Hardware
The leds are connected to the SPI 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.
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 NAND (74HCT00), OR (74HCT32), or AND (74HCT08) ports). Note that it is important to use a chip from the HCT version as these are compatible with the 3.3V inputs of the Pi. 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. Below is a picture of the schematic and pcb layout we created and some photos of the result. We added the option to power the pi using the power supply of the leds by placing an additional fuse, but due to a very bad Chinese power supply we decided to power the Pi and the leds separately and omit the fuse.
