Thread: USB In The Car

  1. #1
    Registered /usr
    Join Date
    Aug 2001
    Newport, South Wales, UK

    Lightbulb USB In The Car

    My car has two cigarette lighters in it, one actually has the press-in button thing and the other (in the armrest) just has a plastic cap.

    I bought a USB charger from the supermarket a while back. Although the blue LED on it can be annoying in the dark, it does the job as it can supply up to 2A to my Android phone.

    The armrest also has a 3.5mm jack that connects to the aux interface on the radio. I use it to play music from time to time.

    All is well in SMurf's world, you would think. But:-

    1. The charger bulges out of the cigarette lighter and, as mentioned, has a bloody blue LED on it;
    2. The 3.5mm cable doesn't seem to make good contact with the car sometimes, I find I have to twiddle it to get stereo sound;
    3. Cables everywhere!

    Being of sound technical mind, I have formulated a plan to remove the cigarette lighter and provide a USB port in its place, to charge my phone. As my phone is capable of USB OTG, I would also like to wire a USB sound card directly to the aux interface of the radio and run a USB cable back to the USB port that is charging the phone.

    So in theory, the 12V DC that was connected to the lighter now connects to a PSU supplying 5V at 3A or so, which is powering both the phone and the USB sound card. The phone becomes a host over the same link and uses the sound card for playback (stock Android kernels generally don't support this, so you will need to root your phone and put something else on there for this to work). I then only have one cable loose, which is fully digital so that's fine.

    But can I have a single USB connection between the phone->sound card that also has some sort of tap for power? This particular phone can't supply any power when it is an OTG host. I think that devices expect to negotiate power draw, which isn't going to work in my theoretical circuit.

    I have thought about doing this with a 4 port self-powered hub, but I can't supply a charging current to the phone through that.

    I'm guessing that I can't charge the phone and have it control other devices at the same time...

  2. #2
    Master Apprentice phantomotap's Avatar
    Join Date
    Jan 2008

    Have you considered hanging a "frankentooth"* device off the telephone running a normal cable to the auxiliary input?


    *) A lot of manufacturers have started making absolutely bizarre Bluetooth devices. One of the devices I've seen is a dongle with a standard issue male 3.5 mm stereo connection powering a Bluetooth audio profile.
    “Salem Was Wrong!” -- Pedant Necromancer
    “Four isn't random!” -- Gibbering Mouther

  3. #3
    Registered User
    Join Date
    Dec 2006
    Power is always supplied from host to device.

    When a device is first plugged in, it is allowed to draw at most 1 unit load (100mA).
    The host then reads the device descriptor from the device, which includes how many unit loads it needs (up to 5, and usually either 1 or 5).
    If the host can supply that much power, it continues the enumeration. If it can't, enumeration fails.
    Once enumeration is done and successful, the device is allowed to draw up to the # of unit loads it requested.

    A device (or OTG port working as a device) should never force a voltage on VBus (5V line). Something will blow up.

    There's also the dedicated charging port spec that allows devices to draw up to 1.8A if it detects that D+ and D- are shorted. Obviously no communication can happen in these cases.

    (And there's also Apple's own special way of negotiating for >500mA power, that everyone should just ignore because it's completely non-standard)

    A phone in OTG host mode cannot be charged by the device.

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