The most basic car alarm system: If you want to think about a car alarm in its simplest form, it is nothing but one or more sensors connected to some sort of siren. The very simplest alarm would have a switch on the driver's door, and it would be wired so that if someone opened the door the siren would start wailing. You could implement this car alarm with a switch, a couple of pieces of wire and a siren. The most modern car alarm system: 1-An array of sensors that can include switches, pressure sensors and motion detectors 2-A siren, often able to create a variety of sounds so that you can pick a distinct sound for your car 3-A radio receiver to allow wireless control from a key fob 4-An auxiliary battery so that the alarm can operate even if the main battery gets disconnected 5-A computer control unit that monitors everything and sounds the alarm -- the "brain" of the system