I have been working on expanding the output of the raspberry pi to control this relay board I ordered from Amazon. I have successfully operated a single 8 port board through a shift register on a bread board straight through the Raspberry Pi.
According to the documentation, the 5v power supply for the relay board requires 400ma for all 8 on. This might cause the Pi to reset or possibly blow a fuse if we were to push it any harder. If I plan on adding any additional relay boards, I need to find another way to power it.
It looks like the 3.3v inputs on the relay only require 0.18ma so it should be ok to power that directly from the GPIO output.
Daisy chaining the shift registers was actually very easy and on the breadboard I was able to control 16 leds using the shift register class I posted previously.
The next step is making it a little more permanent. The top of the board has a 6 pin male header for the input and power to the shift register chips; +3.3v, SER, OE, RCLK, SRCLK, GND. The two port connector is for external 5v power supply that will be routed through the output header to the relay boards.
Running two shift registers in parallel I had a lot of flickering on the leds. Following the Arduino Tutorial recommendations, I used a small cap to even out the latch pin.
I tried to keep the top nice and clean but, that meant the bottom is an ugly mess.
This is my first experience mounting electronics so I did get turned around on the input pins. I had everyone of them backwards and they had to be desoldered and moved. Luckily, I caught it before I applied any power. I hooked it up to a few leds and it worked!
Next I need to try it with an external 5v power supply to see if I can make it control a relay board.