What I'm trying to do is add an auxiliary flashing brake light to a vehicle with an external flasher, but I only want that flasher to be active for about 3 or 4 seconds after the brakes have been applied, and to not activate with successive applications of the brake within a period of about 30-45 seconds.

So I'm thinking of two rising edge triggered 555 timers in monostable mode. Top one would be the 30 second timer, producing a long pulse whose rising edge would trigger the 4 second pulse to activate the flasher.

Would this work?

 

http://forum.allaboutcircuits.com/threads/member-selected-automotive-forums.54400/

 

The owners of All About Circuits have elected not to allow discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

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Here is a site that may provide you with some assistance: Electro Tech Online

Good luck