A few years back I built a home-theater audio system in the slot in my dash where the stereo used to sit, with a 7 inch Android tablet as the face plate. I'll be adding more details and source code in the near future.
The Shaker-500 head unit in my '07 Mustang was having a well-documented bug where it constantly thought there was a cd inserted, and would spend ~30 seconds looking for it in the 5-disc changer. Going through that process every time I turned on the car got a little annoying, and I decided to do something about it.
I had also really wanted to put in a navigation head unit, since I got tired of hooking up my phone every time I wanted to use maps to navigate. This combined with the need to replace my head unit got me thinking.
Eventually I settled on using an Android tablet (the LG v410) as the stereo. This tablet was handy because they're cheap (~$40 on ebay) and they work well on the T-mobile LTE data network. At the time, T-mobile was doing a special where any tablet that could get on their network could register for a 200MB/mo data plan for absolutely free. T-mobile has since discontinued this plan, so I'm glad I got the tablet registered when I did.. as now the plan is grandfathered in and T-mobile has stated they'll to honor it going forward.
I used a USB DAC to provide higher-quality audio than the headphone jack. The one I settled on is Behringer UCA202, runs ~$25 on amazon. This was a cheap way to test out my idea, and provided RCA outputs, which I then connected to the inputs of a 4 channel 400watt amplifier. The outputs of this amp are connected directly to a wiring harness, which plugs in right where the old stereo was.
The next part was a little tricky, since the stock android kernel for the v410 doesn't allow charging while doing USB host mode (meaning it wouldn't be able to charge while outputting USB audio). This I remedied by making a custom kernel driver for USB OTG with host-charge support. Then I used this module to re-compile Cyanogenmod 12.1 from source, producing a flash-able rom that supports simultaneous host-mode and charging. The tablet is powered by a BEC (battery eliminator circuit) commonly used in R.C. aircraft. This is essentially just a DC-DC regulator that takes 8-20 volts, and produces a stable 5v supply to power the USB peripherals and charge the tablet. The input to this is interrupted by a pmos transistor, with the gate tied to the yellow wire on the stereo wiring harness ( the yellow wire is hot when the key is in or the car is running), so that the tablet charges when the car is on. After a few on-device automations, like setting variables for music playing/paused and resuming next time the car starts, the new tablet behaves almost exactly like a normal stereo. Naturally I gave it an appropriate background.
A few weeks later, I decided to upgrade the steering wheel, so I bought a wheel from a '14 mustang, and now I have lots of little buttons on the wheel for media controls. I programmed an Arduino (using a voltage divider for each set of buttons) to detect which button was pressed, and then send serial commands to the tablet over USB.
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| Looks almost professional |
The only tricky part about wiring up the buttons is that the cruise control values had changed. These are resistive buttons just like the media buttons, but between 07 and 14 ford changed the resistance values that each button produces. Fixing this required de-soldering the surface mount resistors, and adding on new ones.
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| Wiring up the new media buttons |
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| Fortunately, the Ford team put an extra plug on the clock springs, which allows you to add anything want and have it cleanly and reliably pass through the steering wheel assembly. Talk about going places |
