Making/Wiring Mono-Pixels

This is a tutorial on how to make a pixel string manually with regular, single-color LEDs instead of 3-color RGB LEDs. It's a rather labor-intensive project but once finished, the result is quite rewarding and along the way, you'll acquire worthwhile knowledge on how pixels work. You'll also acquire much-improved soldering skills!

For this project, the author wanted to replace the incandescent mini-lights in some commercial candy canes with white leds, but also gain the ability to animate each cane individually. The diameter of each cane tube was too small for a normal RGB pixel to fit, and as the tubes were clear, red plastic, showing a green or blue from within wouldn't have worked very well anyway. So instead, the author chose to make his own. Click this link to view the working prototype: http://vimeo.com/214309360

Materials

• Commercial plastic candy canes, about 27" tall. Purchase from Home Depot, Lowe's, Target, etc.
• Common, clear lens white LEDs (12 per candy cane)
• WS2811 driver boards (purchased off eBay) (4 per candy cane)
• LED carrier boards (DIGWDF Store sells these, but you can also use common prototyping boards, 8 per candy cane)
• 5-conductor ribbon cable (about 28" used per candy cane)
• ESPixelStick controller (to power and control them, but most any pixel controller will suffice)

Understanding

To get started, let's understand how the pixel driver board works and how to wire it. At 5/16" wide and 1/2" tall, the driver board is quite tiny and consequently can be a bit testy to work with. This is the same driver board that's in most RGB bullet pixels and uses a single WS2811 chip. The top and bottom sides of the board are shown below along with the assignment of connections. This tutorial will use the 5vdc version of the driver board; pixels that use 12vdc use a variant of the same board but the differences are only to accommodate the larger voltage -- the same concepts presented here apply to the 12vdc version. The WS2811 chip has internal circuitry to serve as a constant-current driver that delivers 18.5ma of current to the three outputs; each chip requires three different channels to control its three outputs.

• The four holes along the top of the board are for the four pins of a common-anode RGB led; the RGB LED is soldered directly into these holes.
• Because the board uses RGB leds of the a common-anode type, switching is done by taking the output control pins to ground, thereby completing the circuit and allowing each respective color to light up.
• Except for the data in and data out pads, the top and bottom sides of the driver board share the same connections.
• The bottom side is clearly marked with an arrow to show the direction of data flow into the chip on the center pad.
• Data is passed through the chip and on to the next chip via the data out pad on the top side of the board.
• On a normal RGB pixel, three wires connect to the bottom side as "incoming power and data" and another three wires are connected to the top side of the board as "outgoing power and data" so that the wires can continue to the next pixel. These wires are often colored to help identify the wires but they may all be one color. In such cases, you must study the pixel carefully to try to read which wire is for which connection because the power and ground must be correct and the data must flow according to the arrow on the first pixel of a string; any other order won't work and may damage one or more pixels in the string.

• To create a mono-pixel string that has only one color (white in this example) yet use all 3 channels of each WS2811 chip, one solders a white LED in place of the red, and then carries the green and blue outputs to two more blank "carrier boards" where the 2nd and 3rd leds will be mounted. Further, the power, ground and data lines must also be brought forward so that the LED in the carrier boards can light as well as feed the next WS2811 chip in the string.
• A five-conductor cable is required; suggested types are colored ribbon cable or cat5 cable to help keep the connections consistent.

• At the led carrier board, all five incoming connections come in on one side and go out the other.
• For consistency, black was used as ground and red the +V line. (While the colors you use really don't matter, it's a good idea to adhere to as many normal colors for DC current as you can or at least be consistent with your choices.) Because the cabling was manufactured with green between the black and red, I chose green to carry the data line out of convenience.
• In this example, the blue wire was connected to the "G" channel output of the WS2811 driver board; the white wire was connected to the "B" channel output. (Again, the cable manufacturer provided the cable colors -- I just kept them consistent throughout the project.)
• At carrier board #1, the LED is soldered across the +V and "green" channel (blue wire, shown with purple lines in the picture).
• All 5 lines should continue on to the 2nd carrier board although only 4 are required. It's just easier to stay consistent.
• At carrier board #2, the LED is soldered across the +V and "blue" channel (white wire, shown with orange lines in the picture).

• This becomes one completed, 3-led, 3-channel section of the mono-pixel string.
• Notice that only three wires are connected to the WS2811 driver board (far right in the picture); these are the feed wires bringing power and data into the first chip of the mono-pixel string.
• Notice that from the WS2811 driver board to the led carriers, the 5-line wire is used per the instructions above.
• Building the string to the desired length is then as simple as connecting these 3-channel sections together. The order of the boards is DRIVER-carrier-carrier-DRIVER-carrier-carrier-DRIVER-carrier-carrier- etc. Connecting a driver board onto the 2nd carrier board of a section is easy: solder ground, data and +V to keep those three lines going to all boards in the string.

• In this example, the strings comprised four WS2811 driver boards and eight led carriers for a total of 12 boards, 12 lights. The total length of the string was 34" and was intended to be inserted into a candy cane. But it was desirable to daisy chain several candy canes together so that they could be sequenced and animated. Therefore, it was necessary to carry the data line through to the next candy cane so that connections could be made at the base of the candy cane.
• The very LAST carrier board of the string included a single wire for the data line that was brought down the candy cane to the base so that a connector could be fashioned to connect to the next candy cane. A thin cat5 wire was used to make the connection.
• The result was a candy cane that required only 12 channels to control (one channel for each led). It's certainly possible to use normal RGB pixels instead, but to produce white, it requires that all 3 channels of each pixel be on, and that's also three times the number of channels required for each candy cane. The mono-pixel string is more efficient in its use of channels.

Construction Tips

• Preparing for assembly is an important part of this project. A small dollop of solder on each of the solder pads helps a lot. Make sure you do both sides of the driver board.

• If you're planning to use ribbon cable, this kind of cable stripping tool is a real lifesaver and strips all the wires simultaneously. Cost is anywhere from $10-$25 on Amazon or eBay.

• You'll want to tin the wires so they fit neatly through holes. Twisting the ends rarely works very well, especially with multi-conductor cable. Notice the blip of solder on the black cable: a simple swipe of the soldering iron from the insulation to the tip will smooth that out so it fits through a hole. Tinned wires also take to the dollops of solder faster which lessens heat to the WS2811 chip, potentially damaging it.

• When soldering the cables to the led carriers, keep the same wire order throughout so you don't get confused. This carrier is ready for installing the LED. (Grelllbbb, DIGWDF's head engineer made these LED carriers capable of 6 lines on both the top and bottom of the 2-sided PCB. We've found them convenient for interconnecting many different types of cables in addition to their original use as a carrier for RGB or other LEDs. Actually, Grelllbbb is more of a head-case than an engineer, but that's a story for another time... )

• Here are a couple views of the WS2811 driver. Note that the inbound cable is soldered to the bottom and the outbound to the top keeping the colors the same to minimize confusion. A typical bullet pixel is soldered so both wires come out the bottom; since I wanted a linear string, I soldered the outbound wires upward so I wouldn't have to bend them upward.

• This WS2811 driver board is waiting for the LED to be soldered. It's a personal preference if you solder the led on either the top or bottom of the board. Instead of installing it flush onto the board, it's a good idea to leave enough of the led's solder legs so you can bend it to face in a different direction if you need to later.

• When you connect the second led carrier to the next WS2811 driver board, only 3 wires are required. This carrier is waiting for its led, too.

• Here's a wiring layout of how the mono-pixel string is built.

• In panels of WS2811 driver boards, the cost gets down to about $.15 per board -- and a benefit is they're already assembled. It's cheaper and easier to buy these than to design and fabricate your own. Purchase from eBay, or you might find them at aliexpress.com or even Amazon.

• If the LEDs you choose are too directional and you need more diffusion, rub the LED casings with some sandpaper to rough them up. This will disperse the light a little better. Clear on the left; sanded on the right.

• To make inbound and outbound pluggable connections for the candy canes, I used an LED carrier as the "interface." This also fit inside the base of each candy cane.