VWCDPIC

Schematic (connector board)

Schematic (main board)

PCB (connector board)

PCB (main board)

(Cadsoft Eagle 4.13 format)

Description:

From the Designer's Website:

"The VWCDPIC is a "lock pick," if you will, that unlocks the CD Changer interface by sending the required data stream, fooling the head unit into thinking a real Volkswagen CD Changer is connected."

This allows you to play your own audio through the head unit, and the head unit simply thinks it's dealing with the CD changer.

I haven't gotten it working yet, but the VWCDPIC also interprets button presses from the head unit and outputs serial data accordingly. This would allow me to control playback on my mp3 box using the head unit controls, just like I was using the CD player. I have gotten to the point where I can see serial data, but it is gibberish, not easily readable text like it should be. I have to find time to take my oscilloscope out and see what's wrong with the serial data stream... My guess is bad bit timing.

Progress:

Done. The only thing I've had to do since I built it months ago is add a diode, capacitor, and auxiliary power jack. It normally receives audio through the CD changer connector but for some reason, it seems to reset itself when the volume is too high on the head unit... my guess is that since the head unit drives the speakers itself, the amplifier in it is causing spikes on the power to the board. Although I'm using a 5v regulator on the nominal 12v line, so I should be good down to about 7v input... But I can't think of any other reason so far... and adding aux power seems to have taken care of it, so it backs up my theory.

The second (smaller) board is the connector board to the CD changer connector. These connectors are pretty much proprietary, and can only be purchased from germany and/or german auto parts suppliers, which is expensive. And they use metric spacing (4mm if memory serves)... So I just did some quick measurements and threw a board design together, drilled the holes a little small, shoved in single 0.1" rectangular header pins, soldered them up, and smeared it with epoxy so nothing could break off easily. The connector takes a little finesse to actually plug in, but otherwise it does work fine.

The serial jack.

A new little issue:

So the VWCDPIC has had this issue for me for some time now: when the volume is set too high, it will intermittently reset, causing the head unit to go back to the radio. I never knew what was causing it, but also never took the time to really investigate. I built one for another person at one point, and he said he had no such problem with his, so I was even more suspicious of mine. I finally took it apart, and saw this:

That's the ground trace, going from the wire on the CD changer connector to the common ground point on the board... Yeah, it's blown off the board in one place, and vaporized for a few millimeters further down. I don't know what caused it (hopefully a one-time wiring error or something), but I can definitely see how a blown ground trace could cause issues; the fact that it still worked at all is surprising, and random resetting when it didnt have a proper ground connection is not a very surprising symptom.