Hardware

Electrical connections

The leds are connected to the SPI interface of the Raspberry Pi. The diagram below (original from [adafruit.com](http://learn.adafruit.com/light-painting-with-raspberry-pi/hardware)) shows how the led string connects to the GPIO header. Note that we prefer a separate power supply for the Pi and the led string in contradiction to what the original Adafruit image shows. The reason for this is that the requested current can vary from near 0.0A (all leds off) to 2.6A (50 leds full white) in a short time. The Raspberry Pi requires a stable input voltage which means that a high quality power supply is needed, or a separate power supply for the Pi and the leds. 

Buffering the data lines?

Most people connect their led string directly to the GPIO of the Raspberry Pi. This appears to be working for (almost?) all people. According to the [datasheet of the WS2801](http://www.adafruit.com/datasheets/WS2801.pdf) the data lines require 0.8*Vdd=4V for an input voltage to be considered as a high bit. This means that the 3.3V output of the GPIO is below the specs. Therefore, we decided to insert a line buffer between the GPIO and the led string based on a simple 74HCT series IC (using cascaded AND/OR/NAND/NOR ports). A similar solution providing the same kind of buffering can be obtained from [HackerspaceShop.com](http://www.hackerspaceshop.com/raspberrypi-ws2801.html) and probably also from other shops.

Physical mounting

Attaching the leds behind the TV can be done in many ways: from simple cardboard frames to solid frames from aluminum profiles. We have chosen to create our frame from tubes for electrical wiring. An advantage of this method is that the leds (not the ones in the corner) can be turned a little to adjust the angle in which they are mounted. A photo showing the result is shown below.