Wiring a Pixel to an external SSR
Historical Project Note
This article documents a creative method of converting a WS2811 pixel driver into a three-channel interface for an external SSR. Modern controllers may provide simpler ways to control remote DC loads, but this project remains useful for understanding WS2811 outputs and for integrating unusual devices into an existing pixel data stream.
It's possible to rewire a pixel to drive 3 channels of a DC SSR instead of the RGB LED. By so doing, you can gain control to other devices by using your normal pixel control data stream. A caveat is that in your sequencing, you need to remember which channels these devices are so you don't get confused. This example uses the Bullet or Square pixel using the WS2811 chip as the example and 5vdc power. Pixels that are powered via 12vdc may not work with this method unless the measured output voltage from the WS2811 chip doesn't exceed what the DC SSR can use as its control signal, which is usually 5vdc.
Compatibility and Safety Note
This project requires modifying a pixel and connecting its WS2811 output channels to an external solid-state relay. Verify the input-voltage and input-current requirements of the specific SSR before connecting it. Pixel construction, output current, channel order, and operating voltage can vary between manufacturers. Test the modified pixel and SSR at low voltage before connecting any external load.
How a WS2811 Chip Works
- First, let's understand how a WS2811 chip works by looking at the PCB for a typical square pixel:
- Notice there are TWO ground pads, TWO 5v pads, a DI (data in) and a DO (data out) pad, and arrows that indicate the direction of the data flow. Also notice the four holes intended for a common anode RGB led (R, +, G, B). Data, power and ground comes into the chip, the chip lights the appropriate color of the LED, and extra data, power and ground goes back out to the next pixel in line.
- A round bullet pixel has the same connection pads and the only difference is that they're on either side of the PCB instead of all on the bottom side.
Wiring Connections
- Here's a look at how a bullet pixel is wired -- if you recall, 3 wires come into the pixel and 3 wires go out to the next one. Notice that there's a DI and a DO connection for data. DI = INCOMING data, DO = OUTGOING data (i.e. to the next pixel).
- Here's a look at how a bullet pixel is wired -- if you recall, 3 wires come into the pixel and 3 wires go out to the next one. Notice that there's a DI and a DO connection for data. DI = INCOMING data, DO = OUTGOING data (i.e. to the next pixel).
- In place of the RGB Led, connect the WS2811 to a cat5 wire as follows:
- How this works is that since a pixel requires three channels to light the LED, connecting the LED outputs to a CAT5 cable instead of to an LED and plugging that cable into a DC SSR gives you 3 channels of complete control, including dimming capability. Slick! The WS2811 outputs are current-sinking outputs, meaning that each activated channel switches the connected load toward ground rather than supplying a positive control voltage. Optionally, if the SSR needs a ground connection on pin 7 so that the SSR's status light pops on, solder the brown/white wire of the cat5 cable to the ground connection on either side of the WS2811 chip. Also note that if the order of "colors" for the WS2811 chip is RGB, the "red" would equate to channel A of the SSR, "green" would be channel B and "blue" would be channel C. If the order is GBR or some other order, then of course the order of the channels for your control would be different and you may wish to wire the cat5 cable differently.
- Does this also work with AC SSRs? Possibly, but compatibility depends on the input requirements and switching design of the SSR. Some AC SSRs may provide only reliable on/off control, while others may not respond correctly to the WS2811 output at all. Dimming performance should not be assumed without testing the specific SSR and load.
Power and Voltage Verification
- Make sure adequate voltage reaches the modified WS2811 pixel. Voltage drop along the pixel string may prevent the WS2811 or the attached SSR input from operating reliably. Measure the supply voltage at the modified pixel while the display is under load. If the voltage is too low, additional power injection into the pixel string may be required. When a string of pixels is lit for full white (all pixels at 100%), if the resulting color is slightly pink, then power injection is a must; the pink color results from low current. Usually, injecting power at both ends of the string solves the problem.
Cat5 Wire Length
The WS2811 driver board used in this example was designed for approximately 18.5mA per channel. Actual output current may vary depending on the WS2811 version, external current-setting components, and pixel-board design. And as such, the control signal should be able to go quite a distance to a remote SSR. When one considers that a typical PIC outputs about 10ma/pin, a signal with nearly twice the current strength should be viable for at least as far as a control signal from a PIC-based controller.
Sample Finished Cable
- Here's a finished cable that's designed to be plugged into the END of a string of pixels. The author chose to plug it into the end as opposed to between strings because it was easier to keep track of the 3 channels. Also note the additional wire used for power injection.
