Today I took a look at the Rasberry PI website and found this video:

It demonstrates XBMC – another free media centre software solution – running incredibly well on hardware that costs $25/$35 (depending on model). I am in awe at what the Raspberry project has achieved, and will forgive the annoyance that had I waited a few months I could have saved myself > £150.

This project has drawn the attention of the media – I discovered it care of The Register – for trying to rekindle computer & electronics hacking (in the true sense of the word) as a hobby. I have little doubt that it will accomplish this and spawn a major community much like the Arduino has.

One of the advantages that this has over the Arduino is that with the $35 model there comes ethernet, HDMI and USB onboard, far simplifying things for people who just want to get going. An ethernet connection makes a lot of sense when it’s likely that people will be using these as frontend devices, or setting up networks of them to communicate and do really cool things.

I’m really looking forward to getting my hands on one, but with the initial batch of just 10,000 units likely to sell out quickly, it may be a while before I can. I sit waiting for the store to open anyway.

Update:

Bit-tech have a good overview of what the Raspberry Pi is all about.

One thing that has been lacking since the first build of the rack is ventilation. With both sets of doors closed, the inside can get quite warm, especially when iron is turned on. Not any more though, having just completed the installation of 2 active ventilation zones, lighting and a low voltage power supply system complete with rack-mount control panel.

Air is drawn in at the bottom of the rack by 5 quiet 80mm 12v fans, positioned behind a perforated 2U panel inside the front panel of the base. The air is blown vertically into the cabinet from the bottom through a similar 1U perforated panel. Although the draught is barely noticeable, the difference it makes to temperatures at the front of the rack is significant.

With this ‘air curtain’ alone though, the back of the rack still gets very warm. To solve this I’ve installed a relatively huge 200mm ‘Big Boy’ fan into the newly-added top of the cabinet. This sucks out the warm air and dispenses it into the containing room. Together these fans keep the entire cabinet cool.

To power the fans I’ve used an old ATX power supply. In order to make replacement easy when the supply fails, no modifications have been made to it. Instead, I bought a 20-pin molex connector so that I could just plug the ATX connector straight into my system.

The PSU is connected to 4 switches that are mounted on a 1U blanking panel in the front of the rack. I wanted some fancy switches, so I splashed out on some nice chromed illuminated ones from China (via eBay). There’s one green DPDT switch, which switches the PSU on and off, and three blue SPDT switches which control power to the two fan systems and some lighting.

The lighting illuminates the front of the rack, and is there purely for decorative purposes. I had considered using cold-cathode tubes, but the seller that I got the switches from also sells strips of LEDs encased in a flexible transparent rubber-like substance. They are sold in various lengths, but I thought 96cm would be OK for what I wanted (at the time I bought it to experiment with, thinking that it could illuminate the wall behind my desk). This strip is now mounted on the left-hand front door.

At the end of last year I tested the prototype of the matrix audio switcher, in the process blowing up part of the amplifier that powers the speakers in the living room. After investigating the damage, I found a great site to buy the replacement parts from. They arrived before Christmas, but I didn’t get around to installing them until recently.

The new transistors are exactly the same as the dead ones, so it was just a matter of unsoldering the old ones and replacing them. The nice chunky pads (compared to all the SMD boards I’m used to handling as computer parts) were lovely to work with, and the work was done in a matter of minutes. Minutes + £6 of parts = one big saving over a new amplifier and a lot of waste electronics. If only all devices were this easy to fault-find and fix.

With a fire extinguisher at the ready and my adrenaline gland just waiting to explode, I switched on the repaired circuit and ….. near silence, just the sound of the fan – it worked! Now that both channels are actually wired to something rather than one of them shorting out, the sound coming out is pretty good. In a few months the whole system should be up and running.

• Temperature
• PIR (motion detector)
• Door contact
• Window contacts
• Light

Room specific sensors could be:

Entrance hall:

• Current meter
• Intruder alarm status (triggered/armed)
• Door bell

Kitchen:

• Back door bolt contact
• Oven/hob state

Each of these nodes can then be queried, via the xAP protocol in this case. Temperature could be recorded, although at present our combi-boiler would probably not allow for remote control. Motion detection and door contacts can be used to determine which rooms are occupied, and along with the window sensors could be used as a secondary security system. The light sensors would be used to control the house lights.

Having one single ‘node’ to talk to would do away with having lots of independent sensors that would probably all communicate differently. Thanks to having picture rail throughout most of the house, there won’t be a problem with hiding the wiring. I’m not sure how big the boxes would be, but I dont think they would be massive. I just need to find the money to build a prototype.

Today I took a look at the damage caused by last weekend’s test of the matrix switcher prototype. I suspect that the cause of the bangs was actually a short because in my haste to test I’d only connected one channel to a speaker and left the other with bare ends – oops!

Removing the case revealed that the damage appears to be limited to two transistors on one of the channels. I’m not the only person who’s tried to fix this model of amplifier so a quick search for the numbers written on the remaining transistors revealed the information I needed to order some replacements. The parts have been ordered, so will hopefuly arrive before Christmas, although delivery times recently seem really slow.

Now I just need to remember where I left my soldering iron…

Possible systems to control include:

• Selecting audio sources and where to play them in the house
• Scheduling recordings on the MythTV distributed A/V system
• Controlling the lights
• Monitoring the security cameras
• Setting reminder alarms (announced)
• Running photo slideshows around the house
• Building shopping lists

To complement the iPod there will be several touch-screen terminals around the house which will give the same control, but in known locations so that the improvised remote doesn’t have to be carried around everywhere (and argued over).

I might mock-up some interface designs to give an idea of how I intend the system to work. Stay tuned!

1. the multiplexers refused to work
2. the amplifier exploded

Of the two problems, the second is clearly the worst because it’s cost me a £50 amplifier. It may be repairable (sounded and smells like a capacitor exploding) but I’m not going to assume that it is. Since the multiplexer didn’t work as I had hoped, another approach to the audio routing needs to be sought. I’ve contemplated buying a commercial product, and a quick look on eBay finds some A/V switchers with RS232 support for £350-£500. Getting one of these would have the advantage of being able to route video signals too. Of course the disadvantage is that it will be significantly more expensive than I had planned on. Remember, on top of the cost of the switcher will be the cost of 4 stereo rack-mountable amplifiers at about £50 each.

So not a great end to an otherwise highly productive weekend. I have managed to get more of the rack cabinet finished (added the locks), tidied up a bit and got mvpmc working.

This project is intended to investigate the possibility of, and hopefully build if successful, a home-made externally-controllable matrix switcher with 8 inputs and 4 outputs using parts that cost a total of < £100 (excluding the external equipment such as amplifiers, speakers and cabling). The quality of the audio must be acceptable, but I'm not expecting it to be perfect - that's why the expensive matrix switchers exist. It just needs to serve the purposes of a small house.

There are many products on the market today to distribute audio around the house. They appear to fall into two categories: MP3 (and similar) wireless streamers targeted at consumers, and hard-wired matrix switchers aimed at professional installers. The former, while relatively cheap at £35-£250 per zone, is limited to playing pre-recorded audio encoded into one of it’s supported formats. These files must then (in most cases) be hosted on a suitable machine that they can be streamed from. The matrix switchers are significantly more expensive, and generally not available directly to consumers. These distribute actual audio signals from a number of inputs to a number of outputs, such that any one output can be connected to any one of the inputs, and outputs can share the same input connection.

Initially the switcher will be connected via a parallel port to a host computer, which will run a dumb network server to relay commands from controllers around the house. The controllers will run some custom-written software (most probably as a web application).Inputs will most probably consist of some of the TVs around the house, plus the DAB radio in the kitchen and a computer to play MP3 and other audio files.

The prototype has been built, and it will soon be plugged into an ATX power supply and brought to life – hopefuly without any accompanying burning smells! The ICs aren’t cheap nor easily obtainable, so if it goes wrong it’ll be a bit of a setback. The prototype mainly exists to test that the audio signal does not degrade significantly as more outputs are added to one input, and that the multiplexer ICs can reliably reproduce a line-level signal.

More details will be posted as this project progresses. Here are some photos of the prototype and amplifier to tide you over.