Anything that's rated for 12vdc and 3a would be suitable.  Just make
sure that it's not the max rating on the part you wish to use.  A relay
would probably work nicely, but that's going to drive a bit of power.
You might be able to find a power mosfet that would work.  How long do
you intend to run the siren?

Here's a 30V 9A mosfet from Vishay.  Heavy looking dude.
http://www.vishay.com/docs/70856/70856.pdf



Lucian wrote:

>Dear Piclisters,
>
>I have yet another question for you. I am still a newbie in electronics
>and PIC's and I feel a little bit shamed that I only come to ask
>questions on this list and rarely know how to answer to other questions,
>but hope that someday I will be able to be more active here.
>My questions is: I want to drive a siren from a microcontroller pin. The
>siren is an alarm one, and consumes 12V and 3A at maximum. Which would
>be the best way to control it (on/off) ? A power MOSFET, a triac maybe,
>a relay ?
>Thank you for your answers.
>
>Lucian
>
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