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How Car Alarms Work (2021 Update)

Most of us know car alarms are an important part of a vehicle security system, but how do car alarms work? Here at Compustar, we’ve been creating quality, dependable car alarms for decades, and we’d like to share some of the secret sauce that powers our systems with you in this article. 

According to the FBI, a motor vehicle is stolen and/or broken into every 44 seconds in the United States. Recent reports show that over $4.3 billion is lost every year due to vehicle theft, which is why more than ever, drivers need to make sure their vehicles are secured with a car alarm.

What is a Car Alarm?

The main purpose of a car alarm is to prevent the theft of your personal belongings within the vehicle and to prevent the theft of the vehicle itself. That said, not all car alarms are the same. Some are triggered by different actions and have different features. OEM car alarms tend to have basic features compared to aftermarket systems.

How Do Car Alarms Work?

A basic car alarm is made of up many components, but the most important parts you should keep in mind are the sensors, the siren, and the control unit. An aftermarket car alarm is wired into your vehicle by a professional installer who connects the door, shock, and any other sensors you need to the control unit. The control module is what sends the signal to the siren to alert you (and the neighborhood) that someone is tampering with your car.

OEM vs Aftermarket

OEM car alarms typically come with basic car alarm features. These features are usually limited to 1-way remote door locks, 1-way remote start, starter/injector kill, and horn trigger. Most are designed only to prevent vehicle theft and are not designed to protect your belongings.

Aftermarket alarm systems offer a wider array of components and features that can be added to beef up your security. The addition of an aftermarket car alarm using just the basic features can significantly increase your vehicle’s security.

Components of a car alarm

There are three main components of a car alarm: The control unit or ‘brain’, the siren and the remote/fob/telematics for controlling the functions. There is a multitude of accessories available to customize the alarm for your needs. These are all outlined below.

It’s important to note that car alarms will not completely eliminate theft and intrusion. However, by installing a security system with additional sensors and features such as 2-way remote alerts, you can significantly deter intruders from messing with your vehicle.

Alarm brain/control unit

The car alarm brain is essentially a computer that receives and sends commands, and is the main component in an aftermarket alarm system. The brain gets its power from the ignition harness or the battery but can also be protected with a battery backup for extra security.  All of the components in the system are connected to the brain; and the brain monitors all of them constantly while in an armed state.


An aftermarket siren provides a significantly louder noise than an OEM car horn. This will bring more attention and lessen the chance of a thief sticking around while the siren is going off.

Remote transmitter

Remotes and key fobs are used to send lock, unlock, and trunk release commands to your vehicle. A 2-way remote or fob can receive confirmations back from the brain confirming commands or alerting you of possible theft in progress.  You may also monitor and control your vehicle by using your phone if you have a telematics module such as DroneMobile.

Check out this post to learn more – Compustar Remote Guide

Tilt/shock sensor

The tilt/shock sensor detects impact and vehicle tilt. The shock sensor will be triggered when there is an impact on your vehicle. The tilt sensor will trigger the alarm if your vehicle is being towed or jacked up. This is a feature that is generally not available with OEM alarms and is available with all Compustar security systems.

Interior LED light

An interior LED light will let people know that you have an aftermarket security system. Car thieves are not as likely to attempt theft of your vehicle if they know you have a security system installed.

Door trigger sensing

Door trigger sensing will trigger the alarm if the doors of your vehicle are opened when the security system is armed. Door trigger sensing is not just limited to driver and passenger doors; it can also be used to detect trunk/hatch entry as well as hood entry.

Learn more about car security systems – How to Choose the Best Car Security System

Additional features that may come with an alarm

There are additional features that may come with or be added to an aftermarket alarm.  Some of these features can be added by simply connecting a wire or turning on a setting in the brain.  Others require additional parts and labor.

Popular Compustar Security Systems

  • CS852-A – 1-way LED with 3,000 ft of range. 
  • CSX7905-A – 2-way LCD with 3,000 ft of range.