Wiring a Pixel to an external SSR
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.
How an 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.
- 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! 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? Yes and no: yes you will have simple on/off control with an AC SSR, but no, you won't have dimming ability.
- It's a good idea to consider injecting power to the WS2811 chip as well because of the potential distance between the chip and the external SSR and the resulting resistance of the wire. Your mileage may vary, but if you find the external SSR doesn't fire on cue, it's probably because power injection is needed.
Cat5 Wire Length
The WS2811 chip puts out 18.5ma per channel, 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.