Fake Car Alarm Circuit

 

 

 

 

This is a circuit for a fake car alarm that I built several years ago. It flashes a red LED mounted on the dash to look like there is an alarm connected. This was designed to be low power, long lasting, and to shut off the light when the ignition switch is turned on. Mine has been going for several years with no problem and doesn’t use enough power to drain the battery even after sitting for several weeks. Since the LM3909* and LED are meant to be used with 5 volts or less I used a LM7805 voltage regulator to drop the 12 volt battery power to 5 volts. The relay is there to turn it off when the ignition switch is turned on. I don’t remember if I used the Slow RC or Fast RC pins so try it and see which works better. You can change the flash rate by changing the size of the capacitor.

 

Parts list:

 

LM3909M (* - Discontinued. Cross References to NTE876 which can be bought from mouser.com)

LM7805A

OUAZ-SS-112D SPDT Relay

330 uF 50v Capacitor

Project box with PC board from Radio Shack

Solder

Wire

Here are some pictures of the completed kit.