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Timed Switch for Car Alarm, How To

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Ok, I need to build this circuit for my car alarm siren output. I can't figure out how to build it:

What I want the circuit to do:
  • I want it to pass the 12V+ signal once it has been received for 3 consecutive seconds
  • It should not pass the signal if it is received for less than 3 seconds
  • Once the signal stops, it should stop passing the signal and reset

I got some air horns because the alarm's siren wasn't loud enough. The alarm dosen't have a dedicated horn honk output. With the horns hooked up at all times the arm/disarm chirps also honk the horns and they're too loud for everyday use. I could disable the arm/disarm chirps but I like them.

With this circuit the siren will, if you guys can help, chirp by itself when arming/disarming the vehicle and the siren will blare continually when the alarm is triggered while the horns honk on and off, because of the flasher, after 3 seconds.

I'll use a standard automotive relay to supply the power to the horns so the circuit only need to supply enough current to switch the relay, not sure exactly how much that is.

Any help or suggestions are appreciated!


I heard this simple circuit would work:

If so, could someone please give me some ballpark values for R1, C1, and Q1. Please be specific for the transistor because I have no expirence with them!
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