Cmos Based Vehicle Anti-Theft Alarm

Posted on Feb 4, 2014

This circuit features automatic exit and entry delays - an optional instant alarm zone - an optional intermittent siren output - and an automatic reset. By adding external relays you can immobilize the vehicle and flash the lights. The alarm is set by opening Sw1. It can be any small 1-amp single-pole change-over switch - or for added security you

Cmos Based Vehicle Anti-Theft Alarm
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could use a key-switch. For automatic operation - use the contacts of a small relay in place of Sw1. Wire the relay coil so that it`s energized while the ignition is on. Then - every time you turn the ignition off - the alarm will set itself. Once Sw1 is opened you have about 10 to 15 seconds to get out of the vehicle and close the door behind you. When you return and open the door the buzzer will sound. You have 10 to 15 seconds to move Sw1 to the off position. If you fail to do so - the siren will sound. The output to the siren is intermittent. That is - it switches on and off. The speed at which it switches on and off is set by C6 & R10. If you want a constant siren output - leave out C6 & R10 - and connect pins 8 & 9 of the Cmos 4011 together. One of the alarm`s inputs is connected to the vehicle`s existing door-switches. This provides the necessary exit and entry delays. It`s usually sufficient to connect a single wire to just one of the door switches. These switches are generally all connected in parallel - with the return through the chassis. You can add extra normally-open switches to the door-circuit if you wish - but note that any additional switches will have to be able to carry the current required by your vehicle`s interior light. Any number of normally-open switches may be connected - in parallel - to the "Instant" input. Since they don`t have to carry the current for the interior light - you can use any type...

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