Car alarm circuit

Designer: Carlos Damian Lopez Herrero
Operating voltage: 12V
Published at 08/2008

NOTE: This circuit design is not tested by staff. The circuit design as submitted by the designer is believed to be correct, but there is no guarantee of the correctness of the circuit design. The contents of the articles below might be totally inaccurate, inappropriate, or misguided. There is no guarantee as to the suitability of said circuits and information for any purpose.

Introduction to car alarms

A car alarm in its simplest form, needs to be 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.

Real world alarms have typically more sensors and controls. The center of the system is often called "brain". The "brain" can be anything from a quite simple electronics circuit (few ICs) up to a small computer (most adavnced systems). . The brain's job is to look for state of the sensors and activate alarm devices (your horn, headlights or an installed siren) when alarm is needed. Alarm is started when certain switches that power sensing devices are opened or closed. Security systems differ mainly in which sensors are used and how the various devices are wired into the brain.

General information

This is a remake of the alarm "cobra" for a car. The alarm cobra was very famous in the 1995 , it was widely sold in supermarkets. This design is a remake of that alarm. This remake allows the user to change all the sounds , times , etcs. The original design did not have any options for those and all the the cars had the same sound.

The alarm work with switch in the doors and the best by fall down voltage.

Orignal device

Original device

Original device

Component list

Circuit diagram

Circuit board

Component layout

Component layout

Component layout

Circuit adjustments and controls

Controls and adjustments

For adjusting the correct voltages on different places in the circuit can be found in Cobra circuit voltages document.

There is a two minute video available on the operation and adjustment of the circuit (video in Microsoft Windows .wmv format, plays with Windows Media Player).

Circuit diagram of speaker driver

Speaker driver