Turn Your Android Tablet Into a Car Head Unit

I recently finished integrating a docking system in my 2005 Subaru Baja for my Asus Nexus 7 (2013). Here's a write-up about my project with some pictures and a parts list for anyone looking to do something similar.

A standard car radio is fine, but what if you want more functionality out of the space in your dashboard? You can choose to install upgrade components, such as a GPS, better sound system, backup camera, even an on-board computer. But these options can cost hundreds, if not thousands, for equipment and labor. I have enjoyed using an Android tablet for some time now, and realized that all of the functionality I wanted in my car was built right into this system. So, why not integrate the tablet into the dashboard of my car instead of messing with expensive upgrades? Well, that's exactly what I've done.

I began this project quite a while ago, and spent plenty of time researching on Google and forums for similar projects, compiling ideas and parts, and learning more about the software and hardware requirements for this build.

My Goals

• Use my tablet as an "Android interface" with my car to replace several devices (Music, FM and internet radio, USB thumb drive, Maps and GPS navigation, and OBDII scanner/car diagnostic info).
• Make the tablet removable to prevent theft and for allow home use.
• Make the interface a seamless, "plug-and-play-and drive" setup with minimal connections and manipulations to be made each time the tablet is plugged in.
• Easily returnable to stock if I decide to sell my car.

Apps to be used in the car

• AutoMate
  • Beta app that acts as the tablet's home screen. Controls music, navigation, and apps. (Props to Kahtaf Alam, who developed this app)
    
• Torque Pro
  • Diagnostic app with an ELM327 Bluetooth OBDII scanner interface
• Google Play Music
• SDR Touch
  • FM radio receiver app. Needs RTL2832U driver for DVB-T to work
• Google Maps(for Navigation)
• Trigger (for writing NFC Tags)
• Phone Tethering (allows my phone to act as a Wifi hotspot which my tablet can connect to)

Parts List

• Nexus 7 Car Installation Parts List (minus the Subaru Baja's dash trim piece, which was found on eBay)

Required Tools and Materials

• Wire Stripper/cutter/crimper
• Multi-meter
• Soldering Iron
• Heat shrink tubing
• Epoxy putty (I like Quick-Steel)
• PlexiGlass
• Silicon Glue
• Screwdrivers/pliers

Thanks for reading!

Step 1: Check Out the Video

It's hard to describe this build in words, so showing you will give you the best idea of what this Instructable is all about. Also, if you feel so inclined, subscribe to my YouTube channel. JakeOfAllTrades. I will post more videos of projects and creations in the future. Enjoy!

Step 2: Basic Steps

Okay, because this process is highly customized to the type of car and tablet you're using, as well as your personal goals and desired operation, I'll outline a rough guide for how you'll want to install a tablet into your car, and follow up with what I personally did and the challenges I faced.

Recipe for a Tablet Head unit Installation

1. Remove your old head unit: Follow your manufacturer's manual for removal steps. Usually this involves removing some trim pieces from the dash and/or console and getting to some hard to reach screws, plus a bit of elbow grease to slide the radio from its housing, then unplugging the stereo wiring harness.
2. Set up a power/charging system: You have a few options here. You can run a line straight from your battery, which will provide constant power unless you add a switch (be sure to use an in-line fuse here), or you can run wiring from an existing unused fuse right off the fuse box. Depending on which fuse you pick, it will either be switched by the ignition or constant. Either way, you'll need some conversion hardware to step the 12 VDC from the battery down to 5 VDC for your device. Standard phone/tablet USB chargers have this circuitry built in, so it's smart to use that. You could also wire up a DC to DC converter. If your tablet supports Qi wireless charging, that could be a viable option to keep the installation a bit cleaner.
3. Set up audio equipment: You're going to need a way to get sound from your tablet to the speakers. Possibilities include Bluetooth adapters, headphone jack audio, or USB audio. Then you'll need to process the sound, so you're going to need an amplifier and possibly an equalizer. Finally, you'll need wiring to get the sound from the amp to the speakers. You can run speaker cable through your car to aftermarket speakers, but if you want to keep your original car speakers you'll need a manufacturer-specific stereo harness to match up to the one you unplugged from the head unit you removed.
4. Set up data equipment: This is entirely optional and dependent on what extra tasks you want your tablet to do. You can use Bluetooth peripherals, a USB flash drives, NFC tags, rear-view cameras, and myriad other things. These are usually hooked up to the tablet's USB port with a hub or via Bluetooth. You need to be sure your tablet can support such things, it will be called USB On the Go (OTG), which basically means it can detect data input, not just send data out of the USB port.
5. Figure out placement and a mounting system. Tons of options here. If you don't want to rip out your head unit, you can install the tablet on your dashboard or over the original head unit. You can even mount it to the glove box or a cup holder. Figure out a placement that won't distract your driving, but is convenient enough to not be a struggle to use. You'll also have to decide where to place your plugs if you're using an aux cable or USB for charging. Think about wiring and avoiding clutter. If you to permanently install your tablet you may choose to fabricate a mounting system into your dashboard. You can make this as stylish or as simple as you'd like.
6. Pick an interface: This has to deal with the apps and themes you want on the tablet itself. Usually a special launcher or automotive app is nice here. Android Auto is an up and coming theme for cars, you may choose to emulate that as closely as you'd like. Pick an interface that gives you easy access to all the options you've created for yourself and minimizes clutter.
7. Test EVERYTHING: A good multi-meter will carry you a long ways here. Check and recheck all connections for proper voltage and current. Compare your amplifier's specifications to your audio outputs to make sure you won't fry anything. Make sure you have solid grounding practices with your electronics so nothing weird happens or shorts out. I remember my car's clock would reset every time I turned the car off and couldn't figure out why. Turns out one of my cable splices on the stereo harness had come loose and was grounding out. So just make sure your hardware and software are solid and mesh well together.

Step 3: Audio Lesson

To meet my goals, I decided I wanted to completely remove my car's radio in lieu of doing a tablet mounting system that connects to the head unit via Bluetooth or a headphone jack, which is what you typically see with these types of builds. I wanted my tablet to BE my head unit, thus minimizing noise and loss of sound quality, as well as reducing the amount of connections and plug ins that had to be made just to get music to play.

I wanted to use the tablet to play music, perform GPS duties, and run OBDII diagnostic information. To play music, I needed a way to get sound from the tablet to the speakers. I didn't want to use the aux port on the tablet. Here's why: I knew I would be using the micro-USB port for charging, so another connection on the opposite side of the dock sounded messy. If I could use the micro USB port for charging AND music output, along with other data output of course, that would keep things simpler and mean a cleaner installation.

Here's a quick lesson on audio signals: Traditionally, audio signals are analog, which is good because speakers require an analog signal to make sound. However, every source of data from a device is digital. This digital signal is converted in something called a DAC (digital to analog converter). When it comes to sound signals, usually a headphone jack is the DAC (the digital sound is converted to analog in the headphone port and an analog signal goes up to your headphones). Now, The Nexus 7's internal DAC is pretty weak and sound quality would suffer when played by large car speakers (I'm no audiophile, but I just wanted to do this right). However, the makers of this tablet allowed for the digital audio signal to be an output of the micro-USB port. This "USB Audio" allows the audio data to travel through digital devices, such as a USB hub, as well as being converted by something other than the headphone jack.

Step 4: Sound Solution

Once I had USB audio figured out, I needed an external DAC to allow for an analog signal to go to my amplifier. Since I was already going to be using a USB hub, a USB DAC was perfect. I went with a TurtleBeach Micro II, because it's small, cheap, and good quality. It plugs into the USB hub, which connects to the tablet, and puts out an analog signal via an Aux port. Now, to amplify the audio coming off of the DAC, I needed an amplifier. I went with the Alpine KTP-445U Power Pack. It's designed to go behind a dash or in a glove compartment. It's small and powerful, has great heat dissipation, and can be powered off of the radio's existing wiring, perfect for a seamless installation. It allows for an RCA input, which meant I needed an aux-to-RCA adapter to come off of the USB DAC. On the output side of the amp, there is a wiring harness that consists of all the wires needed to make music happen (speakers and power). I used a Subaru-specific stereo adapter to get the speaker wires and power (12V constant and 12V ignition) from the amp to match my car's stereo wiring harness. I just spliced the wiring harness off the amp to the stereo adapter wires, which essentially allowed me to plug it into the wiring loom that I unplugged from the original radio.

So, I'm getting sound out of my tablet's micro-USB port, sending it through a USB hub to a DAC, splitting it by channels, amplifying it, and sending it out to the speakers. Usually this is all done within a radio's wiring, but I had to piece it apart myself. The end result is amazing sound quality and a very clean installation of the tablet.

Step 5: Peripherals and Power

Next, I wanted to be able to play FM radio in my car as well as stored and internet music. This is where a DVB-T comes into play. It's a USB stick that receives FM radio and plugs into my 4-port USB hub. It comes with a small antenna that plugs into that back of the stick. But I wanted to use my car's antenna rather than have to mount a tiny antenna on my dashboard. So I used an antenna extension cable and spliced it with the male end of the DVB-T's antenna that plugs into the back. I found the antenna wire that I unplugged from the original radio I removed, and plugged in my makeshift adapter. It worked flawlessly!

I also purchased a USB flash drive to hold music and movies to plug into the hub. So, in my USB hub, there is a USB flash drive, a DVB-T stick for radio, and a USB DAC for converting the audio signal from my tablet.

To connect everything to the N7, I needed an OTG-Y cable. This allows for 1 input into the tablet (micro USB) and splits the charging side and the data side of the build. On the data side, the USB hub plugs into the female end of the OTG-Y cable. This hub holds the USB DAC for signal processing, DVB-T, and flash drive, all mentioned before. If I decide I want to install some sort of backup camera, I can use that 4th port for it. On the charging side, I have a simple USB charging cable that plugs into a USB-cigarette lighter charger. The one I used (and added to the parts list link) draws the right amount of current, and is enough to charge the tablet rather than just maintain or slow the drain on the battery. This is plugged into a cigarette outlet that I added, so as to not take up the console's cigarette outlet and have exposed wiring. I went with a two-port cigarette lighter, just in case I wanted to use the other port to charge my phone. To add the socket, I ran wiring behind the dash and steering column and into the fuse box under the steering wheel. I used an Add-A-Fuse to pull power from a switched 12V source. I used the radio's fuse, just because I knew it worked and the radio wasn't using it anymore.

Step 6: Box It All Up

I wanted all of these devices and wiring to fit neatly inside the dash where the radio used to be. So I made a plexiglass box to hold everything. I just measured the dimensions of the radio and cut the plexiglass and glued it together with silicone glue. There is a small slot in the front of the box that is just big enough for my hand, so I can access certain parts that I want to mess with, like the charging socket and the USB hub. I bought a new dash trim piece for this project, so that I could keep the radio and the stock trim piece together to return everything to stock should I sell this car. I cut up this new trim piece to fit the mounting solution.

I decided to go with a RAM mount made for 7" tablets (see parts list). I used epoxy putty to hold the mount in, and then I used the putty to fix the male end of the OTG-Y cable so that the tablet will slide into the mount and be plugged in. This mount is very secure and allows for the whole screen to be seen. It also allows me to easily install my tablet and find the plug, and then remove it whenever I'm leaving the car.

Step 7: Software and Connectivity

Now for a note on software.

First off, you may not be able to use USB Audio, it's very software-specific. If you're using an Android device, it's software version 5.0 (Lollipop) and above comes with USB Audio native. If not, you may need to use a custom ROM or kernal that allows for it. This gets rather tricky and requires rooting the device and voiding your warranty. So, unless you have experience messing with your tablet's software like that, I would recommend you use another method to make audio work for you.

Next, the nice thing about this project is that you can vastly increase your car's functionality with applications designed for your car. For example, the Torque app allows you to install receive your car's diagnostics and engine information from a Bluetooth OBDII scanner that costs about $15. You just plug it into the OBDII port (usually under your dash on the driver's side) and connect it to the app. It's incredibly useful for engine light scans, fuel economy information, even GPS tracking. Which brings me to my next point.

You can also use your tablet as a GPS system. There are many apps out there that offer maps and turn-by-turn navigation instructions. I personally use Google Maps and Navigation. These apps use your tablet's GPS hardware, but are greatly amplified if your tablet is connected to WiFi. Some tablets have 3G or 4G, but mine is WiFi only. Which brings me to my next point.

If you are into rooting and installing custom ROMs on your device, you are probably familiar with WiFi tethering. I use my phone for tethering when my tablet is installed. It allows the tablet to use high-quality GPS, stream music, and give me voice-activated search and notifications. If your data plan supports it, I highly recommend tethering, or a mobile hot spot.

One last note on connectivity: I've found that i wasted some time after plugging in my tablet with turning on all of these apps and getting it ready for a drive. This is where NFC tags are invaluable. They can be used to run pre-programmed tasks just by touching them to your device. I have one NFC tag in my car to turn on and off my phone's WiFi hot spot. I also have a tag on the tablet's mount that turns on the GPS and Bluetooth, as well as starts the car docking app AutoMate. They're very handy and save you tons of time.

Step 8: Conclusion: Test and Optimize

Through every step of this project, be sure you're testing every component, checking grounds, and ensuring good connections. From experience, I can tell you that there is no bigger headache than getting all of your equipment and wiring shoved into the dash and then realizing it doesn't work or there is some bug you need to work out. Check for continuity through your amplifier and wiring harnesses, ensure your power is on/off when you expect it to be, make sure your sound is behaving at high volumes.

Also, find what works for you. Choose apps that make sense to you and enhance your driving experience, and that don't distract you from the task at hand. Make sure you can safely operate your tablet and your car at the same time. Optimize your build to your specific needs, and have fun with it!

Thank you for reading all the way through this, and I hope you gained something from it all. Good luck on your future projects!