> Thank you all who answered to my siren question. Now I got it and hope
> it will work :)

One extra caution.

In the circuit you attached to your answer (below) you MUST use a "catch
diode" across the load. This will "catch" any inductive current which is
being conducted by the load when the FET turns off and allow it to circulate
until the inductive energy is dissipated. Without this the voltage on the
FET drain will rise to a VERY high value and can destroy the FET and/or
cause noise spikes which get into the microprocessor circuit and cause it to
operate very strangely.

Select a diode which is rated for the FET maximum load current and which
operates OK at the switching frequency concerned. For many purposes cheap
low frequency power diodes such as IN400x (1 amp rated) or or 1N54xx family
(3 amp rated) work fine. You can use several diodes in parallel to get extra
current rating but it is best to use a single properly rated diode if
possible.

Connect the diode across the load such that it does NOT conduct when the FET
is on - that is, diode's anode to the FET drain and the diode's cathode to
the power supply positive.

Even though you are using a motor (siren) here rather thana pure inductor,
failure to use a diode as above will cause you endless problems.



            Russell McMahon



>
>                           +12V          +12V
>                           |              |
>                           |              -------- LOAD
>                           <              -------- LOAD
>                           >  10K         |
>                           <              | d
>                           |___________|--
>                 1K        | c      g  |--   IRF530
> PIC PIN ------/\/\/\----|/               | s
>                      b  |\  2N2222       |
>                           | e            |
>                           |              |
>                          GND            GND
>
>
> This seemed to work very well. The trick is that the MOSFET must
> have its gate switch between fully on (+12V) and fully off (0V)
> otherwise it will get very hot.

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