Homemade alarm for your truck or car

This is a cheap way to put an alarm on your car or truck without spending alot of cash and can be done in 30 to 45 minutes. First, you need to find an old car alarm horn. Lots of cars already have unused ones under there hoods already but if not They can be picked up at pull-your-part junkyards for under 10 bucks. Next you need at least 12 gauge wire(only you know the amount) and a good high amperage all weather toggle switch.

Most vehicles dome lights have constant power and the door jamb switch is just a ground that activates when the door opens. This is what we want to take advantage of. On gm vehicles it is the white wire. Now you need to drill a hole in your inner fender to mount your toggle switch. It has to mount from the engine side  so the toggle part is outside the engine bay above the tire.  Mount your horn on the inner fender as well. Now one side(either as it doesn’t matter) of the horn goes to constant power and if you like you can add a fuse for safety.  The other side of the horn is wired to one side of the toggle switch. The other side of the toggle is wired back into the vehicle to the ground side of your dome lights.

This is the cool part, as you lock your vehicle you reach over the tire and hit the toggle. If someone opens a door or hatch the horn will sound! The alarm horn also will go through the different sounds as they are contained in the horns make up. Before you reenter your vehicle you simply flip the toggle again. Now I know this is cheap, but for my blazer it is just an extra line of defence when my top is off.

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Wednesday, April 8th, 2009 at 18:19 • Automotive InformationRSS 2.0 feed • leave a response or trackback

15 Responses to “Homemade alarm for your truck or car”

  1. Gary says:

    I realize this is an old thread but wanted to see if you could show how to wire a relay for the alarm system. I just installed it and works great just concerned about amp draw as I used a horn instead of siren

  2. Alexander says:

    Hi I am intending to fit this alarm. My question is: when the alarm is on. Does the siren/horn sound until the door is closed or does it keep sounding until the toggle switch is flicked to turn the alarm off.
    I think that the latter would be the best solution and if the diagram mentioned earlier with a relay would enable this, would you draw it for me please?

  3. john says:

    Just fitted to my car
    bought rev sirean and it was not load at all
    hooked to my horn works well
    Waiting on 135db sirean from ebay to wake up the neighborhood

  4. camron26 says:

    can you do this but use your car horn as the device that goes off when the door opens or would the horn just beep once and then stop???

    • miles says:

      Yes you can do that as well. You would just need to wire the dome light circuit to the horn relay. Many dome light circuits have the switches at the doors complete the ground to the light where the horn relay typically switches the +12VDC side of the horn circuit. Therefore you would need to have a relay in between that switches a +12VDC circuit when the ground to the switched side of the relay is completed by the dome light switches. If that doesn’t make sense I can draw you a circuit diagram.

  5. Dustcarts says:

    When you leave your car for the night, step back and open the fuel door, flip the switch then shut the fuel door. Now, if a car door gets opened, the dome light comes on, energizing the relay, and causing your car horn to honk, and will continue to honk till the door are closed.

  6. shaughn says:

    i am going to connect to the dome light an alarm as well as dropping mechanism containing a vile of tear gas… i live in South Africa and the theft is ridiculous… had enough!

  7. Jayson says:

    I like the simplicity of your hookup. Where did you tap into the dome light circuit?

    • jonny blaze says:

      At the dome light switch in the jamb. You can tap in pretty much at any light wether under the dash or the dome above.

      • Jayson says:

        So if I tap into the dome light switch on the drivers door will it work when the passengers door is opened?


      • Captain says:

        Based on the drawing I would think the alarm would not go on if the passanger door was opened but if the dome light wire was the source the alarm would go off no matter which door or hatch was opened.

        • Jonny Blaze says:

          Actually,at least in gm products, all the dome/courtesy lights are linked on the ground side. Thats why if you open a hatch,pass side,or dr. side door all the lights work. Essentially if you ground any of the switches all lights go on. So you can tap into the dome circuit anywhere to make this cheap alarm function.

  8. miles says:

    Looks cool. Too bad CUCV’s don’t have a dome light. The only suggestion I would have would be to trade the toggle switch for a keyed switch so someone who sees you activate it can’t deactivate it as easily, but that would make it more of a pain to activate/deactivate.

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