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Bench Talk for Design Engineers

Bench Talk


Bench Talk for Design Engineers | The Official Blog of Mouser Electronics

Ask an Engineer for Griswold-Style Display Charles Byers

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Electrical engineers often go a bit overboard when it comes to decorating our homes for the winter holidays. As I approached the Clark Griswold state with my own displays, I was dissatisfied that all those strings of “miniature Italian twinkle bulbs” weren’t too original or exciting, so I set off to design something that my neighborhood had never seen before. The result is four generations of holiday snowflake displays that I will describe here, in hopes that you will build upon my ideas and create something truly unique and stunning. If you decide to build something like this, you must take all appropriate electrical safety precautions against overloads, faults, and shock hazards.

Let’s Make a Big Illuminated Snowflake

You can buy ready-made snowflake decorations at your local home center, but those are typically static, with a string of always-on lights attached to a frame. I wanted something more.

Snowflakes in nature exhibit six-fold symmetry, where six axes of symmetry radiate from a central hub. Think of it as six spokes separated by 60 degrees, with crystals arranged in groups equidistant from each spoke. If one takes a number of strands of 12 lights each, carefully arranges them with this symmetric pattern, and then selectively illuminates different combinations of strands, a wide variety of visually pleasing patterns emerge. I have designed four versions of this sort of display and will describe how you can make your own.

First Version: Simplest Version of the Illuminated Snowflake

The simplest version is strictly electro-mechanical. It uses a step-down transformer to create a low voltage. This voltage is applied to a number of strands of 12 white miniature incandescent bulbs. One bulb in each strand is substituted with a “blinker” bulb (you know, the red-tipped extra bulbs with bi-metal interrupters you can put in to make the string blink), creating a number of independently blinking strands. These are arranged on a board in the six-fold symmetry pattern, so all 12 bulbs in a strand are the same radial distance from the center, and also the same distance from the spokes. One that I made is 16” in diameter, has 20 independent strands (240 bulbs) and is assembled from the back of a big polyethylene cutting board that I drilled with 240 holes that friction-fit the bulbs. You get the lights by buying series-connected strings of miniature incandescent lights (not LEDs), and cutting them into 12 light strands. Using 50 or 100 light strings (2.5V bulbs), the power supply required to operate a strand of 12 bulbs is 2.5V * 12 = 30V. Using strings with 3.5V bulbs (as found in 35 or 70 light sets, or “140 lights in motion”), your power supply must be 42V.

Second Version: Illuminated Snowflake with a more Controlled Effect

The second version uses electronics to provide a more controlled effect. It starts with the same bunch of strands of 12 incandescent bulbs each, but instead of using blinker bulbs to randomly switch the strands in uncorrelated patterns, it uses a processor and an array of low-side switching transistors to simultaneously switch all strands. By playing random binary numbers into the switching transistor array, you can illuminate random sets of strands, which form a nearly endless set of snowflake patterns. My prototype has 16 strands of 12 lights each mounted from the back of a 4’ x 4’ piece of plywood that I drilled with 192 holes that friction-fit the light sockets. A simple pseudo-random sequence generator creates a new 16-bit number about twice a second, so the array can do 65,535 unique snowflake images, repeating itself every 9 hours or so. One complication to this design is the spacing between bulbs gets bigger as you get further from the center. If your light string has only 4” between sockets, you could have hundreds of cuts and splices to do to get the bulbs on the outer strands to span the distances between holes. There is much less splicing to do if you start by untwisting a “140 lights in motion” string, which is actually four independent strings of 35 bulbs, with ~16” spacing between bulbs.

Third Version: Illuminated Snowflake with White LEDs

The third version uses white LEDs and measures almost 8’ in diameter (Figure 1). I built its frame out of aluminum tubes and channels. Pairs of high-brightness LEDs are connected in series with a dropping resistor, wired up the radials to a central control box and connected to a controller that has 32 channels of switching transistor driven by a microprocessor. All the corresponding channels from each radial are gathered in the control box and wired to the power switches. A few hundred lines of code allow the creation of some pretty impressive displays.

Figure 1: The image shows a prototype of the illuminated snowflake in operation. (Source: Author)

Fourth Version: Illuminated Snowflake with Red, Green, and Blue LEDs

The fourth version is my project for a future holiday. It will use nearly 400 WS2812-type individually addressable LED pixels to form the pattern. These devices have red, green, and blue LEDs and a control chip that manages their brightness. A serial protocol allows a computer on the end of a string to set each pixel's color individually. I suppose one could snake a single strand around a board in a serpentine pattern and create the symmetry through software, but I am thinking of a more elegant approach. The serial connection between pixels allows a 1:N fanout, where multiple downstream segments will automatically have the same pixel pattern, which creates the symmetry needed for a snowflake. So, a controller is placed at the center of the display. A 1:6 fanout creates six spokes with about six pixels each. The end of each spoke has a 2:1 fanout creating two identical strings of around 30 pixels, each winding in the spaces between spokes. Add a power supply (5V or 12V at about 120W) and a processor to serve as the master pattern generator, and the result should be stunning.

Moving Forward

I hope you find these ideas interesting and are inspired to try a snowflake light display. Happy holidays.

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CHARLES C. BYERS is Associate Chief Technology Officer of the Industrial Internet Consortium, now incorporating OpenFog. He works on the architecture and implementation of edge-fog computing systems, common platforms, media processing systems, and the Internet of Things. Previously, he was a Principal Engineer and Platform Architect with Cisco, and a Bell Labs Fellow at Alcatel-Lucent. During his three decades in the telecommunications networking industry, he has made significant contributions in areas including voice switching, broadband access, converged networks, VoIP, multimedia, video, modular platforms, edge-fog computing and IoT. He has also been a leader in several standards bodies, including serving as CTO for the Industrial Internet Consortium and OpenFog Consortium, and was a founding member of PICMG's AdvancedTCA, AdvancedMC, and MicroTCA subcommittees.

Mr. Byers received his B.S. in Electrical and Computer Engineering and an M.S. in Electrical Engineering from the University of Wisconsin, Madison. In his spare time, he likes travel, cooking, bicycling, and tinkering in his workshop. He holds over 80 US patents.

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