Ok, so I’ve been playing with a couple of Arduinos for a while now. I love these things…. soooo many possibilities. I used to use the Parallax BASIC Stamp 2P24 for similar purposes, but quite frankly, they are just too expensive to compete with the Arduino (or Raspberry PI for that matter). Plus, I don’t really care for the PBasic programming language either. The Arduino essentially uses C/C++ and the libraries are extensive.
Anyway, I bought 2 of these guys. I started with an Arduino Uno and did some experiments and prototyping with it. After playing around I had a cool idea….. remember when I said I got a Raspberry PI a while back? Well, I had decided to build an HTPC using the PI, a Mini-ITX chassis w/power supply, an RTC (real-time clock), and some other goodies. I found that Raspberry Pi + Raspbian + XBMC (Xbox Media Center) = One badass HTPC. I mostly stream video from my TVersity server, but I also added a 250GB 2.5″ SATA HDD using a USB-to-SATA adapter. The power supply that came with Mini-ITX chassis is more than ample considering the Pi only uses 5V @ 0.5A DC. The bulk of the power consumption is spent on the HDD. I also got a SWEET mini wireless keyboard/mouse combo so I can work the whole thing while sittin across the room in bed. I can even surf the web!
1) There is a lot of work to getting XBMC compiled and installed on Raspbian. There are pre-built images that have a stripped-down OS based on Raspbian with XBMC already integrated as a self-contained HTPC OS. They were great, but did not support I2C or the DS1307 (both needed for the RTC module), nor did it have adequate support for my USB-to-SATA controller. I found it best to just build XMBC on top of Raspbian and then customize the whole thing as needed.
2) Power. The Mini-ITX chassis uses an ATX-style power supply. It also has power and reset buttons, as well as power indicator LED in the front bezel just as you’d expect from any other PC. The problem is this: The Pi does not behave like an ATX-class PC. You can soft-off through the OS, but it does not power off the board. You have to disconnect and then reconnect the power in order to get it to boot up again. However, there are 2 solder pads on the Pi used for reset. So then I got the idea: I could build a PMU (power management unit) using an Arduino!!
As it turns out, the ATX specification has a 5V standby pin (purple wire) that is always on as long as it has power. This is what keeps certain functions of your motherboard alive so it can monitor power button presses, which then tells the power supply to turn on by driving the trigger pin (green wire) low, or in other words, ties it to ground. So my thought was to use the Arduino to sense power and reset buttons and then drive relays to to control them.
But that is where I ran into problems. I started off trying to use an ATtiny85 and just program it using my Arduino Uno as an ISP, but that didn’t work out because I couldn’t get the damn thing programmed. I eventually gave up and bought an Arduino Micro from AdaFruit. Not as cheap as the ATtiny85, but far more versatile and a breeze to program. Plus it supported a greater instruction set, which I ended up needing anyway.
Then I ran into another problem: Power draw. The Arduino Micro, plus LED and a couple of relays is just too much power draw from the 5V standby pin. I did not have sufficient power to run the whole circuit. So, plan B: tie the trigger pin low leaving the ATX supply on full-time (or while it to a switch for hard-disconnect). Then I can use the full 12V and 5V rails from the ATX supply. The only thing is, I had to add a third relay. The relay assignments are as follows:
Relay 1 = Reset switch.
Relay 2 = 5V power switch for Raspberry Pi and HDD.
Relay 3 = 12V power switch for HDD.
So the basic operation is simple:
Power button press (momentary):
Power currently on? no-> turn on LED and Relays 2 and 3. Otherwise, turn them all off.
Reset button press (momentary):
Power currently on? yes->turn off LED and turn on Relay 1. Do not switch states back until button release. During this time, ignore power button presses; Otherwise, do nothing.
This was actually quite effective. The prototyping phase is done. I just need to start soldering this stuff to the PCB!! I’ll be posting schematics, source code, etc to github as soon as I get everything put together and tested.