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How To : Remote start carPC using car alarm

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  • How To : Remote start carPC using car alarm

    I just thought I'd post this since a lot of people might be interested in doing something similar. My problem was that I didn't like waiting for my CarPC to boot up when I got in the car. Further complicating things was the fact that the weather in Nebraska where I live gets anywhere from extreme cold to extreme hot. For this reason I did not want to simply wire my computer to the remote start. Having it start separately means that in the winter when it's cold I can remote start the car, then remote start the computer after it warms up while I'm walking out, or during the summer just remote start the computer without wasting gas.

    Now this solution isn't for everybody, you need to have some sort of unused auxillary function on your car alarm (or any method of remotely triggering a constant +12v line). I had door lock and trunk pop and two unused ones. I also have an OPUS power supply for my PC, meaning that it can turn on and off with only a simple trigger wire, most people use the ignition, exactly how an amp turns on and off. You can do this even if you don't have an OPUS power supply with an ignition trigger, the wiring will just be a bit more complicated.

    So now without further hesitation, the simple solution :

    All you need is a standard relay, some wire, and a few different styles of crimp connectors. You need access to a constant +12V wire (low amperage is no problem, I think mine was 22Gauge), your computer-turn-on wire (most people use ignition), and your alarm's trigger wire.

    Step one : Program your alarm function. The extra function I used on my alarm only turned on for a brief time by default, however it could be programmed to turn on and stay on until the ignition is triggered. This was perfect for my purposes so I switched it over to that mode, now when I trigger that function the wire goes live and stays on until the key is turned on.

    Step two : Locate a constant +12V power wire. has a nice wire harness diagram that I used to locate one. A multimeter is your friend.

    Step three : Cut your computer turn on wire. Usually this is the ignition wire.

    Step four and up : Attach your alarm's trigger wire (the one that goes live when you hit the function key) to terminal 86 on the relay. Attach your constant +12v source to both terminals 85 and 87. The ignition wire should run through the relay entering on terminal 87a and exiting on terminal 30.

    Here is the diagram I made to explain this better

    That's it, you're done. Just tidy up your wiring,mount your relay down somewhere, and put everything back together that you might have torn apart to access the wires used for this simple mod. If everything was done correctly when a charge is activated to terminal 86, current from 87 should flow through 30 triggering your computer to start. When there is no current on 86, the relay connects 87a to 30, which means your computer will turn on and off with your key just like normal. In simpler terms, triggering your alarm function should now start your computer and it should still work with the key as well.

    Note : It is important that you program your alarm correctly. The trigger has to go live and stay on, but also turn off when the ignition is turned on. Most alarm functions have this option as well as a timed mode. If you do not set it properly it might only start your computer for a short time, or it might start it and never turn it off unless you trigger the alarm functions again, this would effectively make the key not do anything.

  • #2
    cheers Spoon. Exactly what I was planning on doing. Thanks for this!


    • #3
      No problem, I just finished doing this about an hour ago and was so pleased I thought I'd share. It's too easy to not do it if you already have an alarm and the proper power supply.


      • #4
        nice writeup! thanks for takeing the time
        My Install

        CFiG v1
        iGuidance Plugin for CF

        Working shell for a CarPC

        StopWatch plugin remade for centrafuse RC1


        • #5
          I did mine a little bit differently so I thought I'd share a bit. It's a little bit more involved and requires more relays, But I'm able to activate/deactivate the "ignition" wire going to my power supply by triggering the auxillary on my alarm or a push button within the car. I did this with a series of relays to form a "Latched On/Off Output Using a Single Momentary Pulse" circuit that you can find on

          Originally I only had the push button that only required one push to turn on/off the circuit. So say I was at a gas station and wanted to keep the carpc on but wanted to remove the key, all I needed to do was push the button to keep power going to my pc before removing the key. Later I realized that it only needed a negative pulse to activate it and realized the potential for to integrate it into the alarms auxillary function. Now I have both options.

          Good write up spoon
          "In the beginners mind there are many possibilities, but in the experts mind there are few."- Shunryu Suzuki
          "Do it right or don't do it at all"

          [-------90%-] (New Car=New Build)


          • #6
            Sounds like ours basically work the same way only my alarm had some of the relay work built in. I can keep my pc on when I remove the key only instead of a push button in the car I use my remote so I can even turn it off remotely if I decide to stay in longer than I anticipated. It also times out after 20 minutes automatically just like the remote start since it's hooked through the alarm.


            • #7
              You . I'm so happy I found this forum. Great idea and write-up.


              • #8
                Well I can't say it was entirely my idea, but there wasn't a good writeup on the subject so I thought since I just did it I'd explain the process a bit better.


                • #9
                  one thing is though, your car alarm has the ability to 'latch on'. Most cheapo alarms aka the one I have, only have the trunk pop which is a single signal. I guess I will have to go the route of creating a latch on relay with a bunch of relays.


                  • #10
                    Yeah, it had three modes, a pulse, a timed output, and a latched output. I had to program it to go to latched mode.