System overview

The computer and audio amps are in the trunk, the dashboard radio controller connects to the computer via USB. Only the interface is up on the dashboard.

The audio portion is totally ordinary; a pair of amps drive the two (soon, four) speakers and subwoofer. The amplifier inputs are paralleled and wired to a miniplug that plugs into the computer audio (green) jack. I won't dwell on this stuff here.

The web is full of car/computer projects; most of them suck, as they try to avoid dealing with power management. Never mind the interface issue, which is admittedly partially subjective (though I think that anyone who wants to use a traditional computer interface in a car hasn't thought out the problem, or is an idiot.)

Automobile power management is a pain. Windows is not up to the task; you could probably get OSX to do it. I use linux. With sleep-to-RAM support you could probably arrange to sleep on power off and awake on power on. This takes hardware support; commercial auto 12V power supplies like the M1-ATX below handles this part. Sleep-to-RAM isn't supported on my hardware, so I opted for a cacheing sort of behavior.


The system consists of:

I'll discuss the sub-systems and components first, then later describe how I put them together into a functional system.

The computer

Automobiles are a very difficult environment to operate a computer in. Only while the engine is running is the power supply a nominal 14 volts; it drops to well under 12V while cranking, or dead-battery conditions; it carries huge energy spikes from the engine ignition system, fans, and accessories. Drivers turn cars on and off without notice, especially when starting the engine under less-than-ideal circumstances. Except while driving, there is not a lot of power available; a 25-watt draw will drain even a large car battery in a day.

Temperatures in the trunk or under the dash range from well below freezing to easily 150 degrees in summer, even in northern climates. Vibration and shock are extreme.

Via EPIA 5000 fanless CPU: I chose a slow, fanless CPU to minimize heat production and power consumption, cooled by the fan in the Vroom PC cabinet, which is made of finned aluminum.

The M1-ATX power supply, sold by Mini-Box produces ATX power (+12V, +5V) from car power ranging from 6V (claimed) up to 32V. (However, in my installation I found battery voltage during cranking was dropping low enough for the M1-ATX to reset the computer; I may in fact have a dragging starter motor, though it starts fine; this illustrates further the difficulty of the car environment! (I added a small gel battery, steering diodes and a resistor, which solved this problem.)

The M1-ATX also connects to the motherboard's power-switch connection, and has configurable behavior regarding when it pushes the "power switch". It will press "POWER OFF" either immediately, or 5 second, 30 second, 1 hour, etc delay. I've got it set to 5 seconds since I'm doing my own power management; power-button presses merely generate an ACPI event, which most OS's trap and handle. (Which brings up a problem with ACPI's implementation; ACPI doesn't distinguish between press-means-on and press-means-off. And if a power event/switch press happens during boot, it's lost. Very annoying.)

The Vroom PC chassis is a little annoying and took a lot of work, and some customization, to fit everything in there. It comes with no, zero (0) documentation. I have no idea how a disk drive was supposed to be mounted in there [a later phone call reveals that it mounts under the power supply area], I had to fabricate a bracket/shelf to mount the drive over the board, using RTV silicone rubber to afix it to the PCI socket, and then more silicone to mount the drive to the bracket. It looks a bit kludgey, but it's a good layout for heat, vibration, cable flow, and cabinet clearance. Once assembled however, it's pretty nice, though I sort of wish I had fabricated one myself, making it simply another half-inch taller to accomodate reality.

Memory and disk seem adequate, but I should have got more of both; I was limited by cost.

There is no CD/DVD drive. That would be (1) laughably unreliable in this environment and (2) useless. Ripping music discs in the car is massively inconvenient, and the whole point was to get rid of CDs; that's what networking -- with real operating systems -- is for!

It's very tight in there, so much so that I was worried about cables chafing on the inside of the finned chassis. I laced and cable-tied all cable bundles, covered edges that cables passed over with fish paper, then put a large sheet of fish paper over the entire assembly before sliding it together.

The M1-ATX power supply fits quite nicely in the Vroom chassis; it was designed for it. There exists a jumper PC board for certain CPUs (not the EPIA 5000 unfortunately) that eliminates the power cabling. It would clear up a lot of airflow room.

(Note: I had a tiny VGA display (Xenarc) (don't buy Lilliput -- junk!) and numeric keypad up on the dash while I was prototypin the software and building the radio controller interface. They've since been removed, and wouldn't have been necessary if I hadn't been so impatient.)

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