Rules of Engagement
At the software level, a DPF should be a classic deeply embedded system: Nobody cares about the computer inside. That rules out user education, detailed instructions, tedious care and feeding, and, in particular, Screens of Death. Basically, you don't want yet another PC administration headache.
I decided my users should transfer pictures using ordinary USB memory sticks, as they can simply drag-and-drop images from one folder to another. Remembering to unmount the stick in Windows may pose a problem, though, as it does for everyone. They then simply pop the stick in the DPF, wait for a reboot, yank out the stick, and that's all it takes.
I've seen Windows-based DPFs, but that OS doesn't make sense for this application even if you ignore the ever-present EULA issues. It's simply too easy to blow away a FAT or NTFS filesystem with an inadvertent powercord tug. WinCE may have its points, but friends don't let friends embed consumer-grade Windows, okay?
So we'll build a Linux-based DPF with some attention to hardening the filesystem. You could use any UNIX OS, but Linux wins hands down for being cheap, readily available, and well supported. I used Slackware for this project because it can have a small footprint and offers straightforward configuration; complex system management scripts just aren't appropriate for this task.
On the hardware side, any x86 laptop built in the current millennium should suffice. I started with an IBM Thinkpad 560Z from eBay (24 bucks, delivered, with no hard drive). It was built in 1998, has some limitations that make for a good story, and is about as ancient as you should get. You don't want a new, big, fast, hot laptop hanging on the wall, so fish in the ultracompact end of the pool. Remember: Power costs two bucks per watt per year, so a 20-W laptop burns $40 every year.
DPFs must not have noisy motors, so I added a 2-GB Compact Flash card ($13 after rebate), a CF-to-IDE adapter ($5, eBay), and a 64-MB DRAM card ($22, eBay). Total cost, delivered, under 70 bucks, but your mileage may vary.
You will, however, need a Linux desktop box for the software development tools. If you don't have one, you should, for all the usual reasons in addition to what you'll see in this column. You can boot a live CD version of a Linux distro to avoid installing it or, worst case, buy the cheap laptop and a small hard drive, install Linux, get the DPF code running, then convert the laptop into the DPF.
