Hack46.Turn On Your Car Computer Before You Start Your Car


Hack 46. Turn On Your Car Computer Before You Start Your Car

Most cars today come with keyless entry systems that can be used to turn on your car PC remotely.

"Reduce the Boot Time of Your in-Car Computer" [Hack #44] suggested that one way to reduce the perceived boot time of your car PC is to have it boot when you open the car door. This way, by the time you get the engine started, the computer is fully booted and waiting for your commands.

When you open your car door, the dome light usually comes on. This behavior is controlled by a small plunger between the car door hinges in the frame of the car itself. It turns off when the car door is closed and on when it is open.

Opening the door causes 12V to activate somewhere in the system. This 12V may not be directly connected to the dome lightfor instance, it may trigger some computer that turns on the dome light for a while. Regardless, you can simply use the door opening or dome light 12V signal as a "start" command for the startup controller you selected in "Start Up and Shut Down Your Car PC" [Hack #43], by running a wire from the door button or dome light to the ACC/KEY input on your startup controller. Figure 4-14 shows how this works.

The door opening or dome light turning on will cause the computer to start booting. Depending on your startup controller, you will then have a short while (from 45 seconds to a couple of minutes) to bring the ACC/KEY line high (up to 12V) again after the door is closed or the dome light goes back off. As long as you start your car, or at least turn the key to the accessory position, in this time window, the startup controller will remain on and the computer will continue to receive power, and thus the computer will finish booting without interruption.

Figure 4-14. Booting up your computer using the dome light switch


If you start blending different 12V trigger inputs to the car PC, such as dome light and ACC, you must use a diode between each input and the startup controller to ensure the voltages don't "backwash." Otherwise, the dome light may come on whenever the car is on, or the car radio may try to power up whenever the door opens. Figure 4-15 shows how you can do this.

Once you've completed these steps, your car PC will start to boot the instant a door is opened, because the dome light will go on, which will also make the startup controller think that the car has turned on.

For many cars, using a remote keyless entry device ("key fob") to unlock the doors also activates the dome light, so using such a device may have the side effect of giving you remote activation. As long as you get to the car before your dome light goes out again, you won't lose power to your computer. (The light is usually on a timer, and it will go out a little while after you activate the lock or press the remote's unlock switch.)

You might want your car computer to go on only if you unlock your car. This is almost as straightforward; you need to find a wire that goes to 12V when the car is unlocked. A Chilton's or other shop manual for your car will provide you with wiring diagrams. Although cars vary, the harness of wires going to the driver's door will contain the wire you need. The wire that actually triggers the unlock will most likely be 12V, and you will find it running to a small unlocking motor if you pull the panel off your door.

Figure 4-15. Use diodes to ensure that voltages don't mix


But, if you are using a momentary 12V pulse such as the unlock signal to activate your PC, you need to ensure that the pulse is long enough to turn on the power supply, and that your startup controller doesn't immediately shut the computer back down when the pulse is over. The Opus power supply (described in both "Power Your Car PC" [Hack #42] and "Start Up and Shut Down Your Car PC" [Hack #43]) has the ability to wait up to one hour after the ACC/KEY line goes down before it tries to shut down the PC, so it is good for this application. Even better, the P1260 and P1280 startup regulators from Carnetix (http://www.carnetix.com), also described in "Start Up and Shut Down Your Car PC" [Hack #43], have this "Pulse Start" feature built in, and they are designed to allow you to power up your car PC with momentary door lock, car alarm, or wireless device triggers.

Another option for remote booting is to purchase a remote car starter that has buttons to turn on the car, roll down the windows, open the trunk, and so on. (For more information, Google "remote start.") If you want a dedicated "turn on my computer" button, you can use one of the remote car starter's unused switches for this task. If you configure your startup/shutdown controller to boot up the computer and leave it on for a while after it is triggered, you can tie it to any one of the 12V pulses from a remote starter.

Some aftermarket remotes have extended ranges, allowing you to turn on the car PC from very far away (e.g., 1,000 feet).

4.7.1. See Also

  • "Transfer Data to and from Your Car PC" [Hack #64]



    Car PC Hacks
    Car PC Hacks
    ISBN: 0596008716
    EAN: 2147483647
    Year: 2005
    Pages: 131

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