A good friend of mine recently sent me these great tips about a ready made alarm system. I believe the source was a Yahoo! forum.
CAR KEY TIPS!
Put your car keys beside your bed at night. If you hear a noise outside your home or someone trying to get in your house, just press the panic button for your car. The alarm will be set off, and the horn will continue to sound until either you turn it off or the car battery dies. This tip came from a neighborhood watch coordinator.
Next time you come home for the night and you start to put your keys away, think of this: It’s a security alarm system that you probably already have and requires no installation. Test it. It will go off from most everywhere inside your house and will keep honking until your battery runs down or until you reset it with the button on the key fob chain. It works if you park in your driveway or garage.
If your car alarm goes off when someone is trying to break in your house, odds are the intruder will not stick around. After a few seconds all the neighbors will be looking out their windows to see who is out there and sure enough the criminal won’t want that.
And remember to carry your keys while walking to your car in a parking lot.