Car navigation by smartphone

Fri, 2011-05-13 18:50 by admin · Forum/category:

So you want to use your phone while driving. Good idea! Smartphones are excellent navigators. Google Maps has by far the most timely and the most accurate traffic information and dynamically routes you around traffic jams. If your time is valuable, you have hardly any choice but to use it.

But first you may stumble over two problems.

1. Where to put it

The three good ways to mount a smartphone in your care are these:

a) A suction cup windshield holder

b) A vent grid holder

c) A glue-to-the-dashboard holder

My current favorite is the vent grid holder. It has a whole range of advantages:

  • It does not block your view through the windshield.
  • It is in front of a dark background and therefore easier to read than in front of the windshield.
  • It is closer to you and therefore easier to read.
  • In warm weather the ventilation cools the phone.

A glue-to-the-dashboard holder may be almost as good, but it cannot be removed without trace, unless you can do it with velcro.

2. How to charge it

You will probably notice that the navigation uses a lot of energy to power the GPS receiver, the WiFi receiver (used for navigation by Google Maps), perhaps the track recorder (like Google's My Tracks), and, last, but not least, the display.

So I bought a cigarette lighter socket adapter with two USB sockets. The second one can be used to charge a second smartphone or to drive any other USB-powered device.

However, I noticed that the charger delivered less power than the phone consumed, so the battery still ran towards empty. It took a lot longer, but it would still not last for a long drive, particularly not if the battery was already half empty to begin with. What was wrong?

The problem lies in the USB specification. Computers, if they adhere to the standard (probably a bad thing in this case), limit the power current to 500 mA. If a device exceeds 100 mA, it has to negotiate with the computer first, to get more electricity. But the maximum is still 500 mA, not enough for today's smartphones.

If a device exceeds the maximal current, the computer is supposed to, and many computers actually do, shut down power on that port until the computer is rebooted.

The consequence is that the phone may not even try to draw more current, lest it disable its own energy source.

To circumvent this limitation for quick chargers, the manufacturers had to invent tricks to let the phone know that it is not connected to a standard USB port, but to a charger that can deliver more power.

There may be different schemes to achieve that. I only know one for sure, the one that is used by HTC. The trick for HTC devices is to shortcut the two data lines.

What I did is take a sharp knife and cut out the soft, black plastic on the micro USB plug. I turned it, so the slightly shorter side of the plug faced me and cut a bit nearer to the plug than to the cable side.

After cutting deeper and deeper, I eventually reached the solder points where the wires are soldered to the pins of the plug. Luckily I hit the two data connections first.

In the described orientation with the plug pointing up and the cable pointing down, a micro USB plug has five contacts, numbered 1 to 5 from left to right. Pins number 2 and 3 are the data pins. They are slightly left of center.

It may be easier to do the same operation on the big USB plug on the other side of the cable. That has only 4 pins. The middle two are the data pins.

I took a small soldering iron and placed a tiny drop of solder on the two contacts, melting and connecting them.

Then I went to my car, switched on the electricity, plugged in the charging adapter, stared on its light and very slowly inserted the modified USB cable, expecting sparks and smoke and perhaps a blown fuse, but nothing of the sort happened.

So I connected my Google/HTC Nexus One phone to the micro USB plug and checked the charging type by entering Menu, Settings, About phone, Status. Now it read, "Charging (AC)", which signals that I had won this battle. Before my operation it showed, "Charging (USB)".

Now the phone actually fills up its battery even while navigating.

Needless to say, you can no longer use this cable to transmit data.

I have heard, without confirmation, that other manufacturers use the otherwise unused pin 4 of the micro USB plug to signal the presence of a quick charger, by connecting it to pin 5 (ground).

The iPhone uses a different scheme. It actually talks to its charger via the data wires in a more complex, undisclosed way. Of course that charger is more expensive and forces users in typical Apple style to buy their chargers from Apple.

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