I the dimming hardware does.

To get back to the zero-crossing circuit, I first built this circuit using a wall wart with a switching power supply. Such power supplies are small and efficient, but put a lot of noise into the transformer side. I wasted a lot of time on this, and ultimately switched back to a conventional wall wart (from an old Sony discman I used to have) with a linear power supply. Problem solved.

Back to pictures:

Here’s the completed controller board.

In the center is the AVR 2313V microcontroller. The U-shape is the solid-state relays that switch the AC for the lights. These are nice little Panasonic AQH2223 relays, which switch 600mA (about 75 watts) (though you can get the in versions that do 150 watts), are tiny, and, most importantly, they’re less than $2 each.

Note that these do not have built-in zero-crossing circuits built in. Most solid-state relays do, but you can’t use those to do dimming.

The top has the one transistor used to generate the zero-crossing circuit, a 7805 voltage regulator to provide +5V to the circuit, and a few passive components.

Careful viewers will notice that the upper-right socket is empty. That’s because it’s only a 15-channel controller, but I used 16-pin sockets.  The blue wire that holds the AC socket wires on is wire-wrap wire that I had lying around – these are hot-glued down later on. The two black wires provide the rectified voltage (about 15V IIRC) from the wall-wart.

The controller board is in a small plastic box designed to hold 4×6 photos, and then that’s inside of a larger box. This lets me keep all of the electrical connections inside of the box. It’s not really required for safety, but if you have a lot of exposed plugs and some water on them, you can get enough leakage from them to trip the GFI on your circuit. So having them all inside is better.

The box will be enclosed in a white kitchen garbage bag for weather protection when it’s outside. That seems low-tech, but has worked will in all of my controllers over the years.


Projects like this often come down to cabling. Each light needs a wire that goes from the controller out to the light. I did a random layout of lights on the tree, and put them on 5 different ropes so they could easily be pulled up the tree on a pulled.

Here are the 15 lights and all the extension cords required to hook them up. In this case, I used 24 15′ extension cords because it was cheaper and easier than building the cables up from scratch.

That’s all for now.