Using Raspberry Pi to drive WS2812B LEDs

Question

Hi there, I am pretty new to controlling hardware from the GPIO pins on the Raspberry Pi,

First I will give you a little background to what I am trying to achieve:

I have a 3D Printer which is controlled via my Raspberry Pi, running a distribution of wheezy containing OctoPrint. I have the Raspberry Pi Camera module connected up and a server accessible over the internet outside of my home network so I can see my prints progressing while away from home.

I am trying to use the above quoted LEDs which I purchased here:

20 RGB LED WS2812B Individually Addressable NeoPixel DC5V Arduino & Raspberry Pi | eBay

To illuminate my print bed. I chose these LED's due to them being RGB and (as the listing stated) them being compatible with the Raspberry Pi. I have thus far been unable to make them illuminate. The only light they emit is a quick purple flash when the ground and +5V supplies are first connected to the Raspberry Pi.

I have tried to use the Hyperion package on my Raspberry Pi but again no success. I ideally wanted to be able to be able to control them via the GPIO utilities already built in to the distribution to minimise CPU load (given the nature of 3D printer host software).

If anyone can advise me in this matter that would be fantastic.

Kindest regards,

Nathan

Robert Peter Oakes · Accepted Answer

You have to have the DIN pin connected to PIN 6 of the Arduino as this is the data line used to talk to the chips in the LEDs if you connected to the wrong end of the strip it wont work even though the power is correct (So need to ceck this connection) I can not think of anything else that would need to change, having the num LEDs set to one is not an issue

rew · Answer

Those leds use a serial protocol to communicate their "color" with the host computer. The serial protocol uses a bit-time of 1.25 microseconds, or 800kbps. For each led you need to send 24 bits. If you would write a program that does the GPIO up-and-down things in the right cadence, then the Linux system will mess that up because there are interrupts and maybe other programs that want their slice of CPU-time. The only option I can think of is to write a kernel driver. HO STOP! HACKTIME! Connect the serial in of the WS2812 to the MOSI pin. Configure the SPI interface for 2.4MHz. Then send an SPI packet: a bunch of zeroes (*) (reset) and then you encode: 0x1 0y1 0z1 .... the xyz are the bits you try to send, and the 0.1 (*) are the "protocol bits" that the WS2812 wants for synchronization. Anyway, then the raspberry pi will DMA the data to the SPI port and the signal will be nice and clean without interruptions. (*) I don't remember the polarity of the reset signal and the data bits. The way I've written it here, it can't work, so one or the other needs to be inverted.

Robert Peter Oakes · Answer

Also needs a PI Health Warning here The world is full of demolished PIs where the PI has tried and failed to drive WS28xx LEDs, in frustration the PI launched itself from the highest possible location into the abyss. Seriously though, If you Google this you will find there have been many attempts to drive these LEDs with a PI and all so far have failed with anything but a few LEDs or completely. A kernel driver may as stated by Roger may be your only hope but Linux being Linux, it still may suffer interference or other timing issues with longer strings what I have seen successful is using say an ATMEGA328 to drive the LEDs but a PI to perform the complicated Math of other routines to calculate the patterns, this way the critical timing is un affected by the multitasking operating system of the PI. Communications can easily be set up over the SPI or I2C bus If you are successful in writing and running a kernel mode driver, the world will be knocking at your door for a copy Good luck