Car MP3 Player #2

This is a computer I built for my car, it’s an MP3 music system but can also do other tasks. Based around a super-small “sub-micro ATX” motherboard from a computer known as the BookPC BKi810, I have put the motherboard, hard drive, and a 12 volt power supply in a small case that fits under the seat of my car.

Total size is approximately 11 x 8 x 2 inches. The motherboard has a lot built in. The connectors, from left to right, are mouse, keyboard, 100 MB ethernet, 2x USB, printer, S-Video, composite video, VGA video, joystick/MIDI, audio out, line in, mic in, and then two connectors I added: power in and remote control.

The wired remote control is attached to the face of the rarely used car CD player with Velcro. I used a remote control from the first Nintendo game system for the case of the remote.

The main case has latches to keep it shut, and make it easy to show off the internals. Unfortunately I have not found a source for these plastic enclosures, this was one salvaged from a 15 year-old professional modem!

The CPU fan is also the system fan, and pulls air into the unit through a filter in the top cover.

Also in the top cover are the power supply and 40 GB hard drive. Although not as tough as laptop drives, regular desktop disk drives are fine for use in cars. My other player has been in use over three years and I have not had a single problem. It also uses a Maxtor drive.

Build one for yourself?
Yes, I am documenting this enough for you to build one of your own. I built it for myself, but I used parts that can be obtained or substitutes found easily enough. This is a big project, however, and you must have enough skill and experience to take on such a project, because I won’t help you unless I think you can do it, and the amount of help I give will be limited to parts substitution or clarification here and there.

What you will need
You’ll need to have a BKi810 or equivalent motherboard. This sounds easier than it really is, because this motherboard’s power requirements are special, and the power supply I designed will only power this type of motherboard. If your motherboard has more generic power requirements, please look at my earlier player. Any other parts should be available or substitutable. Remember this is an extreme project, you’ll need to build the power supply completely by hand, there’s no PC board layout available. Take another look at the picture of the power supply above! You’ll need a programmer for PIC microcontrollers. If you want to build one, I have plans and a PC board design listed in the navbar. You also need to be comfortable with the Linux operating system.

Choices and Challenges:
The BKi810 motherboard is small and has great sound onboard. Fast ethernet is nice for updating the music and system, I have a 100 foot cable I drag outside and plug in (wireless would be nice but it’s not in the budget at this time). The motherboard requires no negative voltages, only +12 and +5 volts. The +5 is very low current, it only powers a small amount of the board. The motherboard has a DC-DC converter so the real power requirement is a clean 12 volts at a few amps and +5 volts at 50 milliamps.

The wired remote control is more of a challenge. My previous car MP3 player used a serial port for the remote. This BKi810 motherboard has no serial ports! I used the MIDI port. MIDI is a 31250 baud asynchronous serial protocol very much like RS-232, with the differences being mostly electrical. The real problem was with the operating system. I didn’t want to write my own drivers, and it seems that the use of any kind of hardware is always poorly documented. Drivers for the MIDI port are part of the sound package. I installed SuSE Linux 7.3 and found the ALSA driver for my chipset did not support the MIDI UART. Rather than install OSS sound drivers, I elected to switch Linux distributions. Red Hat Linux uses OSS sound drivers, and I installed Red Hat 7.2 and managed to get the MIDI UART to work. Raw MIDI is supposed to work as a regular serial port, but it doesn’t. The “spells” involved in doing non-blocking I/O took a bit of experimentation and I have it working. The program I wrote is in Perl.

My previous system had a remote based on my lcdterm project, but 31250 baud was too high for it. This remote control has a backlit intelligent LCD display, a PIC microcontroller, and not much else. I switched to a PIC16F628 micro because it has a hardware UART and the baud rate was no problem. It also has a PWM output which I used for controlling backlight brightness.

Power Supply:
The power supply accepts 8 to 16 volts and produces 12 volts. The topology is SEPIC (single-ended primary inductance converter), the advantage being the ability to convert up or down with a minimum of parts and maximum efficiency. The system draws about 1.7 amps while playing, but the power supply is good for about 8 amps output and can power the system while starting the car. There is a linear 5 volt regulator running continuously to power the remote control, this 5 volts is also fed to a transistor and then to the motherboard’s 5 volt input, switched by the main power.

SEPIC topology uses two inductors, but if you couple them your efficiency goes up. Don’t go calling it a transformer, though :) I used an RM10 core, I don’t know the exact material. It is wound in bifilar fashion. I used a bit of sticky label to add a gap between the two halves, and put a spot of epoxy in it to keep it from making noise, see the lower picture for the epoxy shot. RM is a generic classification for this type of ferrite core, and 10 is the size. If you can’t find one, you can use a clamshell open core such as sold by RadioShack for reducing interference. It will look completely different but work just as well. Remember to add the sticky-label gap and glue it with epoxy.

The UC3843 chip is very common and made by many manufacturers using slightly different numbering systems. It comes in an 8 pin DIP package, but I only had 14 pin SOP versions so I used a DIP header and hacked it in. Three type of protection are incorporated in the design. First is a 6 amp Polyswitch fuse, it is self-resetting, and also protects against high temperatures. Next is an 18 volt surge absorber. Finally, the circuit has current limiting using a method I invented that limits based on saturation voltage of the main switching transistor. No current sense resistors are needed. The value of the big electrolytic capacitors is not critical but they must be rated for high frequencies.

Wired Remote Control:
Only four buttons control the unit, each button does something different if held for more than a second. The remote is in charge of the power supply of the main unit. It has a sense lead from the car stereo remote wire, and powers up the computer when the car stereo is on. Computers do not like to have the power turned off unceremoniously, the remote understands. Time is allowed to boot, and the remote waits until the system is halted before powering off (unless it crashes, in which case it will eventually kill the power). I found the BKi810 motherboard standby signal to be a little non-standard when not used with its intended power switch, and it must not be loaded. Here’s where the standby signal is located on the motherboard:

I soldered directly to the bottom of the motherboard for the power and MIDI connections. Here is a diagram showing how I wired the remote and power connectors:

Over the years several people have sent email with questions about the connectors on the bki810 motherboard. I made a reference drawing of them:

Download:
Wired remote control source code rmot3mdi.c version 0.3
Binary version ready to download into a PIC16F628 chip rmot3mdi.hex
Main program mp3book.pl version 0.1

Requirements:
A functional Linux system with Perl installed.
MPG123 patch level “r” or higher.
Sound and MIDI drivers installed.

Installation:
You’ll need to edit mp3book.pl to suit your system. It requires at least one playlist, and a list of available playlists, and expects to see them in /home/bob. Playlists are just like Winamp or XMMS .m3u files, one line per song with full path, such as:
/music/Kate_Bush/Dreaming/01_Sat_In_Your_Lap.mp3
You can automatically create a playlist of all your tunes as simply as:
find /music | grep \.mp3 | sort > all.m3u
You need to start the program when the system boots, so put something like
“perl /home/bob/mp3book.pl &” in your /etc/rc.d/rc.local file.

License:
The hardware design and software are covered under the GNU General Public License.