Quote Originally Posted by pedja089 View Post
Why not to use just 2 diodes?
Now I remember why I went with his transistor design, I need 12V for the siren.

If I use the diodes, the source must always be higher voltage than the battery. I end up with 15V from the main or "about" 12V from the battery. And then I can't add a 7812 voltage regulator at the end.

If I use transistors, I can use a 12V source. The OP AMP doesn't care if it's 12 or 15 volts, as long as it's there.

(and that's why I initially had a 12V source)

Robert